EntranceTypes of chemical reactions in organic chemistry with examples. Main types of organic reactions
Lesson 2. Classification of reactions in organic chemistry. Exercises on isomerism and homologues
CLASSIFICATION OF REACTIONS IN ORGANIC CHEMISTRY.
There are three main classifications organic reactions.
1 Classification according to the method of breaking covalent bonds in the molecules of reacting substances.
§ Reactions proceeding through the mechanism of free radical (homolytic) bond cleavage. Low-polar covalent bonds undergo such cleavage. The resulting particles are called free radicals – chem. a particle with an unpaired electron that is highly chemically active. A typical example of such a reaction is the halogenation of alkanes, For example:
§ Reactions proceeding through the mechanism of ionic (heterolytic) bond cleavage. Polar covalent bonds undergo such cleavage. At the moment of the reaction, organic ionic particles are formed - a carbocation (an ion containing a carbon atom with positive charge) and carbanion (an ion containing a carbon atom with a negative charge). An example of such a reaction is the hydrohalogenation reaction of alcohols, For example:
2. Classification according to the reaction mechanism.
§ Addition reactions - a reaction during which one is formed from two reacting molecules (unsaturated or cyclic compounds enter). As an example, give the reaction of hydrogen addition to ethylene:
§ Substitution reactions are a reaction that results in the exchange of one atom or group of atoms for other groups or atoms. As an example, give the reaction of methane with nitric acid:
§ Elimination reactions – separation of a small molecule from the original organic substance. There is a-elimination (elimination occurs from the same carbon atom, unstable compounds are formed - carbenes); b-elimination (elimination occurs from two neighboring carbon atoms, alkenes and alkynes are formed); g-elimination (elimination occurs from more distant carbon atoms, cycloalkanes are formed). Give examples of the above reactions:
§ Decomposition reactions - reactions that result in one molecule of org. Several simpler compounds are formed. A typical example of such a reaction is the cracking of butane:
§ Exchange reactions - reactions during which molecules of complex reagents exchange their constituent parts. As an example, give the reaction between acetic acid and sodium hydroxide:
§ Cyclization reactions are the process of formation of a cyclic molecule from one or more acyclic ones. Write the reaction for producing cyclohexane from hexane:
§ Isomerization reactions are the reaction of the transition of one isomer to another under certain conditions. Give an example of butane isomerization:
§ Polymerization reactions are a chain process, the sequential combination of low molecular weight molecules into larger high molecular weight ones by attaching a monomer to the active center located at the end of the growing chain. Polymerization is not accompanied by the formation of by-products. A typical example is the reaction of polyethylene formation:
§ Polycondensation reactions are the sequential combination of monomers into a polymer, accompanied by the formation of low molecular weight by-products (water, ammonia, hydrogen halide, etc.). As an example, write the reaction for the formation of phenol-formaldehyde resin:
§ Oxidation reactions
a) complete oxidation (combustion), For example:
b) incomplete oxidation (oxidation is possible with atmospheric oxygen or strong oxidizing agents in solution - KMnO 4, K 2 Cr 2 O 7). As an example, write down the reactions of the catalytic oxidation of methane with atmospheric oxygen and options for the oxidation of ethylene in solutions with different meaning pH:
3. Classification according to the chemistry of the reaction.
· Halogenation reaction – introduction of org. into the molecule. compounds of a halogen atom by substitution or addition (substitutive or addition halogenation). Write the halogenation reactions of ethane and ethene:
· Hydrohalogenation reaction – addition of hydrogen halides to unsaturated compounds. The reactivity increases with increasing molar mass of Hhal. In the case of the ionic reaction mechanism, the addition proceeds according to Markovnikov's rule: a hydrogen ion attaches to the most hydrogenated carbon atom. Give an example of the reaction between propene and hydrogen chloride:
· The hydration reaction is the addition of water to the original organic compound and obeys Markovnikov’s rule. As an example, write the hydration reaction of propene:
· Hydrogenation reaction is the addition of hydrogen to an organic compound. Usually carried out in the presence of metals of Group VIII of the Periodic Table (platinum, palladium) as catalysts. Write the reaction of acetylene hydrogenation:
· Dehalogenation reaction – removal of a halogen atom from an org molecule. connections. As an example, give the reaction for producing butene-2 from 2,3-dichlorobutane:
· Dehydrohalogenation reaction – elimination of a hydrogen halide molecule from organic molecule with the formation of a multiple bond or cycle. Usually obeys Zaitsev's rule: hydrogen is split off from the least hydrogenated carbon atom. Write down the reaction of 2-chlorobutane with an alcohol solution of potassium hydroxide:
· Dehydration reaction – splitting off a water molecule from one or more organic molecules. substances (intramolecular and intermolecular dehydration). It is carried out at high temperatures or in the presence of water-removing agents (conc. H 2 SO 4, P 2 O 5). Give examples of ethyl alcohol dehydration:
· Dehydrogenation reaction – removal of a hydrogen molecule from an org. connections. Write the reaction of ethylene dehydrogenation:
· Hydrolysis reaction – exchange reaction between matter and water. Because hydrolysis is in most cases reversible; it is carried out in the presence of substances that bind reaction products, or the products are removed from the reaction sphere. Hydrolysis is accelerated in an acidic or alkaline environment. Give examples of aqueous and alkaline (saponification) hydrolysis of ethyl acetic acid:
· Esterification reaction - the formation of an ester from an organic or inorganic oxygen-containing acid and an alcohol. Conc. is used as a catalyst. sulfuric or hydrochloric acid. The esterification process is reversible, so the products must be removed from the reaction sphere. Write down the esterification reactions of ethyl alcohol with formic and nitric acids:
· Nitration reaction – introduction of the –NO 2 group into the org molecules. connections, For example, the nitration reaction of benzene:
· Sulfonation reaction – introduction of the –SO 3 H group into org molecules. connections. Write down the methane sulfonation reaction:
· Alkylation reaction – introduction of a radical into org molecules. compounds due to exchange or addition reactions. As an example, write down the reactions of benzene with chloroethane and with ethylene:
Exercises on isomerism and homologues
1. Indicate which of the following substances are homologues with respect to each other: C 2 H 4, C 4 H 10, C 3 H 6, C 6 H 14, C 6 H 6, C 6 H 12, C 7 H 12 , C 5 H 12 , C 2 H 2 .
