EntranceAddition reactions are typical for. Characteristic chemical properties of hydrocarbons
Hydrocarbons
Lecture No. 13
The enol formed during the hydration of an alkyne cannot be isolated, since the tautomeric equilibrium is always strongly shifted towards the keto form.
The rearrangement of enol into the keto form occurs due to the high polarity of the O-H bond, which leads to easy abstraction of the proton of the hydroxyl group.
The proton can then attach either back to the oxygen of the enolate anion or to the carbon. If it attaches to a carbon atom, a less acidic compound is formed in which the proton does not show a pronounced tendency to dissociate. This is how the keto form accumulates in the reaction mixture.
There is only one case when an aldehyde is formed in the Kucherov reaction - the hydration of acetylene itself. All other reactions produce ketones.
Hydration of propine results in acetone rather than propionaldehyde.
Nucleophilic addition reactions. Alkynes are capable of adding nucleophilic reagents in the presence of a catalyst. As a result, vinyl derivatives of different classes are formed. These reactions are called vinylation reactions.
Some nucleophilic addition reactions to alkynes are shown above. They are often used to prepare compounds that serve as monomers in the synthesis of BMC. For example, the reaction of acetylene with n-butanol leads to butyl vinyl ether, the polymerization of which produces vinylin (Shostakovsky balm) is a valuable medicine.
Oxidation. Like all organic compounds containing multiple carbon-carbon bonds, alkynes are easily oxidized by a variety of oxidizing agents, such as potassium permanganate or potassium dichromate. The reaction proceeds with complete cleavage of the triple bond and the formation of carboxylic acids (alkynes with a terminal triple bond form carboxylic acid and carbon dioxide).
These reactions can be used to determine the structure of alkynes.
Some oxidizing agents, such as selenium dioxide, allow selective oxidation of alkynes, during which only p-bonds are cleaved. In this case, disubstituted alkynes are converted into a-diketones, and monosubstituted alkynes into a-keto acids.
Polymerization of alkynes. In the series of alkynes, the reactions of greatest interest are di-, trimerization, and cyclotri- and cyclotetramerization.
Linear di- and trimers of acetylene, which are of great industrial importance, can be obtained in the presence of monovalent copper salts.
Vinylacetylene is the starting compound for the synthesis of chloroprene (2-chlorobutadiene-1,3), the polymerization of which produces chloroprene rubber.
Cyclotrimerization of acetylene leading to benzene was discovered in 1866 by M. Berthelot and modified by N.D. Zelinsky and B.A. Kazansky (1922).
Cyclotetramerization was discovered by W. Reppe (1949).
Chemical properties of alkanes
Alkanes (paraffins) are non-cyclic hydrocarbons in whose molecules all carbon atoms are connected only by single bonds. In other words, there are no multiple - double or triple bonds - in alkane molecules. In fact, alkanes are hydrocarbons containing the maximum possible number of hydrogen atoms, and therefore they are called limiting (saturated).
Due to saturation, alkanes cannot undergo addition reactions.
Since carbon and hydrogen atoms have fairly close electronegativity, this leads to the fact that the C-H bonds in their molecules are extremely low-polar. In this regard, for alkanes, reactions proceeding through the radical substitution mechanism, denoted by the symbol S R, are more typical.
1. Substitution reactions
In reactions of this type, carbon-hydrogen bonds are broken
RH + XY → RX + HY
Halogenation
Alkanes react with halogens (chlorine and bromine) when exposed to ultraviolet light or high heat. In this case, a mixture of halogen derivatives with varying degrees of substitution of hydrogen atoms is formed - mono-, ditri-, etc. halogen-substituted alkanes.
Using methane as an example, it looks like this:
By changing the halogen/methane ratio in the reaction mixture, it is possible to ensure that a specific halogen derivative of methane predominates in the composition of the products.
