. Chemical properties of aldehydes and ketones A) Nucleophilic addition reactions This reaction is the most common reactions of aldehydes and ketones. The carbonyl carbon carries a small degree of positive charge. Nucleophile such as CN − can attack the carbonyl carbon and uses its lone pair to form a new carbon – nucleophile ' σ ' bond, at the same time two electrons from the carbon – oxygen double bond move to the most electronegative oxygen atom.
This results in the formation of an alkoxide ion. In this process, the hybridisation of carbon changes from sp to sp . R R sp hybridised carbon sp hybridised carbon tetrahedral alkoxide ion Nu R Nu R The tetrahedral intermediate can be protonated by water or an acid to form an alcohol. R Nu R R Nu R In general, aldehydes are more reactive than ketones towards nucleophilic addition reactions due to +I and steric effect of alkyl groups.
XII U12-Carbonyl XII U12-Carbonyl - - - - Examples ) Addition of HCN Attack of CN − on carbonyl carbon followed by protonation gives cyanohydrins. H C H C CN H C CN CN ethanal (acetaldehyde) tetrahedral intermediate -hydroxy propanenitrile (acetaldehyde cyano hydrin) The cyanohydrins can be converted into hydroxy acid by acid hydrolysis. Reduction of cyanohydrins gives hydroxy amines ) Addition of NaHSO H C H C OSO H Proton transfer H C H C OSO Na acetone (propanone) bisulphite addition compound H C OSO H This reaction finds application in the separation and purification of carbonyl compound. The bisulphate addition compound is water soluble and the solution is treated with mineral acid to regenerate the carbonyl compounds .
) Addition of alcohol When aldehydes / ketones is treated with equivalents of an alcohol in the presence of an acid catalyst to form acetals. example When acetaldehyde is treated with equivalent of methanol in presence of HCl, , , - dimethoxy ethane is obtained. H C CH HCl H C CH OCH OCH ethanal (acetaldehyde) , - dimethoxy (acetaldehyde dimethyl acetal) ethane XII U12-Carbonyl XII U12-Carbonyl - - - - Mechanisum H C Cl H C H C H C H O H C OCH Cl -Cl H C OCH H C H C H C OCH -H O + H C OCH H O -H O + ) Addition of ammonia and its derivatives When the nucleophiles, such as ammonia and its derivative H N-G .. is treated with carbonyl compound, nuceophilic addition takes place, the carbonyl oxygen atom is protonated and then elimination takes place to form carbon – nitrogen double bond C = N G When G – alkyl, aryl, OH, NH , C H NH, NHCONH etc...
R R H N G -H O R R G R R G R R G G Ammonia derivatives Carbonyl derivatives Product name Hydroxyl amine