2. Compose structural formulas and give names to all isomers of the composition C 4 H 10 O (7 isomers).
3. Products of complete combustion of 6.72 liters of a mixture of ethane and its homologue, which has one more carbon atom, were treated with an excess of lime water, resulting in the formation of 80 g of sediment. Which homologue was more abundant in the original mixture? Determine the composition of the initial mixture of gases. (2.24L ethane and 4.48L propane).
4. Make up the structural formula of an alkane with a relative hydrogen vapor density of 50, the molecule of which contains one tertiary and quaternary carbon atom.
5. Among the proposed substances, select the isomers and compose their structural formulas: 2,2,3,3,-tetramethylbutane; n-heptane; 3-ethylhexane; 2,2,4-trimethylhexane; 3-methyl-3-ethylpentane.
6. Calculate the vapor density in air, hydrogen and nitrogen of the fifth member of the homologous series of alkadienes (2.345; 34; 2.43).
7. Write the structural formulas of all alkanes containing 82.76% carbon and 17.24% hydrogen by mass.
8. For the complete hydrogenation of 2.8 g of ethylene hydrocarbon, 0.896 liters of hydrogen (no.) were consumed. Identify a hydrocarbon if it is known to have a straight-chain structure.
9. When adding which gas to a mixture of equal volumes of propane and pentane, its relative oxygen density will increase; will it decrease?
10. Give the formula for a simple gaseous substance, having the same air density as the simplest alkene.
11. Make up structural formulas and name all hydrocarbons containing 32e in a molecule of 5 isomers).
Exist different systems classifications of organic reactions, which are based on various characteristics. Among them are the following classifications:
- By the final result of the reaction, that is, a change in the structure of the substrate;
- By reaction mechanism, that is, by the type of bond breaking and the type of reagents.
Substances interacting in an organic reaction are divided into reagent And substrate. In this case, the reagent is considered to attack the substrate.
DEFINITION
Reagent- a substance that acts on an object - a substrate - and causes a change in the chemical bond in it. Reagents are divided into radical, electrophilic and nucleophilic.
DEFINITION
Substrate, is generally considered to be a molecule that provides a carbon atom for a new bond.
CLASSIFICATION OF REACTIONS ACCORDING TO THE FINAL RESULT (CHANGE IN THE STRUCTURE OF THE SUBSTRATE)
In organic chemistry, four types of reactions are distinguished according to the final result and change in the structure of the substrate: addition, substitution, detachment, or elimination(from English to eliminate- remove, split off), and rearrangements (isomerizations)). This classification is similar to the classification of reactions in inorganic chemistry according to the number of initial reagents and resulting substances, with or without a change in composition. Classification according to the final result is based on formal criteria, since the stoichiometric equation, as a rule, does not reflect the reaction mechanism. Let's compare the types of reactions in inorganic and organic chemistry.
Type of reaction in inorganic chemistry | Example | Type of reaction in organic chemistry | Variety and example Reactions |
|---|---|---|---|
1. Connection | C l2 + H2 = 2 H C l | Joining by multiple connections | Hydrogenation |
Hydrohalogenation
|
|||
Halogenation
|
|||
Hydration
|
|||
2. Decomposition | 2
H2
O=2 H2
+
O2
| Elimination | Dehydrogenation
|
Dehydrohalogenation
|
|||
Dehalogenation
|
|||
Dehydration
|
|||
3. Substitution | Z n + 2 H C l =ZnCl2+H2 | Substitution |
|
4. Exchange (special case - neutralization) | H2 S O4 + 2 N a O H=N a 2 S O 4 + 2 H 2 O | special case - esterification |
|
5. Allotropization | graphite ⇔ diamond Pred⇔ Pwhite P red ⇔ P white Srhombus.⇔ Splast. Srhomb.⇔Splastic | Isomerization | Isomerization alkanes
|
n) without replacing them with others.
Depending on which atoms are split off - neighboring ones C–C or isolated by two or three or more carbon atoms – C–C–C– C–, –C–C–C–C– C–, compounds can form with multiple bonds and or cyclic compounds. The elimination of hydrogen halides from alkyl halides or water from alcohols occurs according to Zaitsev’s rule.
DEFINITION
Zaitsev's rule: A hydrogen atom H is removed from the least hydrogenated carbon atom.
For example, the elimination of a hydrogen bromide molecule occurs from neighboring atoms in the presence of an alkali, resulting in the formation of sodium bromide and water.
DEFINITION
Regrouping- a chemical reaction that results in a change in the relative arrangement of atoms in a molecule, the movement of multiple bonds or a change in their multiplicity.
Rearrangement can be carried out while maintaining the atomic composition of the molecule (isomerization) or changing it.
DEFINITION
Isomerization- a special case of a rearrangement reaction leading to the transformation of a chemical compound into an isomer through a structural change in the carbon skeleton.
Rearrangement can also occur by a homolytic or heterolytic mechanism. Molecular rearrangements can be classified according to various criteria, for example, by the saturation of the systems, by the nature of the migrating group, by stereospecificity, etc. Many rearrangement reactions have specific names - Claisen rearrangement, Beckmann rearrangement, etc.
Isomerization reactions are widely used in industrial processes, such as petroleum refining to increase the octane number of gasoline. An example of isomerization is the transformation n-octane to isooctane:
CLASSIFICATION OF ORGANIC REACTIONS BY REAGENT TYPE
DISCONNECTION
Bond cleavage in organic compounds can be homolytic or heterolytic. 
DEFINITION
Homolytic bond cleavage- this is such a break, as a result of which each atom receives an unpaired electron and two particles are formed that have similar electronic structure- free radicals.
A homolytic break is characteristic of nonpolar or weakly polar bonds, for example C–C, Cl–Cl, C–H, and requires large quantity energy.
The resulting radicals, which have an unpaired electron, are highly reactive, therefore chemical processes reactions occurring with the participation of such particles are often of a “chain” nature, they are difficult to control, and the reaction results in a set of substitution products. Thus, when methane is chlorinated, the substitution products are chloromethane C H3 C l CH3Cl, dichloromethane C H2 C l2 CH2Cl2, chloroform C H C l3 CHCl3 and carbon tetrachloride C C l4 CCl4. Reactions involving free radicals proceed through the metabolic mechanism of formation chemical bonds.
The radicals formed during such bond cleavage cause radical mechanism the course of the reaction. Radical reactions usually occur at elevated temperatures or radiation (eg light).
