Question

Predict all the alkenes that would be formed by dehydrohalogenation of the following halides with sodium ethoxide in ethanol and identify the major alkene:

1. 1-Bromo-1-methylcyclohexane

2. 2-Chloro-2-methylbutane,

3. 2,2,3-Trimethyl-3-bromopentane.

Answer 1

1. 

In this compound, all β hydrogen are equivalent thus dehydrohalogenation gives only one alkene.

2. 

In the given compound, there are two different sets of equivalent P - hydrogen atoms labelled as ‘a’ and ‘b’ thus dehydrohalogenation yields two alkenes.

Sayt zeff’s rule implies that in dehydrohalogenate reactions, the alkene having a greater number of alkyl groups attached to a doubly bonded carbon atoms is preferably produced.

Therefore, 2-methyl but - 2- ene is the major product in the reaction.

3.

In the given compound, there are two different sets of equivalent P - hydrogen atoms labelled as ‘a’ and ‘b. Thus dehydrogenation of the compound yields two alkenes.

2 -1 thyl – 3, 3 – demethyl but - tene

Again according to Saytzeff’s rule
3,4,4 – trimethylpent - 2- ene is the major product.