Unit XII aldehydes and ketones

Aldehydes and ketones are structurally very similar, both have a carbon-oxygen double bond often called a carbonyl group. Like any double bond, the carbonyl group is composed of a σ bond and a π bond, but unlike carbon-carbon double bonds it is polar due to oxygen's electronegativity.

Aldehydes and ketones differ in that aldehydes have at least one hydrogen atom bonded to the carbonyl group, whereas in ketones the carbonyl group has carbons bonded on each other.

Aldehydes and ketones are quite prevalent in nature. They appear as natural fragrances and flavorings. In addition, carbonyl groups and their derivatives are the main structural features of carbohydrates and appear as functional groups in other natural compounds including dyes, vitamins, and hormones.

Compounds with two aldehyde or two ketone groups are named dials and diones, respectively.

Aldehydes and ketones are structurally similar and consequently they show similar chemical properties. They do differ significantly in one chemical property - susceptibility to oxidation. Aldehydes are easily oxidized under mild conditions, ketones are not.

Aldehydes can be distinguished from ketones using Tollens' reagent which is a solution of silver nitrate in ammonium hydroxide . As the aldehyde is oxidized to the salt of a carboxylic acid, silver ion (Ag+) is reduced to metallic silver. Ketones give no reaction.

Aldehydes and ketones possess a carbon-oxygen double bond and, as we might expect, addition is their most characteristic chemical reaction.

Although aldehydes and ketones add a variety of reagents the reactions are generally not as simple as those of alkenes. This is because the product of straight addition is frequently unstable and either exists in equilibrium with the starting materials or reacts further to form a more stable substance.

Aldehydes and ketones generally react by a nucleophilic addition mechanism. In this type of mechanism a nucleophile (Lewis base) is attracted to and bonds to the partially positive carbonyl carbon. The reaction can be initiated by either an acid or base.

Addition of hydrogen to aldehydes and ketones, catalytically and under pressure, results in the formation of primary and secondary alcohols, respectively. The reaction and its mechanism are analogous to that of addition of hydrogen to alkenes. It does not involve nucleophilic addition as do other reactions of carbonyl compounds.

A second and often more convenient method for the reduction of aldehydes and ketones to alcohols involves the use of metal hydrides such as lithium aluminium hydride, or sodium borohydride. The procedure involves treating a carbonyl compound with lithium aluminium hydride in ether followed by hydrolysis in water or dilute acid.

Aldehydes and ketones are most easily prepared by the oxidation of alcohols, this is especially true for ketones of low molecular weight which may be prepared from inexpensive secondary alcohols.

Primary alcohols can sometimes be oxidized to aldehydes, although this is a delicate reaction since aldehydes are themselves easily oxidized. Special conditions are usually employed, such as dehydrogenation over copper at high temperatures. Aromatic aldehydes are conveniently produced by hydrolysis of 1,1-dichlorocompounds. Both aldehydes and ketones can be formed by the action of ozone on alkenes. Ozonolysis, as this process is called, is useful not only in synthesizing aldehydes and ketones, but also in locating carbon-carbon double bonds.

An important reaction for synthesizing aromatic ketones involves the use of an acid chloride like acetyl chloride in the Friedel-Crafts reaction.