Mechanism Of Some Important Reactions Of Alkenes

Hydrocarbon of Class 12

1. Mechanism of halogen addition:

The mechanism proposed is an ionic mechanism.

In the first step the exposed electrons of the Mechanism Of Some Important Reactions Of Alkenes bond of the alkene attacks the halogen in the following way:

Mechanism Of Some Important Reactions Of Alkenes

As Mechanism Of Some Important Reactions Of Alkenesof 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.

Mechanism Of Some Important Reactions Of Alkenes

On reaction of cyclopentane with bromine in Mechanism Of Some Important Reactions Of Alkenes anti – addition occurs and the products of the reaction are trans – 1, 2 dibromocyclopentane enantiomers (as a race`mate)

Mechanism Of Some Important Reactions Of Alkenes

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.

Mechanism Of Some Important Reactions Of Alkenes

Mechanism Of Some Important Reactions Of Alkenes

2. Mechanism of halohydrin formation

It can be explained by the following mechanism:

Mechanism Of Some Important Reactions Of Alkenes

If the alkene is unsymmetrical, the halogen ends up on the carbon atom with greater number of hydrogen atoms.

Mechanism Of Some Important Reactions Of Alkenes

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 Of Some Important Reactions Of Alkenes

Mechanism in both cases involves formation of cyclic intermediates, then in several steps, the cleavage at oxygen – metal bond takes place producing glycol and Mechanism Of Some Important Reactions Of Alkenesor Os metal.

Mechanism Of Some Important Reactions Of Alkenes

Cis – 2 – butene when treated with cold alkaline Mechanism Of Some Important Reactions Of Alkenesgives meso glycol and trans – 2 – butene gives racemate.

4. Oxidation reactions of alkenes

(i)  With cold dilute Mechanism Of Some Important Reactions Of Alkenes(Baeyer’s reagent) alkenes give 1, 2 – glycols.

Mechanism Of Some Important Reactions Of Alkenes

Propene                     From Mechanism Of Some Important Reactions Of Alkenes    Propylene glycol

                                

(ii)  With hot alkaline Mechanism Of Some Important Reactions Of Alkenes

Cleavage of C = C bond takes place leading to formation of carboxylic acids, ketones and Mechanism Of Some Important Reactions Of Alkenesdepending upon structure of alkene.

Mechanism Of Some Important Reactions Of Alkenes

Hence by identifying the products formed during alkaline Mechanism Of Some Important Reactions Of Alkenesoxidation, 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 Mechanism Of Some Important Reactions Of Alkenes gives aldehydes / ketones or a mixture of these depending upon the structure of alkene.

Mechanism Of Some Important Reactions Of Alkenes

This two step – conversion of alkene into ozonide followed by decomposition with Mechanism Of Some Important Reactions Of Alkenesto give aldehydes / ketones or a mixture of these is called reductive ozonolysis.

If however ozonide is decomposed with Mechanism Of Some Important Reactions Of Alkenes the initially formed aldehydes are further oxidized to the corresponding acids by Mechanism Of Some Important Reactions Of Alkenesproduced in the reaction. This is called oxidation ozonolysis.

The oxidation reactions of alkenes are summed up as follows:

Mechanism Of Some Important Reactions Of Alkenes

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