Properties of Alkenes

PHYSICAL PROPERTIES

At room temperature alkenes differs in their physical state depending upon the number of carbon atom.

$C_2-C_4$ : Gases

$C_5-C_{17}$ : Liquids

$C_{18}-Onwards$ : Solids like alkanes

CHEMICAL PROPERTIES

Alkenes show

(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

e.g.

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.

e.g.

Initiation: $R-O-O-R\underset{Homolyticfission}{\overset{\Delta orh\nu }{\to }}2R\stackrel{\bullet }{O}$

Peroxide

$R\stackrel{\bullet }{O}+H-Br\to ROH+\stackrel{\bullet }{\mathrm{Br}}$

Propagation: (i) $C{H}_{3}CH=C{H}_2+\stackrel{\bullet }{\mathrm{Br}}\to C{H}_3-\stackrel{\bullet }{C}H-C{H}_{2}Br$

$2^{\bullet }$Free radical (more stable)

(ii) $C{H}_3-\stackrel{\bullet }{C}H-C{H}_{2}Br+HBr\to C{H}_3-C{H}_2-C{H}_{2}Br+\stackrel{\bullet }{\mathrm{Br}}$

Termination: (i) $\stackrel{\bullet }{\mathrm{Br}}+\stackrel{\bullet }{\mathrm{Br}}\to Br-Br$

(ii)

Note:

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: