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Describe the following reactions: (i) Cannizzaro's reaction. (ii) Cross aldol reaction. |
Answer» Solution :(i) Cannizzaro.s reaction: Akdehydes which do not have `alpha`- hydrogen atom, undergo self oxidation and reduction (disproportionation) reaction on TREATMENT with concentrated alkali. In this reaction, one molecule of aldehyde is reduced to alcohol while the other is oxidised to carboxylic acid salt. For example, (ii) Cross aldol reaction: When aldol condensation is carried out between two different aldehydes and/or ketones, it is called cross aldol condensation. If both of them CONTAIN `alpha`- hydrogen ATOMS, it gives a mixture of 4 products. Suppose there are two aldehydes RCHO and RCHO both containing `alpha`- hydrogen, one product each will be OBTAINED from (RCHO+RCHO), (R.CHO+R.CHO),(RCHO+R.CHO) and (R.CHO+RCHO). Cross aldol condensation between `CH_(3)CHO` and `CH_(3)CH_(2)CHO` gives the following products. `CH_(3)CHO+CH_(3)CHOoverset("dil" NaOH)(to)underset(OH)underset(|)(CH_(3)CH)-CH_(2)-CHO` `CH_(3)CH_(2)CHO+CH_(3)CH_(2)CHOoverset("dil"NaOH)(to)underset(OH)underset(|)(CH_(3)CH_(2)CH)-overset(CH_(3))overset(|)(CH)-CHO` `CH_(3)CHO+CH_(3)CH_(2)CHOoverset("dil"NaOH)(to)CH_(3)CH_(2)-underset(OH)underset(|)(CH)-CH_(2)CHO` `CH_(3)CH_(2)CHO+CH_(3)CHOoverset("dil"NaOH)(to)underset(OH)underset(|)(CH_(3)CH)-underset(CH_(3))underset(|)(CH)-CHO` Reaction beween benzaldehyde (not containing `alpha`- hydrogen) and acetophenone (containing `alpha` -hydrogen) is given below: 
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