Hydrocarbon of Class 11
The alkenes are unsaturated hydrocarbons that contain one double bond. They have the general formula CnH2n and the double bond is known as olefinic bond or ethylenic bond.
CH2 = CH2 Ethene
CH3 CH = CH2 Propene
CH3CH = CHCH3 But – 2 – ene
METHODS OF PREPARATION
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.
(i) Ease of dehydrohalogenation of alkyl halides is in order 30 > 20 > 10.
(ii) Ease of formation of alkenes is in order
R2C = CR2 > R2C = CHR > R2C = CH2 or RCH = CHR > RCH = CH2 > CH2= CH2 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 < RCl < RBr < 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.
2. Dehydration of Alcohols
Alcohols undergo dehydration to give alkenes.
(i) Vicinal dihalides undergo dehalogenation in the presence of Zn, Ag or Mg.
(ii) Geminal dihalides undergo coupling reaction Via dehalogenation to give alkenes.
R ⎯ CH2 ⎯ CH = CH ⎯ CH2 ⎯ R
(C = 2n)
4. Thermal elimination reaction
The product formation takes place by Hofmann rule. Following compounds give thermal elimination reactions.
(ii) Amine oxide
Thermal elimination of amine oxides is known as copper elimination.
(iii) Quaternary ammonium hydroxide
Thermal elimination of this compound is known as Hofmann elimination.
5. By partial reduction of alkynes:
Alkynes undergo partial reduction to give alkenes in the presence of catalyst
6. Wittig Reaction
Carbonyl compounds react with 10 and 20 alkyl halides in the presence of triphenyl phosphine and strong base (RLi, NaH etc) to give alkenes. This reaction is known as Wittig reaction.
For writing product remove H and X from the α - carbon of alkyl halide and oxygen from- carbonyl carbon and join these two carbons (α - carbon and carbonyl carbon) by double bond.
7. Kolbe hydrocarbon synthesis
Electrolysis of potassium salt of succinic acid gives alkene at the anode
At room temperature alkenes differs in their physical state depending upon the number of carbon atom.
C2 ⎯ C4 : Gases
C5 ⎯ C17 : Liquids
C18 ⎯ Onwards : Solids like alkanes
(a) Free – radical attack (Substitution reaction)
(b) Ionic – attack (Addition reaction)
The Type of reaction depends upon experimental conditions.
Some reactions of alkenes are given below
1. Conversion to alkanes (Heterogenous)
Relative rates of hydrogenation are as follows:
H2C = CH2 > RCH = CH2 > R2C = CH2, RCH = CHR > R2C > R2C = CR2
The rate decreases as steric hinderance increases.
2. Addition reactions
Addition of unsymmetrical reagents such as HX, HOX, H2O, H2SO4 etc to unsymmetrical alkenes such as propene occurs in accordance with Markonikov’s rule which states that the negative part of the addendum (adding regent) gets attached to that carbon atom of the double bond which has least number of hydrogen atoms
Carbocations are the intermediates and the major products always results from more stable carbocations. e.g.
In presence of peroxide, the addition of HBr to unsymmetrical alkenes occurs contrary to Markovnikov’s rule. e.g.
Anti markovnikov’s addition is generally called peroxide effect or kharasch effect. In presence of peroxides, the addition of HBr to unsymmetrical alkenes occurs by a free radical mechanism.
Propagation: (i) 2• Free radical (more stable)
Peroxide effect is not observed with other halogen acids (HF, HCl or HI) since only HBr has both steps exothermic while with HCl second propagation step involving the reaction of carbon radical with HCl is endothermic and with HI, first propagation step involving the addition of iodine radical to alkene is endothermic.
Because of the presence of double bond alkene readily undergoes electrophilic addition reactions. Some important reactions of ethene are given below:
MECHANISM OF SOME IMPORTANT REACTIONS OF ALKENES
1. Mechanism of halogen addition:
The mechanism proposed is an ionic mechanism.
In the first step the exposed electrons of the π- bond of the alkene attacks the halogen in the following way:
As π- e's of the alkene approach the bromine molecules, the electrons of bromine – bromine bond drift to make bromine molecule polarised. The more distant bromine develops a partial negative charge and nearer bromine becomes partially positive. Polarization weakens the bond and cleaves it heterolytically.
In second step, one of the bromide ions predicted in step I attacks one of the carbon atoms of the bromonium ion. The nucleophilic attack results in the formation of a vicinal dibromide by opening the three – membered ring.
On reaction of cyclopentane with bromine in CCl4 anti – addition occurs and the products of the reaction are trans – 1, 2 dibromocyclopentane enantiomers (as a race`mate)
Addition of bromine to cis – 2 – butene gives racemic form of 2, 3 – dibromobutane.
Bromine adds to trans – 2 – butene to form meso compound, thus the reaction is stereospecific in nature.
2. Mechanism of halohydrin formation
It can be explained by the following mechanism:
If the alkene is unsymmetrical, the halogen ends up on the carbon atom with greater number of hydrogen atoms.
3. Syn - hydroxylation
Hydroxylation with permanganate is carried out by reaction at room temperature. It’s a good method for the synthesis of 1, 2 – diols.
Mechanism in both cases involves formation of cyclic intermediates, then in several steps, the cleavage at oxygen – metal bond takes place producing glycol and MnO2 or Os metal.
Cis – 2 – butene when treated with cold alkaline KMnO4 gives meso glycol and trans – 2 – butene gives racemate.
4. Oxidation reactions of alkenes
(i) With cold dilute KMnO4(Baeyer’s reagent) alkenes give 1, 2 – glycols.
Propene From KMnO4 Propylene glycol
(ii) With hot alkaline KMnO4
Cleavage of C = C bond takes place leading to formation of carboxylic acids, ketones and CO2 + H2O depending upon structure of alkene.
Hence by identifying the products formed during alkaline KMnO4 oxidation, it is possible to determine the position of the double bond in an alkene molecule.
(iii) With ozone alkenes first give ozonides which upon reductive cleavage with Zn dust and H2O or H2 /Pd gives aldehydes / ketones or a mixture of these depending upon the structure of alkene.
This two step – conversion of alkene into ozonide followed by decomposition with Zn/H2O to give aldehydes / ketones or a mixture of these is called reductive ozonolysis.
If however ozonide is decomposed with H2O the initially formed aldehydes are further oxidized to the corresponding acids by H2O2 produced in the reaction. This is called oxidation ozonolysis.
The oxidation reactions of alkenes are summed up as follows: