Second Trimester
1. Wolf Kishner Reduction Mechanism:
Purpose: The Wolf Kishner reduction converts carbonyl compounds (aldehydes or ketones) into alkanes by reducing the carbonyl group to a methylene group (-CH₂-).
Mechanism:
Formation of Hydrazone:
Step 1: The carbonyl compound reacts with hydrazine (N₂H₄) to form a hydrazone intermediate.
R₂C=O + N₂H₄ ⟶ R₂C=NNH₂ + H₂O
Formation of Alkoxide Intermediate:
Step 2: Sodium or potassium hydroxide (NaOH or KOH) is added, leading to the formation of an alkoxide ion (R₂C=N-NH₂⁺) through deprotonation.
R₂C=NNH₂ + OH⁻ ⟶ R₂C=N-NH₂⁺ + H₂O
Thermal Decomposition:
Step 3: Heating the reaction mixture induces thermal decomposition, resulting in the loss of nitrogen gas (N₂) and the formation of the reduced alkane.
R₂C=N-NH₂⁺ ⟶ R₂CH₂ + N₂ + H₂O
2. Wittig Reaction Mechanism:
Purpose: The Wittig reaction converts aldehydes or ketones into alkenes using a phosphorus ylide.
Mechanism:
Formation of Phosphonium Ylide:
Step 1: Triphenylphosphine (Ph₃P) reacts with an alkyl halide (R'-X) to form a phosphonium salt (Ph₃P^+R'-X^-).
Ph₃P + R'-X ⟶ Ph₃P^+R'-X^-
Formation of Wittig Reagent:
Step 2: The phosphonium salt reacts with a strong base (e.g., BuLi) to form a phosphorus ylide (Ph₃P=CHR').
Ph₃P^+R'-X^- + BuLi ⟶ Ph₃P=CHR' + LiX + Ph₃P
Olefin Formation (Wittig Reaction):
Step 3: The phosphorus ylide (Ph₃P=CHR') reacts with the carbonyl compound to form an oxaphosphetane intermediate.
R₂C=O + Ph₃P=CHR' ⟶ R₂C=CHR' + Ph₃PO
Olefin Formation:
Step 4: The oxaphosphetane intermediate undergoes an E2 elimination reaction, resulting in the formation of an alkene.
R₂C=CHR' ⟶ R₂CH=CH₂ + Ph₃P=O