Due to their high reactivity, free radicals can have a negative effect on the human body, destroying cell membranes, affecting DNA and causing premature aging. These processes are associated primarily with lipid peroxidation, that is, the destruction of the structure of polyunsaturated acids that form fat inside the cell membrane.
DEFINITION
Heterolytic bond cleavage- this is a gap in which an electron pair remains with a more electronegative atom and two charged particles are formed - ions: a cation (positive) and an anion (negative).
In chemical reactions, these particles perform the functions of " nucleophiles"("phil" - from gr. be in love) And " electrophiles", forming a chemical bond with the reaction partner according to the donor-acceptor mechanism. Nucleophilic particles provide an electron pair to form a new bond. In other words,
DEFINITION
Nucleophile- an electron-rich chemical reagent capable of interacting with electron-deficient compounds.
Examples of nucleophiles are any anions ( C l− , I− , N O− 3 Cl−,I−,NO3− etc.), as well as compounds having a lone electron pair ( N H3 , H2 O NH3,H2O).
Thus, when a bond is broken, radicals or nucleophiles and electrophiles can be formed. Based on this, three mechanisms of organic reactions occur.
MECHANISMS OF ORGANIC REACTIONS
Free radical mechanism: the reaction is started by free radicals formed when homolytic rupture bonds in a molecule.
The most typical option is the formation of chlorine or bromine radicals during UV irradiation.
1. Free radical substitution
methane bromomethane
Chain initiation
Chain growth
Open circuit
2. Free radical addition
ethene polyethylene
Electrophilic mechanism: the reaction begins with electrophilic particles that receive a positive charge as a result heterolytic rupture communications. All electrophiles are Lewis acids.
Such particles are actively formed under the influence of Lewis acids, which enhance the positive charge of the particle. Most often used A l C l3 , F e C l3 , F e B r3 ,ZnC l2 AlCl3,FeCl3,FeBr3,ZnCl2, performing the functions of a catalyst.
The site of attack of the electrophile particle is those parts of the molecule that have increased electron density, i.e., the multiple bond and the benzene ring.
The general form of electrophilic substitution reactions can be expressed by the equation:
1. Electrophilic substitution
benzene bromobenzene
2. Electrophilic connection
propene 2-bromopropane
propyne 1,2-dichloropropene
The addition to unsymmetrical unsaturated hydrocarbons occurs in accordance with Markovnikov’s rule.
DEFINITION
Markovnikov's rule: addition to unsymmetrical alkenes of molecules of complex substances with the conditional formula HX (where X is a halogen atom or hydroxyl group OH–), the hydrogen atom is added to the most hydrogenated (containing the most hydrogen atoms) carbon atom at the double bond, and X to the least hydrogenated.
For example, the addition of hydrogen chloride HCl to a propene molecule C H3 – C H = C H2 CH3–CH=CH2.
The reaction proceeds by the mechanism of electrophilic addition. Due to the electron-donating influence C H3 CH3-group, the electron density in the substrate molecule is shifted to the central carbon atom (inductive effect), and then along the system of double bonds - to the terminal carbon atom C H2 CH2-groups (mesomeric effect). Thus, the excess negative charge is localized precisely on this atom. Therefore, the attack begins with the hydrogen proton H+ H+, which is an electrophilic particle. A positively charged carbene ion is formed [ C H3 – C H − C H3 ] + + , to which the chlorine anion is added C l− Cl−.
DEFINITION
Exceptions to Markovnikov's rule: the addition reaction proceeds against Markovnikov’s rule if the reaction involves compounds in which the carbon atom adjacent to the carbon atom of the double bond partially absorbs the electron density, that is, in the presence of substituents that exhibit a significant electron-withdrawing effect (–C C l3 , – C N , – C O O H(–CCl3,–CN,–COOH and etc.).
Nucleophilic mechanism: the reaction begins with nucleophilic particles having a negative charge, formed as a result heterolytic rupture communications. All nucleophiles - Lewis's foundations.
In nucleophilic reactions, the reagent (nucleophile) has a free pair of electrons on one of the atoms and is a neutral molecule or anion ( H a l– , O H– , R O− , R S– , R C O O– , R– , C N – , H2 O, R O H, N H3 , R N H2 Hal–,OH–,RO−,RS–,RCOO–,R–,CN–,H2O,ROH,NH3,RNH2 and etc.).
The nucleophile attacks the atom in the substrate with the lowest electron density (i.e., with a partial or complete positive charge). The first step in the nucleophilic substitution reaction is the ionization of the substrate to form a carbocation. In this case, a new bond is formed due to the electron pair of the nucleophile, and the old one undergoes heterolytic cleavage followed by elimination of the cation. An example of a nucleophilic reaction is nucleophilic substitution (symbol SN SN) at a saturated carbon atom, for example alkaline hydrolysis of bromo derivatives.
1. Nucleophilic substitution
2. Nucleophilic addition
ethanal cyanohydrin
source http://foxford.ru/wiki/himiya
Municipal budget educational institution
"Secondary school No. 19"
Michurinsk Tambov region
Types chemical reactions
in organic chemistry
Golovkina Svetlana Alexandrovna,
chemistry teacher MBOU Secondary School No. 19, Michurinsk
Content
Abstract……………………………………………………………………………….3
Introduction…………………………………………………………………………………4
Test specification………………………………………………………...5
Tests 9th grade………………………………………………………………15
Tests 11th grade…………………………………………………………………………………24
Information resources……………………………………………………..33
Annotation.
This work reflects the authors' approach to studying the types of chemical reactions in organic chemistry. The proposed material may be of interest to chemistry teachers working in basic and full-time secondary school, since it provides a generalization of the basic concepts of the types of chemical reactions in organic chemistry, which will allow you to prepare for the GIA and Unified State Exam and work out material on this topic.
Introduction.
The material of organic chemistry is difficult to understand, especially in the 9th grade, where very little time is allocated for its study, with a large amount of theoretical material. Questions on organic chemistry are included in the KIMs of the State Academy of Sciences and the Unified State Examination; when preparing students for the final certification, the teacher is often faced with a misunderstanding of this material. You can intensify the teaching process and improve the quality of assimilation of organic chemistry by actively using modern technologies training, for example, the use of ICT, technology test control. In the manual, teachers share their experiences when studying small but complex material.
Specification of tests for preparation for the State Examination and the Unified State Exam
Prescribing tests- estimate general education training students on types of chemical reactions.
Continuity of test material content – show the relationship basic concepts inorganic and organic chemistry.
