CH3−H2 (Ethyl carbocation) :
This is a primary carbocation. It's stabilised by the
+I (inductive) effect of the methyl group and some hyperconjugation from the three alpha-hydrogens. The positive carbon is
sp2-hybridised.
(b)
CH2=CH (Vinyl carbocation): In this carbocation, the positive charge is on an
sp hybridised carbon. An
sp hybridised carbon is more electronegative than an
sp2 or
sp3 hybridised carbon, meaning it's less capable of accommodating a positive charge. The double bond itself cannot provide effective resonance stabilisation to this directly attached positive charge, and in fact, the proximity of the electron-withdrawing
sp hybridised carbon makes it highly unstable. This is due to the electron-withdrawing nature of the double bond.
(c)
CH2=CH−H2 (Allyl carbocation) : This carbocation is highly stabilised by resonance. The positive charge is delocalised over two carbon atoms due to conjugation with the double bond.
(d)
C6H5−H2 (Benzyl carbocation) This carbocation is also highly stabilised by resonance. The positive charge is delocalised into the benzene ring due to conjugation.
Comparing these, the vinyl carbocation
(CH2=H) is the least stable.