Which of the following statements is correct?

+I effect stabilizes a carbocation

The correct answer is option 1. +I effect stabilizes a carbocation.

Let us dive deeper into why statement (1) is correct, based on the effects of +I (positive inductive effect) and –I (negative inductive effect) on carbocations and carbanions.

Carbocations:

+I Effect on Carbocations:

Carbocations are electron-deficient species with a positive charge on a carbon atom. Electron-donating groups (alkyl groups or groups with lone pairs of electrons) adjacent to the carbocation can donate electron density through sigma bonds. This donation of electron density stabilizes the positive charge by spreading it out and reducing the electron deficiency on the positively charged carbon atom. Examples of electron-donating groups (+I effect) include alkyl groups (e.g., methyl, ethyl, etc.) and groups with lone pairs like alkoxyl groups (-OR). Example: In the case of tert-butyl carbocation (t-butyl carbocation, \( (CH_3)_3C^+ \)), the three methyl groups are electron-donating (+I effect), stabilizing the positive charge on the central carbon.

 –I Effect on Carbocations:

Carbocations are destabilized by electron-withdrawing groups that withdraw electron density through sigma bonds. Electron-withdrawing groups (–I effect) increase the positive charge density on the carbocation carbon, making it more electron-deficient and less stable. Examples of electron-withdrawing groups (–I effect) include halogens (e.g., F, Cl, Br), nitro groups (-NO₂), and carbonyl groups (-C(=O)R). Example: In the case of a trichloromethyl carbocation (CCl₃⁺), the three chlorine atoms exert a –I effect, destabilizing the positive charge on the central carbon.

Carbanions:

+I Effect on Carbanions:

Carbanions are negatively charged species with a lone pair of electrons on a carbon atom. Electron-donating groups (alkyl groups, –R) stabilize carbanions by donating electron density through sigma bonds. This donation disperses the negative charge on the carbon atom, making the carbanion less reactive and more stable. Examples of electron-donating groups (+I effect) include alkyl groups (e.g., methyl, ethyl, etc.). Example: In the case of methyl carbanion (CH₃⁻), the methyl group is an electron-donating group (+I effect), stabilizing the negative charge on the carbon.

–I Effect on Carbanions:

Carbanions are destabilized by electron-withdrawing groups that withdraw electron density through sigma bonds. Electron-withdrawing groups (–I effect) increase the positive charge density on the carbanion carbon, making it more electron-deficient and less stable. Examples of electron-withdrawing groups (–I effect) include halogens (e.g., F, Cl, Br), nitro groups (-NO₂), and carbonyl groups (-C(=O)R). Example: In the case of a trifluoromethyl carbanion (CF₃⁻), the three fluorine atoms exert a –I effect, destabilizing the negative charge on the carbon.

Conclusion: Based on the effects described above:

Option (1) is correct: +I effect stabilizes a carbocation by donating electron density and reducing the positive charge density on the carbocation carbon. Options (2), (3), and (4) are incorrect because they describe effects that do not stabilize carbocations or describe effects that stabilize carbanions instead.

Understanding these effects is crucial in organic chemistry for predicting and understanding the reactivity and stability of carbocations and carbanions in various chemical reactions.