Q.With which of the following does acetic acid not form acetyl chloride?

PCl5, PCl3, SOCl2 convert -COOH to -COCl; Cl2 does not.

The -OH of a carboxylic acid is replaced by -Cl by chlorinating agents:

• CH3COOH + PCl5 -> CH3COCl + POCl3 + HCl
• 3 CH3COOH + PCl3 -> 3 CH3COCl + H3PO3
• CH3COOH + SOCl2 -> CH3COCl + SO2 + HCl …

Benzoic acid reacts with ammonia to give benzamide and water; ammonia is the missing reagent.

Carboxylic acids react with ammonia conceptually in two steps: first an acid-base reaction gives the ammonium salt, and on heating (dehydration) this salt loses water to give the amide:

C6H5COOH + NH3 -> C6H5COONH4 (ammonium benzoate) -> (heat, -H2O) -> C6H5CONH2 (benzamide) …

Carboxylic acids react with PCl₅ to replace the –OH group with –Cl, giving an acid chloride.

Phosphorus pentachloride ($PCl_5$) converts the –OH of a carboxylic acid into –Cl, forming the corresponding acid chloride, along with phosphorus oxychloride and HCl gas:

$CH_3COOH + PCl_5 \rightarrow CH_3COCl + POCl_3 + HCl\uparrow$

…

Phosphorus pentachloride converts a carboxylic acid's -OH into -Cl, forming an acid chloride.

PCl5 (and similarly SOCl2, PCl3) is a standard reagent for converting the -COOH group of a carboxylic acid into the corresponding acid chloride (-COCl), replacing the hydroxyl oxygen/hydrogen with chlorine:

$CH_3COOH + PCl_5 \rightarrow CH_3COCl + POCl_3 + HCl$

…

Carboxylic acids react with ammonia to form ammonium salts, which lose water on strong heating to give the corresponding amide.

Identifying the reaction: -COOH converting directly to -CONH2 is acid-to-amide conversion, achieved simply via the ammonium salt.

Reagent and conditions: Ammonia, NH3 (as conc. NH4OH or dry NH3 gas), followed by strong heating (dehydration).

Equations:

$C_6H_5COOH + NH_3 \longrightarrow C_6H_5COONH_4\;(\text{ammonium benzoate})$

$C_6H_5COONH_4 \xrightarrow{\Delta,\,-H_2O} C_6H_5CONH_2\;(\text{benzamide})$

…

Acetic anhydride is made by dehydrating acetic acid with a dehydrating agent such as $P_2O_5$.

Heating acetic acid with a strong dehydrating agent like phosphorus pentoxide ($P_2O_5$) removes a molecule of water from two molecules of the acid, joining them through an anhydride linkage:

$2CH_3COOH \xrightarrow[-H_2O]{P_2O_5,\ \Delta} (CH_3CO)_2O$ …