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Question

The correct decreasing order of stability of alkene formed from the following alkyl halides according to the β elimination reaction with alcoholic KOH will be:
(i)
(ii) CH3CH2Br
(iii) CH3CH2CH2Br

A
(i) > (ii) > (iii)
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B
(iii) > (ii) > (i)
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C
(ii) > (iii) > (i)
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D
(i) > (iii) > (ii)
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Solution

The correct option is D (i) > (iii) > (ii)
There are two types of elimination reaction,
1. α Elimination reaction
2. β Elimination reaction

In α Elimination reaction, the leaving group and the proton are removed from the same α carbon which leads to reactive carbene intermediate.
In β Elimination reaction, the leaving group and the proton are removed from the adjacent atoms i.e., leaving group from α carbon and proton from β carbon. It leads to the formation of alkene compounds.
According to saytzeff rule, more substituted alkenes are more stable.
β Elimination reaction gives more substituted alkenes as the major product.
In given alkyl halides, all will undergo β Elimination to give alkene compounds.
Alkene formed from compound (i) has 2 methyl substituted group, (iii) has 1 methyl group and (ii) has 0 methyl substitutes.

Thus, the correct order of stability is,
(i) > (iii)> (ii).

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