Reaction mechanism
Let us analyze the mechanism of the free radical substitution reaction using the example of the interaction of methane and chlorine. It consists of three stages:
- initiation (or chain nucleation) is the process of formation of free radicals under the influence of external energy - irradiation with UV light or heating. At this stage, the chlorine molecule undergoes homolytic cleavage of the Cl-Cl bond with the formation of free radicals:
Free radicals, as can be seen from the figure above, are atoms or groups of atoms with one or more unpaired electrons (Cl, H, CH 3, CH 2, etc.);
2. Chain development
This stage involves the interaction of active free radicals with inactive molecules. In this case, new radicals are formed. In particular, when chlorine radicals act on alkane molecules, an alkyl radical and hydrogen chloride are formed. In turn, the alkyl radical, colliding with chlorine molecules, forms a chlorine derivative and a new chlorine radical:
3) Break (death) of the circuit:
Occurs as a result of the recombination of two radicals with each other into inactive molecules:
2. Oxidation reactions
Under normal conditions, alkanes are inert towards such strong oxidizing agents as concentrated sulfuric and nitric acids, potassium permanganate and dichromate (KMnO 4, K 2 Cr 2 O 7).
Combustion in oxygen
A) complete combustion with excess oxygen. Leads to the formation of carbon dioxide and water:
CH 4 + 2O 2 = CO 2 + 2H 2 O
B) incomplete combustion due to lack of oxygen:
2CH 4 + 3O 2 = 2CO + 4H 2 O
CH 4 + O 2 = C + 2H 2 O
Catalytic oxidation with oxygen
As a result of heating alkanes with oxygen (~200 o C) in the presence of catalysts, a wide variety of organic products can be obtained from them: aldehydes, ketones, alcohols, carboxylic acids.
For example, methane, depending on the nature of the catalyst, can be oxidized into methyl alcohol, formaldehyde or formic acid:
3. Thermal transformations of alkanes
Cracking
Cracking (from the English to crack - to tear) is a chemical process occurring at high temperatures, as a result of which the carbon skeleton of alkane molecules breaks down with the formation of alkene molecules and alkanes with lower molecular weights compared to the original alkanes. For example:
CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3 → CH 3 -CH 2 -CH 2 -CH 3 + CH 3 -CH=CH 2
Cracking can be thermal or catalytic. To carry out catalytic cracking, thanks to the use of catalysts, significantly lower temperatures are used compared to thermal cracking.
Dehydrogenation
The elimination of hydrogen occurs as a result of the cleavage of C-H bonds; carried out in the presence of catalysts at elevated temperatures. When methane is dehydrogenated, acetylene is formed:
2CH 4 → C 2 H 2 + 3H 2
Heating methane to 1200 °C leads to its decomposition into simple substances:
CH 4 → C + 2H 2
When the remaining alkanes are dehydrogenated, alkenes are formed:
C 2 H 6 → C 2 H 4 + H 2
When dehydrogenating n-butane, butene-1 and butene-2 are formed (the latter in the form cis- And trance-isomers):
Dehydrocyclization
Isomerization
Chemical properties of cycloalkanes
The chemical properties of cycloalkanes with more than four carbon atoms in their rings are, in general, almost identical to the properties of alkanes. Oddly enough, cyclopropane and cyclobutane are characterized by addition reactions. This is due to the high tension within the cycle, which leads to the fact that these cycles tend to break. So cyclopropane and cyclobutane easily add bromine, hydrogen or hydrogen chloride:
Chemical properties of alkenes
1. Addition reactions
Since the double bond in alkene molecules consists of one strong sigma and one weak pi bond, they are fairly active compounds that easily undergo addition reactions. Alkenes often undergo such reactions even under mild conditions - in the cold, in aqueous solutions and organic solvents.
Hydrogenation of alkenes
Alkenes are capable of adding hydrogen in the presence of catalysts (platinum, palladium, nickel):
CH 3 -CH = CH 2 + H 2 → CH 3 -CH 2 -CH 3
Hydrogenation of alkenes occurs easily even at normal pressure and slight heating. An interesting fact is that the same catalysts can be used for the dehydrogenation of alkanes to alkenes, only the dehydrogenation process occurs at a higher temperature and lower pressure.
Halogenation
Alkenes easily undergo addition reactions with bromine both in aqueous solution and in organic solvents. As a result of the interaction, initially yellow bromine solutions lose their color, i.e. become discolored.
