Aldehydes, ketones and carboxylic acids 2

• A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of sp2 hybridised orbitals of carbonyl carbon (Fig. 8.2). • The hybridisation of carbon changes from sp 2 to sp3 in this process, and a tetrahedral alkoxide intermediate is produced. -This intermediate captures a proton from the reaction medium to give the electrically neutral product. The net result is addition of Nu– and H+ across the carbon oxygen double bond.

Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. • Sterically, the presence of two relatively large substituents in ketones hinders the approach of the incoming nucleophile to carbonyl carbon than in aldehydes having only one such substituent. • Electronically, aldehydes are more reactive than ketones because two +I alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively than in former.

The carbon atom of the carbonyl group of benzaldehyde is less electrophilic than carbon atom of the carbonyl group present in propanal. The polarity of the carbonyl group is reduced in benzaldehyde due to resonance as shown below and hence it is less reactive than propanal.

In chemical industry aldehydes and ketones are used as solvents, starting materials and reagents for the synthesis of other products. Acetone and ethyl methyl ketone are common industrial solvents. Many aldehydes and ketones, e.g., butyraldehyde, vanillin, acetophenone, camphor, etc. are well known for their odours and flavours.

Formaldehyde is well known as formalin (40%) solution used to preserve biological specimens and to prepare bakelite (a phenol-formaldehyde resin), urea-formaldehyde glues and other polymeric products.

Acetaldehyde is used primarily as a starting material in the manufacture of acetic acid, ethyl acetate, vinyl acetate, polymers and drugs.

Benzaldehyde is used in perfumery and in dye industries.