Question

Which of the following statements is correct? (1) \(+1\) effect stabilizes a carbocation (2) \(+\mathrm{I}\) effect stabilizes a carbanion (3) \(-\) I effect stabilizes a carbocation (4) \(-\mathrm{I}\) effect destabilizes a carbanion

Short answer

Statements (1), (3), and (4) are correct.

Step-by-step solution

Understanding the +I and -I Effects

The inductive effect refers to the electron-withdrawing or electron-donating properties of substituents due to polarization of sigma bonds. The +I effect involves electron-releasing groups (ERG) which donate electron density through sigma bonds, while the -I effect involves electron-withdrawing groups (EWG) which pull electron density through sigma bonds.

Evaluate Statement (1)

Statement (1) says that a +1 effect stabilizes a carbocation. Since carbocations are positively charged, they are stabilized by electron-donating groups (ERG) through the +I effect. This statement is correct.

Evaluate Statement (2)

Statement (2) suggests that the +I effect stabilizes a carbanion. Carbanions are negatively charged and are stabilized by electron-withdrawing groups, not electron-donating groups. Thus, this statement is incorrect.

Evaluate Statement (3)

Statement (3) states that the -I effect stabilizes a carbocation. Because carbocations are positively charged, anything that withdraws electron density (EWG with a -I effect) will stabilize the positive charge. So, this statement is correct.

Evaluate Statement (4)

Statement (4) claims that the -I effect destabilizes a carbanion. Carbanions are negatively charged and are destabilized by electron-withdrawing groups (EWG). Therefore, this statement is correct.

Final Answer

After evaluating all statements, the correct statements are (1), (3), and (4).

Key concepts

Carbocation Stabilization

A carbocation is a molecule in which a carbon atom bears a positive charge. Carbocations are highly reactive intermediates in many organic reactions because they have an incomplete octet. To stabilize carbocations, electron-donating groups (EDGs) play a crucial role. Through the inductive effect, these groups donate electron density towards the positively charged carbon atom.

EDGs stabilize carbocations through a +I effect, also known as the positive inductive effect. Some common EDGs include alkyl groups such as methyl (CH₃) and ethyl (C₂H₅). The donation of electron density helps to reduce the electron deficiency of the carbocation, making it more stable.

This stabilization is incredibly important in organic chemistry, especially in reactions like nucleophilic substitution and elimination reactions, where carbocations often form as intermediates.

Carbanion Stability

A carbanion is a molecule in which a carbon atom bears a negative charge. Unlike carbocations, carbanions have an octet and are rich in electrons. Because of their extra electron density, carbanions are stabilized by electron-withdrawing groups (EWGs).

When EWGs are present, they pull electron density away from the negatively charged carbon atom through the inductive effect, known as the -I effect or negative inductive effect. Common EWGs include fluorine (F), chlorine (Cl), and nitro groups (NO₂). By pulling electron density away, EWGs make the carbanion more stable by reducing the electron density around the negatively charged carbon.

This concept is vital for understanding various organic reactions, including those involving nucleophiles, where carbanions can act as reactive intermediates.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are groups of atoms that pull electron density away from other parts of a molecule through sigma bonds. This happens due to differences in electronegativity between the atoms of the substituent and the atom it's bonded to.

EWGs stabilize carbocations by removing electron density, thus easing the positive charge. Conversely, they destabilize carbanions by exacerbating the negative charge due to electron withdrawal. Common EWGs include:

• Fluorine (F)
• Chlorine (Cl)
• Bromine (Br)
• Iodine (I)
• Nitro group (NO₂)
• Cyanide group (CN)

Understanding the role of EWGs is important for predicting the behavior of molecules in various chemical reactions. The presence of EWGs can influence reaction mechanisms, rates, and outcomes significantly.

Electron-Donating Groups

Electron-donating groups (EDGs) are groups that push electron density towards other parts of a molecule through sigma bonds, usually due to hyperconjugation or resonance.

EDGs stabilize carbocations by providing additional electron density to the positively charged carbon atom, which reduces its electron deficiency. However, EDGs destabilize carbanions because they add electron density to an already negatively charged carbon atom.

Common EDGs include:

• Alkyl groups (e.g., methyl (CH₃), ethyl (C₂H₅))
• Hydroxyl group (OH)
• Alkoxy group (OR)
• Amino group (NH₂)

The ability of EDGs to influence the stability of carbocations and carbanions is crucial in organic synthesis and reactions. They help predict how different molecules will behave under various conditions.