Explain the following :
(a) Why `MeNH overset (oplus) CH_2` (I) is more stable more than `MeCH_2 overset (oplus) CH_2` (II) when both are `1^@` carbocations ?
(b) Which of the following intermediate is unstable ?

( c) Why allylic free radical `(CH_2 - CH = CH_2) (I)` is more stable than propy1 free radical `(dot C H_2 - CH_2 - CH_3) (II)`, when both are `1^@` free radicals ?

Explanation

(a) In `(I)`, positve charge on `C` atom is stabilised by resonance by the `L P overline e ^, s` on `N` atom.

So, `(I)` is more stable than (II)
(b) (I)
In `(I)`, positive charge is present on bridgehead, so corbocation cannot attain planarity , hence `(I)` is less stable than `(II)`.
( c) `(I)` is resonance stabilised, whereas `(II)` is not.
`overset(*)CH_(2)-CH=CH_(2)harrCH_(2)=CH-overset(*)CH_(2)`
`-=overset(*delta)(CH_(2))overset(--)_ CH_(2)overset(--)_ overset(*delta)CH_(2)`
(d) `EDG` stabilities carbocation, whereas `EWG` destabilises. `ED` power of : `ddot NH_2 ( + R` and `- I) gt -Me( + I` and H.C effect), but `+R gt H.C gt : dot C H`, triplet carbene have least tendency to release electron)` gt - NO_2 (- I` and `- R`, net `EW` power).
So stability order is : `IV gt III gt I gt II`
( e) `(IV)` is not feasible, because `N` is pentavalent with a positive charge.
(f) `(II)` is more stable since it has five `alpha-H-`atoms whereas `(I)` has three `alpha-H-`atoms. That is why `(I)` undergoes ring expansion by `1,2 - H^(Ө)` shift. The shifts are always `1,2` not `1,3`.