Characteristics of test content – Each version of the test control consists of three parts and tasks. Tasks of the same level of complexity and presentation form are grouped in certain parts of the work.
Part A contains 10 tasks for choosing an answer of a basic level of complexity A1, A2 .... A10
Part B contains 3 tasks for choosing an answer of increased difficulty level B1, B2, B3
PartC contains 1 task high level of complexity.
Table 1 Distribution of tasks by parts of work.
Multiple Choice Questions check the main part of the studied material: language chemical science, chemical bonds, knowledge of properties organic matter, types and conditions of chemical reactions.
Tasks of an increased level of difficulty test at an increased level of knowledge about redox reactions. The work offers multiple-choice tasks.
Completing tasks of an increased level of complexity allows you to differentiate students according to their level of preparation and, on this basis, give them higher grades.
Long answer questions- the most difficult in the test. These tasks test the mastery of the following content elements: amount of substance, molar volume and molar mass substances, mass fraction of dissolved substance.
4.Distribution of tasks test work by content, tested skills and types of activities.
When determining the content of the test tasks, the volume of each content block occupied in the chemistry course was taken into account.
5. Lead time
45 minutes are allotted for completing the test (1 lesson)
Approximate distribution of time allocated for completing individual tasks:
for each task of part A up to 2 minutes.
for each task of part B up to 5 minutes.
for each task of part C up to 10 minutes.
6. Evaluation system for individual tasks and work as a whole
Correct completion of each task in Part A is scored 1 point.
Correct completion of each task in Part B is scored 2 points;
An error was made in one of the answer elements - 1 point.
Completion of tasks in part C is variable, correct and complete completion of task C1 - 4 points,
The points received by students for completing all tasks are summed up. The rating is given on a five-point scale.
7. Gradation of assessment:
0% - 25% - from the points scored “1”
26% - 50% - from “2” points scored
51% - 75% - from the points scored “3”
76% - 85% - from the points scored “4”
86% - 100% - from the points scored “5”
Types of chemical reactions in organic chemistry
Chemical reaction - this is a change in substances in which old chemical bonds are broken and new chemical bonds are formed between the particles (atoms, ions) from which substances are built.
Chemical reactions are classified:
1. By the number and composition of reagents and products
This type of reaction may include isomerization reactions, which proceed without changes not only qualitatively, but also quantitative composition molecules of substances.
Decomposition reactions in organic chemistry, in contrast to decomposition reactions in inorganic chemistry, have their own specifics. They can be considered as processes inverse to addition, since they most often result in the formation of multiple bonds or cycles.
CH3-CH2-C=-CH CH3-C=-C-CH3
ethyl acetylene dimethyl acetylene
In order to enter into an addition reaction, an organic molecule must have a multiple bond (or cycle), this molecule will be the main one (substrate). A simpler molecule (often an inorganic substance, a reagent) is added at the site where the multiple bond is broken or the ring opens. Most often, multiple bonds or cycles are formed.
Their distinguishing feature is the interaction of a simple substance with a complex one. The concept of “substitution” in organic chemistry is broader than in inorganic chemistry. If in the molecule of the original substance any atom or functional group is replaced by another atom or group, these are also substitution reactions.
Exchange reactions are reactions that occur between complex substances, in which their components exchange places. Typically these reactions are considered ionic. Reactions between ions in electrolyte solutions proceed almost completely towards the formation of gases, precipitation, and weak electrolytes.
2. By thermal effect
Exothermic reactions occur with the release of energy.
These include almost all compound reactions.
Exothermic reactions that occur with the release of light are classified as combustion reactions. The hydrogenation of ethylene is an example of an exothermic reaction. It runs at room temperature.
Endothermic reactions occur with the absorption of energy.
Obviously, almost everyone will relate to them decomposition reactions,
CH 2 =CH 2 + H 2 → CH 3 -CH 3
3. According to the use of catalyst
They run without a catalyst.
Since all biochemical reactions occurring in the cells of living organisms occur with the participation of special biological catalysts of a protein nature - enzymes, they are all catalytic or, more precisely, enzymatic.
4. By direction
They flow simultaneously in two opposite directions.
The overwhelming majority of such reactions are.
In organic chemistry, the sign of reversibility is reflected by the names - antonyms of the processes:
hydrogenation - dehydrogenation,
hydration - dehydration,
polymerization - depolymerization.
All reactions are reversible esterification (the opposite process, as you know, is called hydrolysis) and protein hydrolysis, esters, carbohydrates, polynucleotides. The reversibility of these processes underlies most important property living organism - metabolism.
Under these conditions they flow in only one direction.
These include all exchange reactions accompanied by the formation of a precipitate, gas or slightly dissociating substance (water) and all combustion reactions.
5. According to the state of aggregation
Reactions in which the reactants and reaction products are in different states of aggregation (in different phases).
Reactions in which the reactants and reaction products are in the same state of aggregation(in one phase).
6. By changing oxidation states chemical elements, forming substances
Reactions that occur without changing the oxidation states of chemical elements. These include, for example, all ion exchange reactions, as well as many joining reactions, many decomposition reactions, esterification reactions
Reactions that occur with changes in the oxidation states of elements. These include many reactions, including all substitution reactions, as well as those reactions of combination and decomposition in which at least one simple substance is involved.
HCOOH + CH 3 OH → HCOOCH3 + H2O
7. According to the flow mechanism.
They go between the radicals and molecules formed during the reaction.
As you already know, in all reactions old chemical bonds are broken and new chemical bonds are formed. The method of breaking the bond in the molecules of the starting substance determines the mechanism (path) of the reaction. If a substance is formed due to covalent bond, then there can be two ways to break this bond: hemolytic and heterolytic. For example, for molecules Cl2, CH4, etc., hemolytic cleavage of bonds is realized; it will lead to the formation of particles with unpaired electrons, that is, free radicals.
They go between ions that are already present or formed during the reaction.
Typical ion reactions is the interaction between electrolytes in solution. Ions are formed not only during the dissociation of electrolytes in solutions, but also under the action of electrical discharges, heating or radiation. Ŷ-Rays, for example, convert water and methane molecules into molecular ions.
According to another ionic mechanism, reactions of addition of hydrogen halides, hydrogen, halogens to alkenes, oxidation and dehydration of alcohols, replacement of alcohol hydroxyl with halogen occur; reactions characterizing the properties of aldehydes and acids. In this case, ions are formed by the heterolytic cleavage of polar covalent bonds.
