Question

Which of the following statements is correct ?

• A. The reaction of methyl magnesium iodide with acetone followed by hydrolysis gives secondary butanol.
• B. Primary alcohols are dehydrated easily than secondary and tertiary alcohols
• C. Tertiary alcohol is more acidic than primary alcohol
• D. Tertiary butyl alcohol gives turbidity fastest with Lucas reagent.

Answer 1

Answer: (D)

A. The reaction of methyl magnesium iodide with acetone followed by hydrolysis gives tertiary butanol as:

$C{H}_{3}MgI+C{H}_{3}COC{H}_3\stackrel{H_{2}O}{\to }C{H}_3-\stackrel{\stackrel{OH}{|}}{\underset{\underset{C{H}_3}{|}}{C}}-C{H}_3$

B. The dehydration reaction is as: $Alcohol+H_{2}S{O}_4\to alkene$

is a elimination reaction that goes via carbocation formation. Since tertiary carbocations are most stable due to + Inductive effect of alkyl groups, then is the secondary and least stable is primary.

Therefore the reactivity of alcohol is governed by the stability of carbocation that follows the order as

Tertiary >secondary>primary

Primary alcohols are dehydrated with most difficulty than secondary and tertiary alcohols

C. Tertiary alcohol has high electron density on the oxygen atom of -OH due to + Inductive effect of three alkyl groups. Thus they are less acidic than primary alcohol

D. Lucas test is test to recognize whether the given alcohol is primary, secondary or tertiary in nature. Lucas reagent is a mixture of conc $HCl$ and dry anhydrous $ZnC{l}_2$. In this reaction carbocation is formed as an intermediate and thus the stability and ease of carbocation formation determines the rate of the reaction as that's the slow step of the mechanism (see example). Hence order of reactivity is tertiary alcohol>secondary alcohol by $S_{N}1$ mechanism.

Thus with Lucas reagent, tertiary alcohol reacts fastest and forms turbid solution easily.

Tertiary butyl alcohol gives turbidity fastest with Lucas reagent.