Example 8.2

Example . Since aldehydes and ketones both possess the carbonyl functional group, they undergo similar chemical reactions. . Nucleophilic addition reactions Contrary to electrophilic addition reactions observed in alkenes, the aldehydes and ketones undergo nucleophilic addition reactions. (i) Mechanism of nucleophilic addition reactions A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of sp hybridised orbitals of carbonyl carbon (Fig. . ). The hybridisation of carbon changes from sp to sp 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 as shown in Fig. . . . Arrange the following compounds in increasing order of their boiling points. CH CHO, CH CH OH, CH OCH , CH CH CH Fig. . : Nucleophilic attack on carbonyl carbon Would you expect benzaldehyde to be more reactive or less reactive in nucleophilic addition reactions than propanal? Explain your answer. 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.