8. By the type of energy initiating the reaction.
They are initiated by high-energy radiation - X-rays, nuclear radiation (Ý-rays, a-particles - He2+, etc.). With the help of radiation reactions, very rapid radiopolymerization, radiolysis (radiation decomposition), etc. are carried out.
For example, instead of the two-stage preparation of phenol from benzene, it can be obtained by reacting benzene with water under the influence of radiation emissions. In this case, radicals [·OH] and [·H·] are formed from water molecules, with which benzene reacts to form phenol:
C6H6 + 2[OH] -> C6H5OH + H20
Vulcanization of rubber can be carried out without sulfur using radiovulcanization, and the resulting rubber will be no worse than traditional
They are initiated by thermal energy. These include all endothermic reactions and many exothermic reactions, the initiation of which requires an initial supply of heat, that is, initiation of the process.
They are initiated by light energy. In addition to the photochemical processes of HCl synthesis or the reaction of methane with chlorine discussed above, these include the production of ozone in the troposphere as a secondary atmospheric pollutant.
This type of reaction also includes the most important process, flowing in plant cells, - photosynthesis.They are initiated by electricity. In addition to the well-known electrolysis reactions, we will also indicate electrosynthesis reactions, for example, the reactions industrial production inorganic oxidizing agents.
Test tasks for 9th grade
Option 1.
Part A
A1. What models correspond to alkene molecules?
a) everything except A
b) everything except B
c) everything except B
d) everything except G
A2. What reagent can alkanes react with:
a) Br 2 (solution)
b) Cl 2 (light)
c) H 2 SO 4
d) NaOH
A3. In the reaction of 1,3-butadiene with HCl cannot be formed
a) 3-chlorobutene-1 c) 1-chlorobutene-2
b) 4-chlorobutene-1 d) 2,3-dichlorobutane
A4. The substance with which formic acid, under appropriate conditions, enters into a redox reaction is:
a) copper;
b) copper (II) hydroxide;
c) copper (II) chloride;
d) copper (II) sulfate.
A5. The interaction of an ester with water can be called:
a) hydration;
b) dehydration;
c) hydrolysis;
d) hydrogenation.
A6. In a chain of transformations
reactions “a” and “b” are respectively:
a) hydration and oxidation;
b) oxidation and hydration;
c) hydration and hydration;
d) oxidation and oxidation.
A7. The reaction caused by the presence of a double bond in the molecules of carbonyl compounds is the reaction:
a) accession;
b) decomposition;
c) substitution;
d) exchange.
A8. Using an ammonia solution of silver oxide it is impossible to recognize:
a) ethanol and ethanal;
b) propanal and propanone;
c) propanal and glycerin;
d) butanal and 2-methylpropanal.
A9. When propenal is exposed to excess hydrogen, it forms I:
a) limiting alcohol;
b) unsaturated alcohol;
c) unsaturated hydrocarbon;
d) saturated hydrocarbon.
A10. Acetaldehyde formed upon hydration:
a) ethane;
b) ethene;
c) etina;
d) ethanol.
Part B
IN 1. Match the type of reaction with the equation
REACTION TYPE
AT 2. Acetylene weighing 10.4 g added hydrogen chloride weighing 14.6 g. The formula of the reaction product is _____.
AT 3. From technical calcium carbide weighing 1 kg, acetylene with a volume of 260 l (n.s.) was obtained. The mass fraction of impurities (in %) contained in the calcium carbide sample is ____ _____. (Write your answer to the nearest hundredth).
Part C.
C 1. Write the reaction equations that can be used to carry out the following
BaCl2
transformations: C O
Option 2.
Part A For each of the tasks A1-A10, four answer options are given,
only one of which is correct. Circle the answer number.
A1. Reaction characteristic of alkanes
a) accession
b) substitution
c) hydration
d) exchange
A2. What hydrocarbons are characterized by polymerization reactions?
a) CH
4b) C 2 H 4
c) C 6 H 6
d) C 2 H 5 OH
A3. A substance with which methane undergoes a displacement reaction.
a) CL 2 (light)
b) H 2 O
c) H 2 SO 4
d) NaOH
A4. Which substance is easily oxidized by potassium permanganate.
a) C 2 H 6
b) C 2 H 2
c) C 2 H 5 OH
d) C 6 H 6
A5. What substance can be subjected to dehydration reaction.
a) C 2 H 4
b) C 2 H 5 OH
c) CH 4
d) C H 3 COH
A6. In a chain of transformations C 2 H 6 – acetylene – ethane reactions “a” and “b” - this corresponds
a) hydration and hydrogenation
b) hydration and oxidation
c) dehydrogenation and hydrogenation
d) oxidation and hydration
A7. What is the name of the reaction that produces esters?
a) accession
b) substitution
c) esterification
d) decomposition
A8. When ethylene reacts with water, it is formed.
a) limiting alcohol
b) unsaturated alcohol
c) saturated hydrocarbon
d) unsaturated hydrocarbon
A9. Acetic acid is formed from:
a) ethane
b) ethene
c) etina
d) ethanol
A10. What reaction is typical for fats?
a) accession
b) oxidation
c) hydrolysis
d) substitution
Part BWhen completing tasks B1, establish correspondence. Q2 and Q3, make calculations and write down the answer.
IN 1. Match the type of reaction with the substance
Reaction type
AT 2. The volume of oxygen required for complete combustion is 50 liters. methane (n.s.) is equal to ___l.
AT 3. The hydrocarbon contains 16.28% hydrogen. Determine the formula of a hydrocarbon if its vapor density for hydrogen is 43.
Part C. For answers to task C1, use a separate form (sheet)
Write down the task number and the answer to it.
C1. Calculate volume carbon dioxide, released during the combustion of 56 liters of methane in 48 liters of oxygen
ANSWERS
Option 1
Part A
Part B
Part C
Write the reaction equations that can be used to carry out the following BaCl 2
transformations: C O 2 → Na 2 CO 3 → X → CO 2. For the second process, write an ionic equation for the reaction.
Answer
Option 2
Part A
A1
Part B
Part C
Test tasks for grade 11
Option 1.
Part A For each of the tasks A1-A10, four answer options are given,
only one of which is correct. Circle the answer number.
