Question

Explain the terms Inductive and Electromeric effects. Which electron displacement effect explains the following correct orders of acidity of the carboxylic acids? (a) \(\mathrm{Cl}_{3} \mathrm{CCOOH}>\mathrm{Cl}_{2} \mathrm{CHCOOH}>\mathrm{ClCH}_{2} \mathrm{COOH}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}>\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCOOH}>\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C.COOH}\)

Short answer

The inductive effect explains the orders of acidity in both parts (a) and (b).

Step-by-step solution

Understanding Inductive Effect

The inductive effect is the electron-withdrawing or electron-releasing effect exerted by substituents through a sigma bond. It diminishes with increasing distance from the source group, usually an electronegative atom.

Understanding Electromeric Effect

The electromeric effect involves the complete transfer of electrons between atoms connected by a double or triple bond in response to an attacking reagent. It is a temporary effect as it depends on the presence of the external reagent.

Analyzing Order of Acidity in Part (a)

In part (a), the presence of chlorine atoms in the structure increases the acidity of the carboxylic acid due to chlorine's strong electron-withdrawing inductive effect. More chlorine atoms result in a more pronounced effect, increasing acidity.

Analyzing Order of Acidity in Part (b)

In part (b), the presence of alkyl groups decreases acidity due to their electron-releasing inductive effect. As the number and size of alkyl groups increase, their bond donation effect becomes stronger, leading to decreased acidity.

Conclusion: Identifying the Effect

The observed orders of acidity in both cases (a and b) are explained by the inductive effect, as this effect considers the intrinsic nature of substituents without needing an external reagent as required for the electromeric effect.

Key concepts

Electromeric Effect

The electromeric effect is a fascinating phenomenon that occurs in organic chemistry when electrons are transferred fully between atoms connected by multiple bonds, such as double or triple bonds. This electron displacement is temporary and only happens in the presence of an attacking reagent. It is important to note that during the electromeric effect, the shift of electrons is complete, resulting in a temporary polarization of the molecule.
This effect is often observed in reactions where an external reagent targets a molecule with unsaturated bonds. As the reagent approaches, electrons shift towards one atom within the molecule, creating a new reactive site.
This electron movement can significantly impact reaction mechanisms and explains why certain reactions occur the way they do.

• Transient electron shift
• Occurs in presence of external reagents
• Impacts reactivity of unsaturated bonds

By understanding the electromeric effect, we can gain insights into how molecules respond to certain conditions, allowing us to predict and influence chemical reactions.

Acidity of Carboxylic Acids

Carboxylic acids are organic compounds known for their acidic nature, which can vary based on the structure and substituents present in the molecule. The acidity of these compounds is primarily influenced by electron-withdrawing or electron-donating effects from substituents attached to the carboxyl group. For instance, in part (a) of the original exercise, chlorine atoms, known for their strong electron-withdrawing capability through the inductive effect, increase the acidity of carboxylic acids. This happens because electron-withdrawing groups stabilize the negative charge of the conjugate base, making the acid more willing to release its hydrogen ion.

On the other hand, electron-donating groups, such as alkyl chains in part (b), can decrease acidity by making the release of a hydrogen ion less favorable. The presence of more or larger alkyl groups leads to a stronger release effect, hence lowering the acidity.

• Electron-withdrawing groups increase acidity
• Electron-donating groups decrease acidity
• Acidity affected by substituent's effect on electron distribution

Therefore, the order of acidity in carboxylic acids can often be easily explained by assessing the nature and position of substituents within the molecule.

Electron Displacement Effects

Electron displacement effects are crucial in understanding the reactivity and properties of organic molecules. These effects describe how electrons can be shifted within a molecule due to different influences, such as nearby substituents or reagents. The inductive effect is one of the most common electron displacement effects and involves the pull or push of electrons along a sigma bond due to electronegativity differences. It diminishes with distance from the source group.
Another example is the electromeric effect, which is more transient and involves a complete electron shift when an external reagent is present. These effects are integral because they explain how molecules can react differently based on their surroundings and inherent atomic properties.

• Inductive effect involves permanent electron push or pull
• Electromeric effect is temporary and reagent-dependent
• These effects influence molecular stability and reactivity

Understanding electron displacement effects brings clarity to complex chemical behaviors and helps in predicting reactions and properties of organic compounds.