Explain the following with one example
(a) Aldol condensation
(b) Cannizzaro reaction
(c) Esterification
(d) Decarboxylation

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Solution

(a) Aldol condensation
It is a reaction between two molecules of carbonyl compounds having alpha hydrogen atoms to form beta hydroxy aldehyde (aldol) or beta hydroxy ketone (ketone). It occurs in presence of dilute base NaOH, KOH or Na $_{2}$ CO $_{3}$ . Thus, ethanal and propanal will give following products.
$2 C H_{3} - C H O d i l N a O H - ----- \to C H_{3} - C H O H - C H_{2} - C H O (i) H^{+} - -- \to (i i) Δ C H_{3} - C H = C H - C H O$
(b) Cannizzaro reaction
Aldehydes (not having alpha H atom) undergo self oxidation and reduction (disproportionation) in presence of concentrated aqueous or alcoholic alkali. The products are a mixture of alcohol and salt of carboxylic acid. One aldehyde molecule is reduced to alcohol and other aldehyde molecule is oxidized to salt of carboxylic acid.
Thus when formaldehdye is treated with aq NaOH, a mixture of methyl alcohol and sodium formate is obtained.
$2 H C H O + N a O H \to C H_{3} - O H + H C O O N a$
(c) Esterification
A carboxylic acid (or acid chloride or acid anhydride) reacts with an alcohol in presence of an acid cataylst.
$C_{2} H_{5} - O H + C H_{3} C O O H H^{+} - - \to C H_{3} - C O O - C_{2} H_{5} + H_{2} O$
$C_{2} H_{5} - O H + (C H_{3} C O)_{2} O H^{+} - - \to C H_{3} - C O O - C_{2} H_{5} + C H_{3} C O O H$
$C_{2} H_{5} - O H + C H_{3} C O C l pyridine - ---- \to C H_{3} - C O O - C_{2} H_{5} + H C l$
(d) Decarboxylation
When anhydrous sodium salt of carboxylic acid is fused with soda lime (sodium hydroxide and calcium oxide in the ratio 3:1) itgives hydrocarbon containing one carbon atom less than the carboxylic acid. The net result is loss of a molecule of carbon dioxide from carboxylic acid.
$C H_{3} C O O N a + N a O H C a O - - \to Δ C H_{4} + N a_{2} C O_{3}$

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