Question

1) Draw the structure of the product formed by addition of vitamin B₁ and benzaldehyde.

2) Ammonia adds to carbonyl groups but is ineffective as a catalyst for the benzoin condensation. Explain.

3) Write a balanced oxidation-reduction equation for the oxidation of benzoin by ammonium nitrate (the cupric ion catalysis need not to be considered).

4) In the benzil oxidation the initial blue color changes to gren as the reaction proceeds. Why?

5) In dilantin sodium it is the imide proton that is abstracted rather than the amide proton. Why?

Answer 1

The reaction of two moles of benzaldehyde to form a new carbon-carbon bond is known as the benzoin condensation. It is catalyzed by two different catalysts, cyanide ion and the vitamin B1, but both are seen to function in exactly the same way.

1) Benzaldehyde condenses to form benzoin when heated with a catalytic amount of sodium cyanide.

2) Two molecules of benzaldehyde self-condense to form benzoin when stirred in ethanol with a catalytic amount of sodium hydroxide and thiamine. Carbonyl carbon of one benzaldehyde molecule has somehow been turned into a nucleophile, and has attacked a second benzaldehyde molecule in a nucleophilic carbonyl addition reaction. This reaction is possible due to thiamine catalyst. The important part of the thiamine molecule is the thiazole ring thus we will draw thiamine using R groups to depict the unreactive parts of the molecule.

2) In the benzoin condensation, the first step is addition of, cyanide ion a nucleophile, to an aldehyde carbonyl carbon. In the second step, the proton that used to be the aldehyde proton before addition of the nucleophile is removed by another cyanide ion acting as a base. The anion that is formed when the proton is removed is alpha to the cyanide carbon and can be stabilized by a resonance structure

Ar-C(OH)=C=N-.

Ammonia will also add to an aldehyde as a nucleophile. Since there is no pi bond on the product formed from ammonia addition, no such resonance structure can be drawn, and the aldehyde H is, therefore, not acidic. Since the aldehyde H cannot be removed, no nucleophile is generated to add to another molecule of aldehyde and make the new C-C bond which is formed in the benzoin condensation. So ammonia cannot serve as a catalyst for the benzoin condensation