Propose mechanisms for the following reactions. (a) Ph-
-H
Ph-
-H
(b) CH_3-
-H
CH_3-
-H
(c)
(d)
The Correct Answer and Explanation is:
Let’s analyze and propose mechanisms for each reaction shown in the image:
(a) Acid-catalyzed acetal formation
Reaction:
Benzaldehyde (Ph-CHO) + CH₃OH → Acetal (Ph-CH(OCH₃)₂) under acidic conditions.
Mechanism:
- Protonation of the carbonyl oxygen increases the electrophilicity of the carbonyl carbon.
- Methanol attacks the carbonyl carbon, forming a hemiacetal.
- Proton transfer activates the hydroxyl group, converting it to a better leaving group.
- Water leaves, forming a resonance-stabilized carbocation.
- A second methanol attacks, forming the acetal.
- Deprotonation gives the final product.
(b) Formation of hydrazone derivative
Reaction:
Acetaldehyde + PhNHNH₂ → Hydrazone (CH₃CH=N–NHPh)
Mechanism:
- Nucleophilic attack by hydrazine nitrogen on the carbonyl carbon.
- Proton transfer yields a carbinolamine intermediate.
- Protonation of the hydroxyl group turns it into a good leaving group.
- Loss of water gives an iminium ion.
- Deprotonation at nitrogen forms the C=N hydrazone.
(c) Hydrolysis of imine to aldehyde
Reaction:
Imine + H₂O → Aldehyde + NH₃ under acidic conditions.
Mechanism:
- Protonation of the imine nitrogen increases electrophilicity.
- Water attacks the imine carbon, forming a tetrahedral intermediate.
- Proton transfers rearrange bonds.
- Ammonia leaves and is replaced by a carbonyl group.
(d) SN2 reaction (nucleophilic substitution)
Reaction:
Cyclic diether + EtNH₂ → Amine product via SN2.
Mechanism:
- Ethylamine acts as a nucleophile.
- It attacks the electrophilic carbon bonded to the leaving group (OCH₃).
- Methanol leaves in an SN2 reaction.
- Product is formed with inversion of configuration at the carbon center.
Conclusion:
These reactions demonstrate key organic principles: acetal formation (a), imine/hydrazone formation (b, c), and SN2 substitution (d). Acid catalysis and nucleophile-electrophile interactions drive these transformations. Understanding each mechanism helps predict reactivity and product formation in synthetic organic chemistry.
