Give reactions of carboxylic acids in which $C-OH$ bond breaks. $OR$ Give methods of preparation for $(a)$ anhydride $(b)$ ester $(c)$ acid chloride and $(d)$ amide from carboxylic acids.

(N/A) Formation of anhydride: Carboxylic acids on heating with mineral acids such as $H_{2}SO_{4}$ or with $P_{2}O_{5}$ give corresponding anhydride.
$(b)$ Esterification: Carboxylic acids are esterified with alcohols or phenols in the presence of a mineral acid such as concentrated $H_{2}SO_{4}$ or $HCl$ gas as a catalyst.
$RCOOH + R'OH \rightleftharpoons H^{+} \rightleftharpoons RCOOR' + H_{2}O$
In this method,the $C-OH$ bond of the acid and the $O-H$ bond of the alcohol break to form an ester and remove one molecule of water.
$(c)$ Formation of acid chloride: The hydroxyl group $(-OH)$ of carboxylic acids is easily replaced by a chlorine atom on treating with $PCl_{5}$,$PCl_{3}$ or $SOCl_{2}$.
$RCOOH + PCl_{5} \rightarrow RCOCl + POCl_{3} + HCl$
$3RCOOH + PCl_{3} \rightarrow 3RCOCl + H_{3}PO_{3}$
$RCOOH + SOCl_{2} \rightarrow RCOCl + SO_{2} + HCl$
$(d)$ Formation of amide: Carboxylic acids react with ammonia to give ammonium salts which on further heating at high temperature give amides.
$RCOOH + NH_{3}$ $\rightarrow RCOONH_{4}$ $\xrightarrow{\Delta} RCONH_{2} + H_{2}O$