Which of the following statements is not correct?

Primary carbonium ions are more stable than secondary ones.

The correct answer is option 1. Primary carbonium ions are more stable than secondary ones.

Let us analyze the stability of carbonium ions (carbocations) and free radicals to understand why certain statements about their stability are correct or incorrect.

Carbonium Ions (Carbocations)

A carbonium ion is a positively charged carbon ion (carbocation) with a general formula \(R_3C^+\). The stability of carbocations depends on the number of alkyl groups attached to the positively charged carbon. Alkyl groups are electron-donating groups that stabilize the carbocation through inductive and hyperconjugation effects.

Primary Carbocation (Primary Carbonium Ion): \(CH_3^+\) or \(RCH_2^+\)

Has one alkyl group attached. Least stable due to the lack of significant electron-donating groups.

Secondary Carbocation (Secondary Carbonium Ion): \(R_2CH^+\)

Has two alkyl groups attached. More stable than primary carbocations due to the increased electron-donating effects from the two alkyl groups.

Tertiary Carbocation (Tertiary Carbonium Ion): \(R_3C^+\)

Has three alkyl groups attached. Most stable due to the maximum electron-donating effects from the three alkyl groups.

The order of stability is: tertiary > secondary > primary.

Free Radicals

Free radicals are species with an unpaired electron, represented as \(R_3C^\cdot\). The stability of free radicals also depends on the number of alkyl groups attached to the carbon with the unpaired electron.

Primary Free Radical: \(RC^\cdot H_2\)

Has one alkyl group attached. Least stable due to the lack of significant electron-donating groups.

Secondary Free Radical: \(R_2C^\cdot H\)

Has two alkyl groups attached. More stable than primary free radicals due to increased electron-donating effects from the two alkyl groups.

Tertiary Free Radical: \(R_3C^\cdot\)

Has three alkyl groups attached. Most stable due to maximum electron-donating effects from the three alkyl groups.

The order of stability is: tertiary > secondary > primary.

Analysis of options

1. Primary carbonium ions are more stable than secondary ones.

The statement is incorrect. As explained, secondary carbocations are more stable than primary carbocations.

2. Secondary free radicals are more stable than primary free radicals.

The statment is correct. Secondary free radicals are more stable than primary free radicals due to increased stabilization from the alkyl groups.

3. Tertiary free radicals are more stable than secondary ones.

The statement is correct. Tertiary free radicals are more stable than secondary free radicals because of the greater stabilizing effects of three alkyl groups.

4. Tertiary carbonium ions are more stable than primary ones.

The statement is correct. Tertiary carbocations are more stable than primary carbocations due to the strong electron-donating effects of the three alkyl groups.

Therefore, the statement that is not correct is (1) Primary carbonium ions are more stable than secondary ones.