Question

Write the structures of A, B, and C in the following sequence of reactions:

Hint:

In this reaction sequence, DIBAL-H is used to reduce nitriles to aldehydes, and aldol condensation with NaOH under heat leads to the formation of β-hydroxy aldehydes or ketones, which can undergo further transformations to form the final product.

Answer 1

To solve the problem, we need to determine the structures of intermediates A, B, and C in the given reaction sequences.

Sequence (a):

1. Reaction of CH₃COOH with SOCl₂:
Acetic acid (CH₃COOH) reacts with thionyl chloride (SOCl₂) to form acetyl chloride (A):

$ CH_3COOH \xrightarrow{SOCl_2} CH_3COCl $
A: $ CH_3COCl $ (Acetyl chloride)

2. Reduction of Acetyl Chloride with H₂/Pd-BaSO₄:
Acetyl chloride (CH₃COCl) is reduced to acetaldehyde (B) using hydrogen gas and palladium on barium sulfate (Rosenmund reduction):

$ CH_3COCl \xrightarrow{H_2, Pd-BaSO_4} CH_3CHO $
B: $ CH_3CHO $ (Acetaldehyde)

3. Reaction of Acetaldehyde with Hydrazine (H₂N-NH₂):
Acetaldehyde reacts with hydrazine to form acetaldehyde hydrazone (C):

$ CH_3CHO + H_2N-NH_2 \rightarrow CH_3CH=N-NH_2 $
C: $ CH_3CH=N-NH_2 $ (Acetaldehyde hydrazone)

Sequence (b):

1. Reduction of CH₃CN with DIBAL-H:
Acetonitrile (CH₃CN) is reduced by diisobutylaluminum hydride (DIBAL-H) to form acetaldehyde (A):

$ CH_3CN \xrightarrow{1. (DIBAL-H)} CH_3CHO $
A: $ CH_3CHO $ (Acetaldehyde)

2. Aldol Condensation with Dilute NaOH:
Acetaldehyde undergoes aldol condensation in the presence of dilute NaOH to form 3-hydroxybutanal (B):

$ 2 CH_3CHO \xrightarrow{Dil. NaOH} CH_3CH(OH)CH_2CHO $
B: $ CH_3CH(OH)CH_2CHO $ (3-Hydroxybutanal)

3. Dehydration upon Heating (Δ):
3-Hydroxybutanal undergoes dehydration upon heating to form crotonaldehyde (C):

$ CH_3CH(OH)CH_2CHO \xrightarrow{\Delta} CH_3CH=CHCHO $
C: $ CH_3CH=CHCHO $ (Crotonaldehyde)

Final Answers:

Sequence (a):
A: $ CH_3COCl $ (Acetyl chloride)
B: $ CH_3CHO $ (Acetaldehyde)
C: $ CH_3CH=N-NH_2 $ (Acetaldehyde hydrazone)

Sequence (b):
A: $ CH_3CHO $ (Acetaldehyde)
B: $ CH_3CH(OH)CH_2CHO $ (3-Hydroxybutanal)
C: $ CH_3CH=CHCHO $ (Crotonaldehyde)