A1. The Wurtz reaction corresponds to the description:
1. acetylene hydration
2. elongation of the carbon skeleton
3. reduction of nitro derivatives with metals in an acidic environment
4. simultaneous dehydration and dehydrogenation of ethanol
A2. Glucose and sucrose can be distinguished by:
1. nitric acid
2. ammonia solution of silver oxide
3. water
4. sodium hydroxide.
A3. Ethanol can be produced from ethylene through the reaction
1. hydration
2. hydrogenation
3.Halogenation
4. Hydrohalogenation
A4. The reaction with an ammonia solution of silver oxide is characteristic of
1. propanol-1
2. propanal
3. propanol-2
4. dimethyl ether
A5. Alkaline hydrolysis of ethyl formate produces
1. formaldehyde and ethanol
2. formic acid and ethanol
3. formic acid salt and ethanol
4. formaldehyde and formic acid
A6. A distinctive feature of the Kucherov reaction is the interaction of substances with
1. with hydrogen
2. with chlorine
3. with water
4. with acid
A7. Zinin reaction, characteristic of aromatic hydrocarbons, has a different name
1. chlorination
2. bromination
3. nitration
4. hydrogenation
A8. Qualitative reaction on polyhydric alcohols is their interaction
1. with copper oxide ( II)
2. with copper hydroxide ( II)
3. with copper
4. with copper oxide ( I)
A9. During the reaction of ethanol with hydrochloric acid in the presence of sulfuric acid is formed
1. ethylene
2. chloroethane
3. 1,2-dichloroethane
4. vinyl chloride
A10. Unlike ethanal, acetic acid reacts with
1. magnesium
2. copper hydroxide ( II)
3. oxygen
4. hydrogen
Part B
write them in ascending order
IN 1. The products of hydrolysis of esters of composition C 5 H 10 O 2 can be
1. pentanal and methanol
2. propanoic acid and ethanol
3. ethanol and butanal
4. butanoic acid and methanol
5. ethanoic acid and propanol
6. formaldehyde and pentanol
AT 2. Reacts with formic acid
1. Na 2 CO 3
2.HCl
3.OH
4.H 2 S
5. CuSO 4
6. Cu(OH)2
AT 3. Substances with which α-aminopropanoic acid can interact
1. ethane
2. potassium hydroxide
3. potassium chloride
5. dimethyl ether
6. hydrogen chloride
Part C. For answers to task C1, use a separate form (sheet)
Write down the task number and the answer to it.
C1. As a result of the catalytic oxidation of propane, propionic acid weighing 55.5 g was obtained. The mass fraction of the reaction product yield is 60%. Calculate the volume of propane taken (no.).
Option 2
Part A For each of the tasks A1-A10, four answer options are given,
only one of which is correct. Circle the answer number.
A1. Each of two substances reacts with bromine water under normal conditions:
1. benzene and toluene
2. cyclohexane and propene
3. ethylene and benzene
4. phenol and acetylene
A2. Ethylene is formed as a result of the reaction:
1. acetylene hydration
2. chloromethane with sodium
3. acetylene with hydrogen chloride
4. ethanol dehydration
A3. Ethanol can be produced from ethylene by the reaction
1. hydration
2. hydrogenation
3. halogenation
4. hydrohalogenation
A4. As a result of the acetylene trimerization reaction, the following is formed:
1. hexane
2. hexene
3. ethane
4. benzene
A5. During the oxidation of ethylene with an aqueous solution of CM nO 4 is formed:
1. ethane
2. ethanol
3. glycerol
4. ethylene glycol
A6. Alkaline hydrolysis of 2-chlorobutane predominantly produces:
1. butanol-2
2. butanol-1
3. butanal
4. butanone
A7. A substitution reaction with chlorine involves:
1. ethene 2. ethin 3 . butene-2 4. butane
A8. Characteristic reaction for polyhydric alcohols is interaction with:
1. H 2
2. Cu
3. Ag 2 O (NH 3 solution)
4. Cu(OH)2
A9. The monomer for producing artificial rubber using the Lebedev method is:
1. butene-2
2. ethane
3. ethylene
4. butadiene-1,3
A10. Butanol-2 and potassium chloride are formed by the interaction:
1. 1-chlorobutane and 2-chlorobutane
2. 2-chlorobutane and alcohol solution of KOH
3. 1-chlorobutane and alcohol solution of KOH
4. 2-chlorobutane and aqueous KOH solution
Part B When completing tasks B1-B3, choose three answer options and
write them in ascending order
IN 1. The products of hydrolysis of esters of composition C 6 H 12 O 2 can be
1 . ethanal and dimethyl ether
2 . propanoic acid and propanol
3 . methyl acetate and butane
4 . ethanoic acid and butanol
5. pentanoic acid and methanol
6. propanal and ethanediol
AT 2. Alkenes interact with:
1 . [ Ag(NH 3) 2 ]OH
2 . H2O
3 . BR 2
4 . KMnO4(H+)
5 . Ca(OH)2
6 . Cu(OH)2
AT 3. Methylethylamine interacts with:
1 . ethane
2 . potassium hydroxide
3. hydrobromic acid
4 . oxygen
5 . propane
6 . water
Part C. For answers to task C1, use a separate form (sheet)
Write down the task number and the answer to it.
C1. Ammonia gas released when 160 g of a 7% solution of potassium hydroxide is boiled with 9.0 g. Ammonium chloride, dissolved in 75 g of water. Determine the mass fraction of ammonia in the resulting solution.
ANSWERS
Option 1
Part A
A1
Part B
IN 1
Part C
Contents of the correct answer and instructions for assessment (other wording of the answer is allowed that does not distort its meaning)
Response elements:
1. The reaction equation has been compiled
3C 2 H 2 
C 6 H 6
2. The amounts of acetylene and benzene were determined
n(C 2 H 2) = 10.08/22.4 = 0.45 mol
according to the reaction equation n(C 2 H 2) : n(C 6 H 6) =3:1
n(C 6 H 6) = 0.45/3 = 0.15 mol
3. The theoretical mass of benzene is calculated
m(C 6 H 6) = 0.15 mol * 78 g/mol = 11.7 g
4. Calculated practical weight benzene
m(C 6 H 6) pr = 0.7 * 11.7 = 8.19 g
Option 2
Part A
Part B
Part C
C 1 1. Ammonia gas released when 160 g of a 7% solution of potassium hydroxide is boiled with 9.0 g. Ammonium chloride, dissolved in 75 g of water. Determine the mass fraction of ammonia in the resulting solution.
