The order of reactivity of following alcohols with halogen acids is……….
(A) `CH_(3)CH_(2)-CH_(2)-OH` (B) `CH_(3CH_(2)-underset(CH_(3))underset(|)"CH"-OH`
`(C ) CH_(3)CH_(2)-underset(CH_(3))underset(|)overset(CH_(3))overset(|)C-OH`

Options

A. (i) `gt` (ii) `gt` (iii)
B. (iii) `gt` (ii) `gt` (i)
C. (ii) `gt` (i) `gt` (iii)
D. (i) `gt` (iii) `gt` (ii)

Answer

B

Explanation

Reaction between alcohols and halogen acid follows `S_N^1` mechanism. In `S_N^1` mechanism carbocations are formed as intermediates.
Let us consider the formation of carbocations with the given three alcohols
`CH_3-CH_2-CH_2-OH rarr CH_3-CH_2-overset(+)(C)H+OH^(-)` In this case, `1^@` carbocation is formed. It is least stable. So, here `S_N^2` mechanism is followed. In this `S_N^2` mechanism a transitory state is observed in `alpha` -carbon is linked with two nucleophiles.
`CH_3-CH_2-underset(CH_3)underset(|)(CH)-OH rarr CH_3-CH_2-underset(2^@" carbocation(more stable than "1^@" carbocation)")underset(CH_3)underset(|)overset(+)(CH)+OH^(-)`
`CH_3-CH_2-overset(CH_3)overset(|)underset(CH_3)underset(|)(C)-OH rarr H_3C-CH_2-overset(CH_3)overset(|)underset(3^@" carbocation(most stable)")underset(CH_3)underset(|)(C^+)+OH^(-)`
The reaction proceeded with stable carbocation. Higher the stability of carbocation, higher will be the possibilities of attack of − X ion to the carbocation.
As, the tertiary carbocation is most stable so the possibilities of attack of `X^(-)` ion are more prominent in case of tertiary carbocations. Thus, attack of `X^(-)` ion to carbocation is proceeded with tertiary carbocation as follows
`H_3C-CH_3-overset(CH_3)overset(|)underset(3^@" carbocation")underset(CH_3)underset(|)(C^(oplus))+X rarr H_3C-CH_2-overset(CH_3)overset(|)underset(CH_3)underset(|)(C)-X`