Question

The correct statement is (a) chloral forms stable hydrate (b) methanal is more reactive than ethanal towards nucleophilic addition (c) protonated carbonyl group is more reactive towards KCN (d) all of them

Short answer

The correct statement is (d) all of them.

Step-by-step solution

Understand the Statements

Before solving, let's break down and understand the given statements. (a) Chloral (trichloroacetaldehyde) is known to form a stable compound when it reacts with water, forming chloral hydrate. (b) Methanal (formaldehyde) is often more reactive in nucleophilic addition reactions compared to ethanal (acetaldehyde). (c) A carbonyl group gains increased electrophilicity when protonated, making it more susceptible to nucleophilic attack by KCN (potassium cyanide). (d) This suggests all statements might be correct if each individual statement holds true.

Evaluate Chloral's Hydrate Formation

Chloral is known to form chloral hydrate effectively due to the nature of its electrophilic carbon atom, which readily reacts with water. This leads to a stable chloral hydrate formation, making statement (a) correct.

Compare Reactivity of Methanal and Ethanal

Methanal, with its lesser steric hindrance and higher electrophilicity compared to ethanal, undergoes nucleophilic addition reactions more readily. This makes statement (b) correct.

Assess Protonated Carbonyl Group's Reactivity

Protonation increases the electrophilicity of the carbonyl carbon, enhancing its reactivity towards nucleophiles such as KCN. This makes statement (c) correct.

Conclusion

All individual statements (a), (b), and (c) are indeed correct when evaluated separately. Therefore, statement (d) which suggests that all of them are correct, holds true.

Key concepts

Chloral Hydrate

Chloral hydrate is a fascinating compound that results from the reaction between chloral (trichloroacetaldehyde) and water. When chloral encounters water molecules in a nucleophilic addition reaction, it forms chloral hydrate, which is notably stable.
This stability is due to:

• The presence of three electronegative chlorine atoms in chloral, which increases the electrophilicity of the carbonyl carbon, making it highly susceptible to attack by water molecules.
• The formation of strong hydrogen bonds between the hydroxyl groups in chloral hydrate, which contribute to its structural stability.

As a hydrate, chloral hydrate has been historically used for its sedative properties, though it now primarily serves more niche applications due to safer alternatives.

Electrophilicity

Electrophilicity refers to the ability of an atom or molecule to attract electrons or electron-rich species. It plays a crucial role in guiding nucleophilic addition reactions, which are common in organic chemistry.
Several factors influence electrophilicity:

• Presence of electronegative atoms: Atoms like chlorine, oxygen, and fluorine can withdraw electron density, making nearby carbon atoms more electrophilic.
• Charge: A positively charged atom is more electrophilic because it naturally attracts negatively charged species.
• Resonance stabilization: Electrophilicity can increase if an electron-deficient state can be stabilized through resonance.

For instance, when a carbonyl group (c=O) is protonated, its electrophilicity rises, intensifying its reactivity towards nucleophiles like KCN.

Carbonyl Reactivity

The reactivity of carbonyl compounds such as aldehydes and ketones is largely determined by the nature of the carbonyl group. This group features a carbon atom double-bonded to oxygen, creating a region of unsaturation and a significant dipole moment.
Reactivity of carbonyl compounds depends on:

• Electrophilicity: The carbon atom in the carbonyl group is electrophilic, favoring nucleophilic attacks that often lead to addition reactions.
• Steric effects: Smaller substituent groups on the carbonyl carbon allow easier access for nucleophiles, making compounds like methanal more reactive than bulkier ethanal.
• Protonation: When a carbonyl group is protonated, as in an acidic environment, electrophilicity increases significantly, enhancing its reactivity with nucleophiles.

Understanding these factors helps explain why certain carbonyls react faster or differently, essential knowledge for mastering organic chemistry reactions.