Question

Which of the following is the correct order for ease of dehydration-

• A. I > III > II > IV
• B. IV > III > II > I
• C. IV > II > III > I
• D. III > IV > II > I

Answer 1

Answer: (B)

The ease of dehydration can be determined by comparing the stability of intermediate carbocation.
$\mathrm{CH}{}_3$ $\underset{\text{(I)}}{\mathrm{CH}{}_2}$ $\mathrm{CH}{}_2\mathrm{CH}{}_2$ $\mathrm{CH}{}_3$ $\underset{\text{(II)}}{\mathrm{CH}{}_2}$ $\mathrm{CH}\mathrm{CH}{}_3$
$\mathrm{CH}{}_2$ $\underset{\text{(III)}}{\mathrm{CH}}$ $\mathrm{CH}\mathrm{CH}{}_3$
(IV) intermediate carbocation is most stable due to resonance followed by (III) which is allylic carbocation. Next is (II) as it is a secondary carbocation and least stable is (I) as it is a primary carbocation. Alternatively, ease of dehydration can be determined from the stability of the product formed.