The ketone carbonyl group of pyruvic acid (or 2-oxopropanoic acid) has a stretching frequency of a typical ketone, 1710 cm⁻¹. When hydroxylamine is added to a solution of pyruvic acid, this stretching frequency slowly disappears. Later, a new IR absorption appears at 1400 cm⁻¹. What happens?

You can probably apply something of what you know from Chapters 6 and 10 about the reactivity of carbonyl compounds towards nucleophiles to work out what is happening in this reaction between a carbonyl compound and an amine. The hydroxylamine fi rst adds to the ketone to form an unstable intermediate similar to a hemiacetal.

Notice that it is the more nucleophilic nitrogen atom, and not the oxygen atom, of hydroxyl amine that adds to the carbonyl group. Like hemiacetals, these intermediates are unstable and can decompose by loss of water. The product is known as an oxime and it is this compound, with its C N double bond, that is responsible for the IR absorption at 1400 cm⁻¹.

We know that the oxime is formed via an intermediate because the 1400 cm⁻¹ absorption hardly appears until after the 1710 cm⁻¹ absorption has almost completely gone. There must really be another curve to show the formation and the decay of the intermediate. The only difference is that the intermediate has no double bond to give an IR absorbance in this region of the spectrum. We come back to oximes later in the chapter.

مواضيع ذات صلة

Modifying reactivity using acetals

Cyclic acetals are more stable than acyclic acetals

Acetals hydrolyse only in the presence of acid

Amines from imines: reductive amination

Making acetals

Acetals are formed from aldehydes or ketones plus alcohols in the presence of acid

Acid or base catalysts increase the rate of equilibration of hemiacetals with their aldehyde and alcohol parents

Aldehydes can react with alcohols to form hemiacetals

Making the products less reactive

Making the starting materials more reactive

A bit of shorthand

Making alcohols instead of ketones