Conversion of multiple bonds, such as a double or a triple bond, into other functional groups are usually achieved using addition reactions. The reaction of multiple bonds will convert an unsaturated compound to more saturated and functionalized species. In this module, a number of examples of electrophilic addition to electron rich double bonds are presented. When a double bond is activated by attaching it with electron withdrawing groups, conjugated addition is observed.
The carbonyl carbon is sp2-hybridized, and it and the atoms attached to it are coplanar. Aldehydes and ketones are polar molecules. Nucleophiles attack CPO at carbon (positively polarized) and electrophiles, especially protons, attack oxygen (negatively polarized).
Aldehydes and ketones have higher boiling points than hydrocarbons, but have lower boiling points than alcohols. The numerous reactions that yield aldehydes and ketones are sufficient for most syntheses.
The characteristic reactions of aldehydes and ketones involve nucleophilic 17.5–17.13 addition to the carbonyl group and are summarized in Table 17.5. Reagents of the type HY react according to the general equation
Aldehydes undergo nucleophilic addition more readily and have more favorable equilibrium constants for addition than do ketones. The step in which the nucleophile attacks the carbonyl carbon is rate-determining in both base-catalyzed and acid-catalyzed nucleophilic addition. In the base-catalyzed mechanism this is the first step.
Under conditions of acid catalysis, the nucleophilic addition step follows protonation of the carbonyl oxygen. Protonation increases the carbocation character of a carbonyl group and makes it more electrophilic.
Examples of nucleophilic addition and nucleophilic addition elimination reactions:
- Addition of hydrogen cyanide (HCN): Cyanohydrins are produced when aldehydes and ketones reacts with hydrogen cyanide. In pure HCN these reactions are very slow. When it is catalysed by a base then the cyanide ion adds to carbonyl compounds and gives cyanohydrins.
- Addition of sodium hydrogensulphite: Some addition products are formed when sodium hydrogensulphite is added to aldehydes and ketones. Due to steric reasons, the position of equilibrium for most of the aldehydes is right hand side and towards the left for the ketones.
- Addition of alcohols: To yield alkoxyalcohol intermediates also known as hemiacetals, aldehydes react with monohydric alcohol in the presence of dry hydroelectric acid. Cyclic product also known as ethylene glycol ketals is formed when ketones react with ethylene glycol. The oxygen of carbonyl carbon is protonated by dry hydrogen chloride which increases the electrophilicity of the carbonyl carbon by increasing the nucleophilic attack of ethylene glycol.