Question

The false statements among the following are:
i.A primary carbocation is less stable than a tertiary carbocation.
ii.A cyclopropyl methyl carbocation is less stable than allyl carbocation.
iii.A tertiary free radical is more stable than a primary radical.
iv.Isopropyl carbanion is more stable than ethyl carbanion.
A.I and II
B.II and III
C.I and IV
D.II and IV

Answer 1

Hint: The stability of cations and radicals depend upon the size of carbocation, more the number of substituents on the charged carbon atom more these are stable but in carbanions, more the number of substituents on the charged carbon atom lesser it is stable.

Complete step by step answer:
We have given four statements, we will try to understand every statement.
The first statement is that a primary carbocation is less stable than a tertiary carbocation. This statement is true because the distribution of charge is more stable than the compound. In primary carbocation the charge is distributed among only two carbon atoms but in tertiary carbocation the charge is distributed over four carbon atoms.
The second statement is that cyclopropyl methyl carbocation is less stable than allyl carbocation. This is the false statement because this is an exceptional case. The cyclopropyl methyl carbocation performs dancing resonance which makes it more stable than allyl carbocation.
The third statement is that a tertiary free radical is more stable than a primary radical. This is also a true statement. The radicals have the same properties as the cations so a tertiary radical is stabilized by the $ + I$ effect of the three methyl groups whereas a primary radical is stabilized by the $ + I$ effect of only one methyl group so tertiary free radicals are more stable than the primary radicals.
The last statement is, Isopropyl carbanion is more stable than ethyl carbanion. This is a false statement because as the number of methyl groups attached increases the $ + I$ effect also increases which lowers the stability of the anion. Lesser the number of methyl groups attached to the charged atom, more stable is the carbanion.
Hence option (D) is correct , II and IV .

Note:
The stability of carbocations and carbanions depend upon the degree of the charged carbon atom. If it is cation, more is the degree of charged carbon higher it is stable but in anions it is opposite, more the degree of carbon lesser the stable anion.