Question

The compound which gives the most stable carbonium ion on dehydration is

• A. $CH _{3} CHOHCH _{3}$
• B. $\left( CH _{3}\right)_{3} COH$
• C. $CH _{2}= CH - CH _{2} CH _{2} OH$
• D. $CH _{3} OH$

Answer 1

Answer: (B)

The dehydration of tertiary alcohol proceeds via tertiary carbocation intermediate which is most stable.
$\underset{\text{tertiary alcohol}}{(CH_3)_3)C-OH} \xrightarrow [-H_2O]{+H^+} \underset{3^{\circ} \text{carbocation more stable}}{(CH_3)_3 \overset{+}{C}}$
$CH_3CHOHCH_3 \xrightarrow [-H_2O]{+H^+}\underset{2^{\circ}\text{carbocation less stable}}{CH_3\overset{+}{CH} - CH_3}$
$CH _{2}= CH - CH _{2}- CH _{2} OH \xrightarrow[-H_2O]{+H}^{+} CH _{2}= CH - \underset{1^{\circ}\text{ carbocation (least stable)}}{CH _{2}- CH _{2}^{+}}$
Decreasing order of stability of carbocation is tertiary $>$ secondary $>$ primary $>$ methyl.