CH 2 =CH 2 + Br 2 → CH 2 Br-CH 2 Br
Hydrohalogenation
As is easy to see, the addition of a hydrogen halide to a molecule of an unsymmetrical alkene should, theoretically, lead to a mixture of two isomers. For example, when hydrogen bromide is added to propene, the following products should be obtained:
However, in the absence of specific conditions (for example, the presence of peroxides in the reaction mixture), the addition of a hydrogen halide molecule will occur strictly selectively in accordance with Markovnikov’s rule:
The addition of a hydrogen halide to an alkene occurs in such a way that a hydrogen is added to a carbon atom with a greater number of hydrogen atoms (more hydrogenated), and a halogen is added to a carbon atom with a fewer number of hydrogen atoms (less hydrogenated).
Hydration
This reaction leads to the formation of alcohols, and also proceeds in accordance with Markovnikov’s rule:
As you can easily guess, due to the fact that the addition of water to an alkene molecule occurs according to Markovnikov’s rule, the formation of a primary alcohol is possible only in the case of ethylene hydration:
CH 2 =CH 2 + H 2 O → CH 3 -CH 2 -OH
It is through this reaction that the bulk of ethyl alcohol is carried out in large-scale industry.
Polymerization
A specific case of an addition reaction is the polymerization reaction, which, unlike halogenation, hydrohalogenation and hydration, proceeds through the free radical mechanism:
Oxidation reactions
Like all other hydrocarbons, alkenes burn easily in oxygen to form carbon dioxide and water. The equation for the combustion of alkenes in excess oxygen has the form:
C n H 2n + (3/2) nO 2 → nCO 2 + nH 2 O
Unlike alkanes, alkenes are easily oxidized. When alkenes are exposed to an aqueous solution of KMnO 4, discoloration occurs, which is a qualitative reaction to double and triple CC bonds in molecules of organic substances.
Oxidation of alkenes with potassium permanganate in a neutral or weakly alkaline solution leads to the formation of diols (dihydric alcohols):
C 2 H 4 + 2KMnO 4 + 2H 2 O → CH 2 OH–CH 2 OH + 2MnO 2 + 2KOH (cooling)
In an acidic environment, the double bond is completely broken and the carbon atoms that formed the double bond are converted into carboxyl groups:
5CH 3 CH=CHCH 2 CH 3 + 8KMnO 4 + 12H 2 SO 4 → 5CH 3 COOH + 5C 2 H 5 COOH + 8MnSO 4 + 4K 2 SO 4 + 17H 2 O (heating)
If the double C=C bond is located at the end of the alkene molecule, then carbon dioxide is formed as a product of oxidation of the outermost carbon atom at the double bond. This is due to the fact that the intermediate oxidation product, formic acid, easily oxidizes itself in an excess of oxidizing agent:
5CH 3 CH=CH 2 + 10KMnO 4 + 15H 2 SO 4 → 5CH 3 COOH + 5CO 2 + 10MnSO 4 + 5K 2 SO 4 + 20H 2 O (heating)
The oxidation of alkenes in which the C atom at the double bond contains two hydrocarbon substituents produces a ketone. For example, the oxidation of 2-methylbutene-2 produces acetone and acetic acid.
The oxidation of alkenes, in which the carbon skeleton is broken at the double bond, is used to determine their structure.
Chemical properties of alkadienes
Addition reactions
For example, the addition of halogens:
Bromine water becomes discolored.
Under normal conditions, the addition of halogen atoms occurs at the ends of the 1,3-butadiene molecule, while the π-bonds are broken, bromine atoms are added to the extreme carbon atoms, and the free valences form a new π-bond. Thus, a “movement” of the double bond occurs. If there is an excess of bromine, another molecule can be added at the site of the formed double bond.
Polymerization reactions
Chemical properties of alkynes
Alkynes are unsaturated (unsaturated) hydrocarbons and therefore are capable of undergoing addition reactions. Among the addition reactions for alkynes, electrophilic addition is the most common.
Halogenation
Since the triple bond of alkyne molecules consists of one stronger sigma bond and two weaker pi bonds, they are capable of attaching either one or two halogen molecules. The addition of two halogen molecules by one alkyne molecule proceeds through an electrophilic mechanism sequentially in two stages:
Hydrohalogenation
The addition of hydrogen halide molecules also occurs via an electrophilic mechanism and in two stages. In both stages, the accession proceeds in accordance with Markovnikov’s rule:
Hydration
The addition of water to alkynes occurs in the presence of ruti salts in an acidic medium and is called the Kucherov reaction.
