Chemistry > Hydrocarbons > 7.0 Methods of Preparation Alkenes
Hydrocarbons
1.0 Introduction
2.0 Alkanes
3.0 Methods of Preparation Alkanes
4.0 Physical Proparties
5.0 Chemical Properties
6.0 Alkenes
7.0 Methods of Preparation Alkenes
7.1 Dehydrohalgoenation
7.2 Dehydration of Alcohols
7.3 Dehalogenation
7.4 Thermal elimination reaction
7.5 By partial reduction of alkynes:
7.6 Wittig Reaction
7.7 Kolbe hydrocarbon synthesis
8.0 Physical Proparties
9.0 Chemical Properties
10.0 Mechanism Of Some Important Reaction Of Alkenes
10.1 Mechanism of halogen addition
10.2 Mechanism of halohydrin formation
10.3 Syn - hydroxylation
10.4 Oxidation reactions of alkenes
11.0 Alkynes
12.0 Methods of Preparation Alkynes
12.1 Industrial source
12.2 Kolbe’s method
12.3 Dehydrohalogenation of 1, 2 – dihalides
12.4 Dehydrohalogenation of 1, 1 – dihalides
12.5 Dehalogenation of tetrahalides or trihalides
12.6 Alkylation of acetylene and terminal alkynes
13.0 Physical Properties
14.0 Chemical Properteis
14.1 Electrophilic addition reactions
14.2 Acidity of Alkynes
14.3 Aromatic Hydrocarbons
14.4 Structure of Benzene
15.0 Modern Concept
15.1 Aromaticity in Benzene and Related Systems
15.2 Huckel’s rule or $\left( {{\bf{4n}}{\text{ }} + {\text{ }}{\bf{2}}} \right)\pi $ electron rule
16.0 Properteis
17.0 Mechanism of Electrophilic Substitution Reactions
17.1 Nitration
17.2 Friedel – Craft Alkylation
17.3 Friedel – Craft Acylation
17.4 Reactions of side chains
18.0 Toluene
19.0 Alkenyl Benzene
7.1 Dehydrohalgoenation
7.2 Dehydration of Alcohols
7.3 Dehalogenation
7.4 Thermal elimination reaction
7.5 By partial reduction of alkynes:
7.6 Wittig Reaction
7.7 Kolbe hydrocarbon synthesis
10.2 Mechanism of halohydrin formation
10.3 Syn - hydroxylation
10.4 Oxidation reactions of alkenes
12.2 Kolbe’s method
12.3 Dehydrohalogenation of 1, 2 – dihalides
12.4 Dehydrohalogenation of 1, 1 – dihalides
12.5 Dehalogenation of tetrahalides or trihalides
12.6 Alkylation of acetylene and terminal alkynes
14.2 Acidity of Alkynes
14.3 Aromatic Hydrocarbons
14.4 Structure of Benzene
15.2 Huckel’s rule or $\left( {{\bf{4n}}{\text{ }} + {\text{ }}{\bf{2}}} \right)\pi $ electron rule
17.2 Friedel – Craft Alkylation
17.3 Friedel – Craft Acylation
17.4 Reactions of side chains
It is possible to form alkenes by base – induced elimination from alkyl halides
Alcoholic KOH converts alkyl halide into alkene by a reaction called dehydrohalogenation which involves removal of the halogen atom together with a hydrogen atom from a carbon adjacent to the one having the halogen.
Note:
(i) Ease of dehydrohalogenation of alkyl halides is in order 30 > 20 > 10.
(ii) Ease of formation of alkenes is in order
${R_2}C{\text{ }} = {\text{ }}C{R_2} > {\text{ }}{R_2}C{\text{ }} = {\text{ }}CHR{\text{ }} > {\text{ }}{R_2}C{\text{ }} = {\text{ }}C{H_2}or{\text{ }}RCH{\text{ }} = {\text{ }}CHR{\text{ }} > {\text{ }}RCH{\text{ }} = {\text{ }}C{H_2} > {\text{ }}C{H_2} = {\text{ }}C{H_{2}}$
and same is the stability order of alkenes.
Thus by general rule: More stable the alkene, the more easily it is formed
Increasing rate of dehydrohalogenation is in the order.
$RF{\text{ }} < {\text{ }}RCl{\text{ }} < {\text{ }}RBr{\text{ }} < {\text{ }}RI$
Also greater the conjugation, greater is the stability (due to resonance) hence easier is the dehydrohalogenation.
As II is more stable than IV hence dehydrohalogenation of I is easier than that of (III)
In case of elimination of HBr from 1 – bromo – 1 – methyl cyclohexane, loss of bromide provides a tertiary cation. This species is symmetrical and loss of proton from either of the adjacent methylene groups leads to the same product, 1 – methyl – 1 cyclohexene in which the double bond is in the ring (endocyclic) on the other hand loss of proton from the methyl group produces methylene cyclohexane (Y) in which the double bond is outside the ring (exocyclic). By saytzeff rule, the more highly substituted an alkene, the more stable it is hence formation of 1 – methyl – 1 – cyclohexene is favoured.
Formation of less substituted alkene in an elimination reaction is referred to as a Hofmann elimination.
Note:
Hindered base gives Hofmann product as major isomer.