Contents of the correct answer and assessment instructions Response elements:- The reaction equation is drawn up:
- The mass and amount of alkali substance in the solution, as well as the amount of ammonium chloride substance, are calculated:
- The substance that is in excess in the solution is indicated:
- The mass of ammonia and its mass fraction in the solution were determined
*Note. If the answer contains an error in calculations in one of the answer elements, which led to an incorrect answer, the score for completing the task is reduced by only 1 point.
Informational resources.
Artemenko A.I. Amazing world organic chemistry. – M.: Bustard, 2004.
Gabrielyan O.S., Ostroumov I.G. Teacher's handbook. Chemistry. 10th grade. – M.: Bustard, 2004.
Koroshchenko A.S., Medvedev Yu.N. Chemistry GIA standard test tasks– M.: “Exam”, 2009.
Kuznetsova N.E., Levkina A.N., Problem book in chemistry, 9th grade. – M.: Publishing center “Ventana – Graf”, 2004.
Kuznetsova N.E., Titova I.M., Gara N.N., Zhegin A.Yu. Chemistry. – 9th grade. – M.: Publishing center “Ventana – Graf”, 2002.
Potapov V.M. Organic chemistry. – M.: Education, 1976.
Encyclopedic dictionary of a young chemist. – M.: Pedagogy – Press, 1997.
Pichugina G.V. Chemistry and everyday life person. – M.: Bustard, 2005.
http://www.fipi.ru/
Nucleophilicis a reaction in which a reagent attacks the substrate with its nucleophile; it is denoted by an index N (nucleophlle).
In electrophilic reactions, the reagent is usually called an electrophile. In organic chemistry, the electrophilicity of a reagent characterizes its ability to interact with a carbon atom of the substrate that carries a full or partial negative charge.
In fact, the mechanism and result of any electrophilic-nucleophilic reaction is determined not only by the properties of the reagent, but also by the properties of the substrate, the resulting reaction products, the solvent and the conditions for its implementation. Therefore, the division of electrophilic-nucleophilic reactions into nucleophilic and electrophilic only based on the properties of the reagent is conditional. In addition, as can be seen from the above diagrams, in these reactions the electrophiles and nucleophiles contained in the substrate and reagent always interact with each other. In many reactions, only conditionally one component can be considered a substrate and the other a reagent.
Free radical reactions. Homolytic decay is characteristic of non-polar or low-polar bonds. It is accompanied by the formation of free radicals - particles with an unpaired electron.
Homolysis of a covalent bond can be considered as the cleavage of this bond by an exchange mechanism. To carry out homolysis of a bond, energy (heat, light) is required sufficient to break this bond. The presence of an unpaired electron is the reason for the low stability of free radicals (the lifetime in most cases is a fraction of a second) and high reactivity in free radical reactions. The presence of a free radical (R۰) in the system can lead to the formation of new radicals due to its interaction with existing molecules: R۰ + A – B → R – A + ۰B
Free radical reactionsare accompanied by the interaction of free radicals with molecules or with each other with the formation of new free radicals (nucleation or development of a chain) or only molecules (chain termination).
Free radical reactions are characterized by a chain mechanism, which includes three stages: initiation, development and chain termination. These reactions stop when free radicals disappear from the system. Free radical reactions are designated by the index R (radical).
Radical particles, depending on their electron affinity, can both accept electrons (i.e., be oxidizing agents) and donate electrons (i.e., be reducing agents). In this case, the affinity of a radical for an electron is determined not only by its properties, but also by the properties of its reaction partner. Features of free radical oxidation-reduction processes occurring in the body are considered separately when describing the properties of certain classes organic compounds.
In complexation reactions, radicals can be both complexing agents and ligands. In the case of charge transfer complexes, radical formation can occur within the complex due to intramolecular oxidation-reduction between the complexing agent and the ligand.
The formation of radicals most easily occurs during the homolysis of nonpolar simple bonds between atoms of the same element:
C1 2 → C1۰ + ۰С1 HO-OH → СО۰ + ۰ОН
R-O-O-R" → RO۰ + ۰OR" R-S-S-R" →RS۰ + ۰SR"
With homolysis of a low-polar S-N connections alkyl radicals are formed in which the unpaired electron is located at the carbon atom. The relative stability of these radicals depends on the type of substitution of the carbon atom bearing the unpaired electron, and increases in the series: CH 3< CH 2 R < CHR 2 < CR 3 . Это объясняется положительным индуктивным эффектом алкильных групп, который, повышая электронную плотность на атоме углерода, способствует стабилизации радикала.
The stability of free radicals increases significantly when it is possible to delocalize the unpaired electron due to the π-electrons of neighboring multiple bonds. This is especially clearly observed in the allylic and benzyl radicals:
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allylic radical benzyl radical
When familiarizing yourself with possible reaction mechanisms in substrate and reagent molecules, reaction centers should be distinguished by their nature: nucleophilic, electrophilic And radical.
According to the final result of the chemical transformation, the simplest organic reactions are classified into reactions: substitution, addition, elimination (elimination) And regrouping.
Substitution reactions. Substitution refers to the replacement of an atom or group with another atom or group. In a substitution reaction, two different products are always formed. This type of reaction is designated by the symbol S (substitution).
Substitution reactions include: halogenation and nitration of alkanes, esterification and alkylation of carboxylic acids, as well as numerous reactions of simple polar molecules (H 2 O, NH 3, NGal) with ethers, alcohols and halogen derivatives.
Addition reactions. By addition we mean the introduction of atoms or groups into the molecule of an unsaturated compound, accompanied by the breaking of π bonds. In this case, double bonds turn into single bonds, and triple bonds into double or single bonds. This type of reaction is indicated by the symbol A (addition).
Elimination reactions. Elimination refers to the removal of atoms or groups from an organic molecule to form a multiple bond. Therefore, elimination reactions are the opposite of addition reactions. This type of reaction is designated by the symbol E (elimination).
Each of the organic reactions of substitution (S), addition (A) or elimination (E) can be electrophilic (E), nucleophilic (N) or radical (R). Thus, in organic chemistry there are nine typical reactions, denoted by the symbols S, A or E with the subscripts R, N or E:
The given types of organic reactions should be considered model ones, since they are not always realized in their pure form. For example, substitution and elimination can occur simultaneously:
With further acquaintance with specific classes of organic compounds, we will consider the following: Chemical properties: acid-base, complexing, redox, electrophilic-nucleophilic, as well as the ability for free radical interaction. Particular attention will be paid to the peculiarities of the occurrence of the reactions under consideration in biological systems.
