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Chemistry Class 12 NCERT Solutions: Chapter 10 Haloalkanes and Haloarenes Part 6

Q: 10. 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-methylbutane

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

Answer:

(i)

Illustration: Chemistry Class 12 NCERT Solutions: Chapter 10 Haloalkanes and Haloarenes Part 6

1 − bromo − 1 − methylcyclohexane

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

Illustration: Chemistry Class 12 NCERT Solutions: Chapter 10 Haloalkanes and Haloarenes Part 6

(ii)

Illustration: Chemistry Class 12 NCERT Solutions: Chapter 10 Haloalkanes and Haloarenes Part 6

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.

Illustration: Chemistry Class 12 NCERT Solutions: Chapter 10 Haloalkanes and Haloarenes Part 6

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)

Illustration: Chemistry Class 12 NCERT Solutions: Chapter 10 Haloalkanes and Haloarenes Part 6

2,2, 3-Trimethyl-3-bromopentane

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

Illustration: Chemistry Class 12 NCERT Solutions: Chapter 10 Haloalkanes and Haloarenes Part 6

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.