Tishchenko Reaction: Ester Formation from Aldehydes
The Tishchenko Reaction is an organic chemical reaction that involves the disproportionation of two molecules of an aldehyde (with or without $\alpha$-hydrogen) to form an Ester. It is essentially a modified Cannizzaro reaction carried out in the presence of an aluminium alkoxide catalyst.
1. General Reaction
Usually, two molecules of the same aldehyde react. One molecule is oxidized to a carboxylic acid, and the other is reduced to an alcohol. However, unlike the Cannizzaro reaction, these two fragments combine immediately to form an ester.
Conditions:
- Substrate: Aldehydes (e.g., Acetaldehyde, Benzaldehyde).
- Reagent: Aluminium Ethoxide ($Al(OEt)_3$) or Aluminium Isopropoxide.
- Product: Ester.
2. Detailed Mechanism
The reaction proceeds via a concerted hydride transfer mechanism facilitated by the Lewis acidic Aluminium atom.
Step 1: Coordination
The carbonyl oxygen of the aldehyde coordinates with the aluminium atom of the alkoxide catalyst, increasing the electrophilicity of the carbonyl carbon.
Step 2: Nucleophilic Addition
A second molecule of aldehyde adds to the complex, leading to a hemiacetal-like intermediate coordinated to the aluminium.
Step 3: Hydride Transfer (Key Step)
An intramolecular 1,3-Hydride Shift occurs. A hydride moves from the carbon of the "alcohol-to-be" part to the carbonyl carbon of the "acid-to-be" part. This regenerates the carbonyl bond in one fragment (forming the acyl part) and creates an alkoxide in the other.
Step 4: Product Release
The ester is released, and the catalyst is regenerated.
3. Examples
A. Synthesis of Ethyl Acetate
Reaction of two molecules of Acetaldehyde ($CH_3CHO$).
$$ 2CH_3CHO \xrightarrow{Al(OEt)_3} \underbrace{CH_3COOCH_2CH_3}_{\text{Ethyl Acetate}} $$B. Synthesis of Benzyl Benzoate
Reaction of two molecules of Benzaldehyde ($C_6H_5CHO$).
$$ 2C_6H_5CHO \xrightarrow{Al(OEt)_3} \underbrace{C_6H_5COOCH_2C_6H_5}_{\text{Benzyl Benzoate}} $$4. Comparison with Cannizzaro
Key Differences
- Substrate Scope: Cannizzaro is strictly for aldehydes without $\alpha$-hydrogens. Tishchenko works for all aldehydes (even those with $\alpha$-H like Acetaldehyde).
- Product: Cannizzaro gives Salt of Acid + Alcohol (separate). Tishchenko gives an Ester (combined).
- Reagent: Cannizzaro uses strong base ($NaOH$). Tishchenko uses Aluminium Alkoxide ($Al(OR)_3$).
5. Intramolecular Tishchenko
If the reaction is carried out on a dialdehyde, it leads to the formation of cyclic esters known as Lactones.
Tishchenko Reaction Quiz
Test your concepts on Ester formation. 10 MCQs with explanations.
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