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:

(i)1-Bromo-1 methylcyclohexane 

(ii)2-Chloro-2-methyl butane 

(iii)2,2,3-Trimethyl-3-bromopentane

Answer 1

(i)

1−bromo−1−methylcyclohexane In the given compound,all β-hydrogen atoms are equivalent.Thus, dehydrohalogenation of this compound gives only one alkene.

(ii) 

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

Saytzeff’s rule implies that in dehydrohalogenation reactions,the alkene having a greater number of alkyl groups attached to a doubly bonded carbon atoms is preferably produced.Therefore,alkene(I) i.e.,2-methylbut-2-ene is the major product in this reaction.
(iii)

2,2,3-Trimethyl-3-bromopentane  In the given compound,there are two different sets of equivalent β-hydrogen atoms labeled as a and b. Thus, dehydrohalogenation of the compound yields two alkenes.According to Saytzeff’s rule,in dehydrohalogenation reactions,the alkene having a greater number of alkyl groups attached to the doubly bonded carbon atom is preferably formed.

Hence, alkene (I)i.e.,3,4,4-trimethylpent-2-ene is the major product in this reaction.