As a result of hydration, the addition of water to acetylene produces acetaldehyde (acetic aldehyde):
For acetylene homologues, the addition of water leads to the formation of ketones:
Hydrogenation of alkynes
Alkynes react with hydrogen in two steps. Metals such as platinum, palladium, and nickel are used as catalysts:
Trimerization of alkynes
When acetylene is passed over activated carbon at high temperature, a mixture of various products is formed from it, the main of which is benzene, a product of acetylene trimerization:
Dimerization of alkynes
Acetylene also undergoes a dimerization reaction. The process takes place in the presence of copper salts as catalysts:
Alkyne oxidation
Alkynes burn in oxygen:
C nH 2n-2 + (3n-1)/2 O 2 → nCO 2 + (n-1)H 2 O
Reaction of alkynes with bases
Alkynes with a triple C≡C at the end of the molecule, unlike other alkynes, are able to enter into reactions in which the hydrogen atom at the triple bond is replaced by a metal. For example, acetylene reacts with sodium amide in liquid ammonia:
HC≡CH + 2NaNH 2 → NaC≡CNa + 2NH 3 ,
and also with an ammonia solution of silver oxide, forming insoluble salt-like substances called acetylenides:
Thanks to this reaction, it is possible to recognize alkynes with a terminal triple bond, as well as to isolate such an alkyne from a mixture with other alkynes.
It should be noted that all silver and copper acetylenides are explosive substances.
Acetylenides are capable of reacting with halogen derivatives, which is used in the synthesis of more complex organic compounds with a triple bond:
CH 3 -C≡CH + NaNH 2 → CH 3 -C≡CNa + NH 3
CH 3 -C≡CNa + CH 3 Br → CH 3 -C≡C-CH 3 + NaBr
Chemical properties of aromatic hydrocarbons
The aromatic nature of the bond influences the chemical properties of benzenes and other aromatic hydrocarbons.
The unified 6pi electron system is much more stable than ordinary pi bonds. Therefore, substitution reactions rather than addition reactions are more typical for aromatic hydrocarbons. Arenes undergo substitution reactions via an electrophilic mechanism.
Substitution reactions
Halogenation
Nitration
The nitration reaction proceeds best under the influence of not pure nitric acid, but its mixture with concentrated sulfuric acid, the so-called nitrating mixture:
Alkylation
A reaction in which one of the hydrogen atoms at the aromatic ring is replaced by a hydrocarbon radical:
Alkenes can also be used instead of halogenated alkanes. Aluminum halides, ferric halides or inorganic acids can be used as catalysts.<
Addition reactions
Hydrogenation
Chlorine addition
Proceeds via a radical mechanism upon intense irradiation with ultraviolet light:
A similar reaction can only occur with chlorine.
Oxidation reactions
Combustion
2C 6 H 6 + 15O 2 = 12CO 2 + 6H 2 O + Q
Incomplete oxidation
The benzene ring is resistant to oxidizing agents such as KMnO 4 and K 2 Cr 2 O 7 . There is no reaction.
Substituents on the benzene ring are divided into two types:
Let us consider the chemical properties of benzene homologues using toluene as an example.
Chemical properties of toluene
Halogenation
The toluene molecule can be considered as consisting of fragments of benzene and methane molecules. Therefore, it is logical to assume that the chemical properties of toluene should to some extent combine the chemical properties of these two substances taken separately. This is often what is observed during its halogenation. We already know that benzene undergoes a substitution reaction with chlorine via an electrophilic mechanism, and to carry out this reaction it is necessary to use catalysts (aluminum or ferric halides). At the same time, methane is also capable of reacting with chlorine, but via a free radical mechanism, which requires irradiation of the initial reaction mixture with UV light. Toluene, depending on the conditions under which it is subjected to chlorination, can give either products of substitution of hydrogen atoms in the benzene ring - for this you need to use the same conditions as for the chlorination of benzene, or products of substitution of hydrogen atoms in the methyl radical, if it, how chlorine acts on methane under ultraviolet irradiation:
As you can see, the chlorination of toluene in the presence of aluminum chloride led to two different products - ortho- and para-chlorotoluene. This is due to the fact that the methyl radical is a substituent of the first kind.
