Q.(a) How is acetic acid prepared using CH3MgBr? What happens when CH3COOH reacts with C2H5OH in presence of conc. H2SO4 ? [2+2=4]

Acetic acid is made from $CH_3MgBr+CO_2$ then hydrolysis, and is esterified with ethanol to ethyl acetate. OR: aniline is diazotised to benzenediazonium chloride, which couples with phenol to give an azo dye.

(a) Acetic acid from $CH_3MgBr$:

Methylmagnesium bromide (a Grignard reagent) reacts with dry ice / gaseous carbon dioxide; the highly nucleophilic carbanion-like methyl group attacks the electrophilic carbon of $CO_2$ to form a magnesium salt of the acid, which on acid hydrolysis gives acetic acid:

$CH_3MgBr + CO_2 \rightarrow CH_3COO-MgBr \xrightarrow[H_3O^+]{\text{hydrolysis}} CH_3COOH + Mg(OH)Br$

When acetic acid reacts with ethanol in the presence of concentrated $H_2SO_4$ (acting as both catalyst and dehydrating agent), a Fischer esterification occurs, forming a fruity-smelling ester, ethyl acetate:

$CH_3COOH + C_2H_5OH \underset{\Delta}{\xrightarrow{conc.\ H_2SO_4}} CH_3COOC_2H_5 + H_2O$

(b) Two uses of acetic acid:

1. Dilute (about 5-8%) acetic acid is used as vinegar, a common food preservative and condiment.
2. It is used as a solvent and as a starting material in the manufacture of cellulose acetate (used in making photographic film, rayon), various esters, and other industrial organic chemicals.

OR — (a) Benzenediazonium chloride from aniline:

Aniline is diazotised by treating it with sodium nitrite and dilute hydrochloric acid at low temperature (273-278 K, i.e. 0-5°C, since the diazonium salt is unstable above this range):

$C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{273-278\ K} C_6H_5N_2^+Cl^- + NaCl + 2H_2O$

(b) Coupling reaction of phenol with benzenediazonium chloride: …