Question

The order of reactivity of alcohols with halogen acids is ______.

(A) \[\ce{CH3CH2 - CH2 - OH}\]

(B) \[\begin{array}{cc}
\phantom{}\ce{CH3CH2 - CH - OH}\\
\phantom{...}\phantom{}|\\
\phantom{......}\ce{CH3}
\end{array}\]

(C)  \[\begin{array}{cc}
\phantom{........}\ce{CH3}\\
\phantom{.....}\phantom{}|\\
\phantom{}\ce{CH3CH2 - C - OH}\\
\phantom{.....}\phantom{}|\\
\phantom{........}\ce{CH3}
\end{array}\]

Options

• (A) > (B) > (C)

• (C) > (B) > (A)

• (B) > (A) > (C)

• (A) > (C) > (B)

Answer 1

The order of reactivity of alcohols with halogen acids is (C) > (B) > (A).

Explanation:

(1) \[\ce{CH3CH2-CH2-OH -> \underset{1° Carbocation (least stable)}{CH3CH2\overset{+}{C}H + OH-}}\]

(2) \[\begin{array}{cc}
\ce{CH3CH2-CH-OH -> CH3CH2-\overset{+}{C}H + OH-}\phantom{....}\\
\phantom{}|\phantom{.......................}|\phantom{...}\\
\phantom{............}\ce{CH3}\phantom{.........}\ce{\underset{2° Carbocation (more stable than 1°)}{CH3}}\phantom{}
\end{array}\]

(3) \[\begin{array}{cc}
\phantom{......}\ce{CH3}\phantom{..................}\ce{CH3}\phantom{}\\
\phantom{...}|\phantom{......................}|\phantom{}\\
\ce{CH3CH2-C-OH -> CH3CH2-C^+-OH-}\\
\phantom{...}|\phantom{......................}|\phantom{}\\
\phantom{..............}\ce{CH3}\phantom{...........}\ce{\underset{3° Carbocation (most stable)}{CH3}}\phantom{}
\end{array}\]

The reactivity of alcohols depends on the type of alcohol (i.e. primary/secondary/tertiary). It is due to the presence of nature of carbocation present in its structure. Greater the stability of carbocation bonded with OH⁻, greater will be its ease of formation from alky halide and faster will be the rate of reaction. The order of reactivity of alcohols with a given haloid is acid is 3° > 2° > 1°.