Question

(1)Phenylglyoxal. Phenylglyoxal is in equilibrium with its hydrate as shown: H + H2O HO OH (a) Propose a mechanism for this reaction under acidic conditions. (b) Propose a mechanism for this reaction under basic conditions. The IR spectrum of the hydrate is shown below MICRONS NICOLET 20SX FT-IR 21 22 25 2.6 2.72.8 2.9 100 3 3.5 4.5 5.5 10 12 14 15 18 17 18 19 80 0.1 70 40 A 05 08 0.7 09 10 20 4000 3800 3600 3400 3200 3000 200 2600 2400 2200 1800 1600 WAVENUMBERS 1400 1200 1000 800 600 (c) Explain why this spectrum is consistent with the structure of the hydrate but not the dicarbonyl compound (d) Explain why this spectrum is consistent with hydration of the aldehyde but not the ketone.

Answer 1

a) & b)

a) & b) conversion of phenylglyoxal to corresponding hydrate a) In Acidic Medium: b)basic medium c) The given spectrum shows peaks corresponding to the Hydrate as it is the predominant species in an aqueous solution of Phenyl-glyoxal. The stability of diol is due to the ease of hydrogen bonding that is formed between Carbonyl C = O and any one of the 2�-OH of diol. As in aqueous solution, the predominant species is hydrate form, IR absorptions corresponding to hydrate form are appeared & not the diketone (Ketonic + Aldehydic). d) The hydration at aldehydic C = O but not the ketonic C = O occurred. i) From the mechanism of hydration of Carbonyl functionality, in both acidic & basic medium shows that there is a nucleophilic addition to Carbonyl Carbon.

c) The given spectrum shows peaks corresponding to the Hydrate as it is the predominant species in an aqueous solution of Phenyl-glyoxal.

The stability of diol is due to the ease of hydrogen bonding that is formed between Carbonyl C=O and any one of the 2 ^-OH of diol.

As in aqueous solution, the predominant species is hydrate form, IR absorptions corresponding to hydrate form are appeared & not the diketone (Ketonic + Aldehydic).

d) The hydration at aldehydic C=O but not the ketonic C=O occurred.

i) From the mechanism of hydration of Carbonyl functionality, in both acidic & basic medium shows that there is a nucleophilic addition to Carbonyl Carbon.

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