Saturday, 15 September 2018

ALDEHYDES, KETONES AND CARBOXYLIC ACIDS NOTE

Aldehydes and ketones undergo nucleophilic substitution reaction. In nucleophilic substitution, a species that has extra electron or lone pair of electron attacks the carbonyl carbon of the ketone or the aldehyde leading to an alkoxide intermediate as shown in the middle figure below. This alkoxide intermediate can capture a proton to produce a neutral species. Reactivity of aldehyde vs ketone: Aldehydes are usually more reactive than Ketones for two reasons.



Steric and 2 electronic reasons Sterically, the presence of two bulky (large) groups in ketones will hinder the attack of nucleophile to carbonyl carbon in ketone. Aldehydes have only one bulky group around the carbonyl carbon and it is easier for the nucleophile to attack the carbonyl carbon as compared to ketones. Electronically, aldehyde is allso more reactive than ketone because the presence of two alkyl groups in ketones will reduce the electrophilicity (partial positive charge ) of the carbonyl carbon due to inductive effect by the alkyl groups. Therefore, attack of nucleophile on carbonyl carbon in ketone is harder than attack of carbonyl carbon in aldehyde making the aldehyde more reactive. Reduction:Reduction of aldehydes and ketones to primary or secondary alcohol using sodium borohydride or lithium aluminum hydride.

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