Question

Which of the following carbonium ions has the least stability?
(A) methyl
(B) ethyl
(C) isopropyl
(D) tert-butyl

Answer 1

For this problem, we have to study about the carbonium ion and how its stability is affected by the electron-withdrawing and electron-donating group. And then we can easily determine the correct answer.

Complete step by step solution:
-In the given question, we have to choose the correct molecule which will have the least stable carbonium ion.
-The carbonium ion is the ion which consists of positive charge on the molecule.
-Now, as we know that there are two types of molecules that are electron-withdrawing and electron-donating groups.
-Electron withdrawing groups are those which have the ability to accept a pair of electrons from the adjacent atom. For example, CN, COOH, etc.
-Whereas electron-donating groups are those which can easily donate a pair of electrons to the adjacent atom. For example, alkyl groups.
-So, when the alkyl group is attached to carbonium ion, the stability of the carbonium ion will increase because of the donation of the negative charge to the positive charge.
-That's why more is the alkyl group attached to the carbonium ion, more will be stability.
-So, as we know that the chemical formula of methyl, ethyl, isopropyl and tert-butyl is
$\text{CH}_{3}^{+},\ \text{C}{{\text{H}}_{3}}\text{CH}_{2}^{+},\ \text{C}{{\text{H}}_{3}}\text{CH(C}{{\text{H}}_{3}}\text{)CH}_{2}^{+},\,\ {{(\text{C}{{\text{H}}_{3}})}_{3}}{{\text{C}}^{+}}$
-So, methyl will have the least stability because it lacks an alkyl group.

Therefore, option A is the correct answer.

Note: In the given problem, the stability of the carbonium ion will decrease when the electron-withdrawing group will be attached because it will withdraw electrons from the carbon due to which the positive charge on the carbon will increase.