In which of the following pairs of carbocations, the first carbocation is more stable than the second ?
(i) `CH_(2) = CH - CH_(2) and CH_(2) = CH - CH_(2) - overset(+)(CH_(2))`
(ii) `CH_(3) - NH - overset(+)(CH_(2)) and overset(+)(CH_(2)) - OH`
(iii) `CH_(3) - O - CH_(2) - overset(+)CH_(2) and CH_(3) - O - overset(+)(CH_(2))`
(IV) `CH_(3) overset(+)(CH) - CH_(2) CH_(2) CH_(3) and CH_(3) CH_(2) - overset(+)(CH) - CH_(2) CH_(3)`

Options

A. (i),(ii) and (iii)
B. (i) , (ii) and (iv)
C. (ii) and (iii)
D. (iii) and (iv)

Answer

B

Explanation

(i)First carbocation `(CH_(2) = overset(+)(CHCH_(2)) ) ` is stabilized by resonance but the second `(CH_(2) = "CHCH"_(2) CH_(2)^(+))` is not.
Therefore, first carbocation is more stable than the second

In both the cases, +ve charge is dispersed due to resonance but in the second carbocation +ve charge is located on a more electronegative O-atom. Therefore, first carbocation is more stable than second.

(iii) Second carbocation
is stabilized by resonance but first `(CH_(3) - O - CH_(2) - overset(+)(CH_(2)))` is not. Therefore, carbocation second is more stable than the frst. (iv) First carbocation `(CH_(3) overset(+)("CHCH"_(2)) CH_(2) CH_(3))` has five a-hydrogen atoms and hence five hyperconjugation structures are possible while the second carbocation `(CH_(3) CH_(2) overset(+)("CHCH"_(2)) CH_(3))` has only four `alpha`-hydrogen atoms and hence has four hyperconjugative structures. Therefore, first carbocation is little more stable than the second. Thus, the correct answer is option (b).