Types of reactions characteristic of different classes of hydrocarbons, the mechanism of their occurrence and biological significance processes are presented in Table 10.
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Reactions of organic substances can be formally divided into four main types: substitution, addition, elimination (elimination) and rearrangement (isomerization). It is obvious that the entire variety of reactions of organic compounds cannot be reduced to the proposed classification (for example, combustion reactions). However, such a classification will help establish analogies with the reactions that occur between inorganic substances that are already familiar to you.
Typically, the main organic compound involved in the reaction is called substrate, and the other reaction component is conventionally considered as reagent.
Substitution reactions
Substitution reactions- these are reactions that result in the replacement of one atom or group of atoms in the original molecule (substrate) with other atoms or groups of atoms.
Substitution reactions involve saturated and aromatic compounds such as alkanes, cycloalkanes or arenes. Let us give examples of such reactions.
Under the influence of light, hydrogen atoms in a methane molecule can be replaced by halogen atoms, for example, by chlorine atoms:
Another example of replacing hydrogen with halogen is the conversion of benzene to bromobenzene:
The equation for this reaction can be written differently:
With this form of writing, the reagents, catalyst, and reaction conditions are written above the arrow, and the inorganic reaction products are written below it.
As a result of reactions substitutions in organic substances are formed not simple and complex substances, as in inorganic chemistry, and two complex substances.
Addition reactions
Addition reactions- these are reactions as a result of which two or more molecules of reacting substances combine into one.
Unsaturated compounds such as alkenes or alkynes undergo addition reactions. Depending on which molecule acts as a reagent, hydrogenation (or reduction), halogenation, hydrohalogenation, hydration and other addition reactions are distinguished. Each of them requires certain conditions.
1.Hydrogenation- reaction of addition of a hydrogen molecule through a multiple bond:
2. Hydrohalogenation- hydrogen halide addition reaction (hydrochlorination):
3. Halogenation- halogen addition reaction:
4.Polymerization- a special type of addition reaction, during which molecules of a substance with a small molecular weight combine with each other to form molecules of a substance with a very high molecular weight - macromolecules.
Polymerization reactions are processes of combining many molecules of a low molecular weight substance (monomer) into large molecules (macromolecules) of a polymer.
An example of a polymerization reaction is the production of polyethylene from ethylene (ethene) under the action of ultraviolet radiation and a radical polymerization initiator R.
The covalent bond most characteristic of organic compounds is formed when atomic orbitals and the formation of common electron pairs. As a result of this, an orbital common to the two atoms is formed, in which a common electron pair is located. When a bond is broken, the fate of these shared electrons can be different.
Types of reactive particles
An orbital with an unpaired electron belonging to one atom can overlap with an orbital of another atom that also contains an unpaired electron. In this case, a covalent bond is formed according to the exchange mechanism:
The exchange mechanism for the formation of a covalent bond is realized if a common electron pair is formed from unpaired electrons belonging to different atoms.
The process opposite to the formation of a covalent bond by the exchange mechanism is the cleavage of the bond, in which one electron is lost to each atom (). As a result of this, two uncharged particles are formed, having unpaired electrons:
Such particles are called free radicals.
Free radicals- atoms or groups of atoms that have unpaired electrons.
Free radical reactions- these are reactions that occur under the influence and with the participation of free radicals.
In the course of inorganic chemistry, these are the reactions of hydrogen with oxygen, halogens, and combustion reactions. Reactions of this type are characterized by high speed and release of large amounts of heat.
A covalent bond can also be formed by a donor-acceptor mechanism. One of the orbitals of an atom (or anion) that has a lone pair of electrons overlaps with the unoccupied orbital of another atom (or cation) that has an unoccupied orbital, and a covalent bond is formed, for example:
The rupture of a covalent bond leads to the formation of positively and negatively charged particles (); since in this case both electrons from a common electron pair remain with one of the atoms, the other atom has an unfilled orbital:
Let's consider electrolytic dissociation acids:
It can be easily guessed that a particle having a lone pair of electrons R: -, i.e. a negatively charged ion, will be attracted to positively charged atoms or to atoms on which there is at least a partial or effective positive charge.
Particles with lone pairs of electrons are called nucleophilic agents (nucleus- “nucleus”, a positively charged part of an atom), i.e. “friends” of the nucleus, a positive charge.
Nucleophiles(Nu) - anions or molecules that have a lone pair of electrons that interact with parts of the molecules that have an effective positive charge.
Examples of nucleophiles: Cl - (chloride ion), OH - (hydroxide anion), CH 3 O - (methoxide anion), CH 3 COO - (acetate anion).
Particles that have an unfilled orbital, on the contrary, will tend to fill it and, therefore, will be attracted to parts of the molecules that have an increased electron density, a negative charge, and a lone electron pair. They are electrophiles, “friends” of the electron, negative charge, or particles with increased electron density.
Electrophiles- cations or molecules that have an unfilled electron orbital, tending to fill it with electrons, as this leads to a more favorable electronic configuration of the atom.
Not any particle is an electrophile with an unfilled orbital. For example, alkali metal cations have the configuration of inert gases and do not tend to acquire electrons, since they have a low electron affinity.
From this we can conclude that despite the presence of an unfilled orbital, such particles will not be electrophiles.
Basic reaction mechanisms
Three main types of reacting particles have been identified - free radicals, electrophiles, nucleophiles - and three corresponding types of reaction mechanisms:
- free radical;
- electrophilic;
- zeroophilic.
In addition to classifying reactions according to the type of reacting particles, in organic chemistry four types of reactions are distinguished according to the principle of changing the composition of molecules: addition, substitution, detachment, or elimination (from the English. to eliminate- remove, split off) and rearrangements. Since addition and substitution can occur under the influence of all three types of reactive species, several can be distinguished mainmechanisms of reactions.
In addition, we will consider elimination reactions that occur under the influence of nucleophilic particles - bases.
6. Elimination:
A distinctive feature of alkenes (unsaturated hydrocarbons) is their ability to undergo addition reactions. Most of these reactions proceed by the electrophilic addition mechanism.
Hydrohalogenation (addition of halogen hydrogen):
When a hydrogen halide is added to an alkene hydrogen adds to the more hydrogenated one carbon atom, i.e. the atom at which there are more atoms hydrogen, and halogen - to less hydrogenated.