If the chlorination of toluene in the presence of AlCl 3 is carried out in excess of chlorine, the formation of trichloro-substituted toluene is possible:
Similarly, when toluene is chlorinated in the light at a higher chlorine/toluene ratio, dichloromethylbenzene or trichloromethylbenzene can be obtained:
Nitration
The replacement of hydrogen atoms with a nitro group during the nitration of toluene with a mixture of concentrated nitric and sulfuric acids leads to substitution products in the aromatic ring rather than the methyl radical:
Alkylation
As already mentioned, the methyl radical is an orienting agent of the first kind, therefore its alkylation according to Friedel-Crafts leads to the substitution products in ortho- and para-positions:
Addition reactions
Toluene can be hydrogenated to methylcyclohexane using metal catalysts (Pt, Pd, Ni):
C 6 H 5 CH 3 + 9O 2 → 7CO 2 + 4H 2 O
Incomplete oxidation
When exposed to an oxidizing agent such as an aqueous solution of potassium permanganate, the side chain undergoes oxidation. The aromatic core cannot oxidize under such conditions. In this case, depending on the pH of the solution, either a carboxylic acid or its salt will be formed.
; in this case, one p-bond is broken and one or two s-bonds are formed. To indicate the addition of reactions, use the symbol Ad (from the English addition - accession); For cycloadditions, such a symbol is not used.
Depending on the nature of the substrate, reaction additions are distinguished by isolated or conjugated multiple bonds, for example: C=C, C=C, C=C-C=C, C=O, C=N, C=N. homolytic (Ad R) and heterolytic. accession. The latter, depending on the charge of the attacking reagent, are divided into electroph. (Ad E) and nucleoph. (Ad N)attachments. The behavior of the reagent depends on the type of substrate and the conditions of the reaction (reagent, the presence of a catalyst, the effect of UV irradiation, etc.). Mn. reagents may exhibit different effects under different conditions. types of reaction abilities, eg. halogens can act as radicals, electroph. and even nucleoph. agents.
Naib. addition reactions at multiple carbon-carbon bonds were studied. These processes proceed according to a stepwise (staged) or synchronous (coordinated) mechanism. With a stepwise mechanism, the first stage is the attack of a nucleophile, electrophile or free. radical, the second is the recombination of the resulting intermediate with positive, negative. or a neutral particle, for example:
Electroph. or nucleoph. the particles do not have to be ions; they can represent an electron-withdrawing or electron-donating part (group) of a molecule. Ad N reactions are possible only with C=C bonds activated by electron-withdrawing substituents; To implement Ad E, either unsubstituted C=C bonds or activated by electron-donating substituents are needed. For the Ad R solution, the nature of the substituent at the C=C bond is not of great importance.
Stereochem. the result of stepwise addition depends on the mechanism of the reaction and the nature of the reacting compounds. Yes, electroph. addition to olefins can occur as a son-addition - particles Y and W attack the molecule from one side of the double bond plane, or as an anti-addition - particles attack from different sides of the plane; in some cases, the reactions are non-stereospecific. Nucleof. addition involving carbanions proceeds, as a rule, non-stereospecifically. In addition reactions involving triple bonds, syn addition leads to a cis isomer, anti-addition leads to a trans isomer.
In the case of a synchronous mechanism, the attack on both C atoms occurs simultaneously and the reaction proceeds as a dipolar addition (see Cycloaddition), while additions of the reaction at a double or triple bond proceed as a son-addition (see, for example, Reppe reactions).
P addition reactions at conjugated double bonds, proceeding according to a stepwise mechanism, lead to the formation of 1,2- and 1,4-addition products:
Synchronous 1,4-addition to dienes proceeds as follows. way:
A special type of addition reaction is conjugate addition. The occurrence of such reactions is accompanied by the binding of a solvent (or a specially added reagent) at the final stage of the process. For example, conjugated electroph. the addition of halogens to alkenes in CH 3 COOH leads, along with 1,2-dihalides, to b-acetoxyalkyl halides:
Examples of conjugated nucleoph. accession - Michael reaction and interaction. activated alkenes with cyanide anion in protic solutions SH:
In the case of addition of reactions through multiple carbon-hetero-atom bonds, in which it is put. the charge is localized on the C atom (C=O, C=N, C=N and C=S bonds), nucleophiles always attach to the C atom, and electrophiles to the heteroatom. In max. Nucleophilic addition reactions at the carbonyl group have been studied to the extent:
P The reaction at the C atom may be one of the stages of aromatic substitution. row, for example: