Q.(i) Give structures of A, B and C in the following reactions : a benzene ring bearing a Br substituent and a NO2 substituent reacts with Mg in ether to give A, A reacts with CO2/dry ice to give B, B on treatment with H3O+ gives C. (structure drawn: benzene ring --Br, Mg/ether--> A --CO2, dry ice--> B --H3O+--> C, ring also carries a NO2 substituent) [3 marks]

Water protonates both Grignard reagents, and since both carry a propyl group, each gives propane.

Cyclohexyl bromide reacts with magnesium in dry ether to form a Grignard reagent, which is then hydrolysed by water to give the parent hydrocarbon.

Step 1: Cyclohexyl bromide reacts with magnesium metal in dry ether (an aprotic solvent, essential since Grignard reagents react violently with any trace of water or protic solvent):

C6H11-Br + Mg --(dry ether)--> C6H11-MgBr (X, cyclohexylmagnesium bromide, the Grignard reagent)

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A Grignard reagent is formed by reacting an alkyl (or aryl) halide with magnesium metal in dry ether; CH3MgBr is a simple example.

CH3-Br + Mg --(dry ether)--> CH3-MgBr

CH3MgBr (methylmagnesium bromide) is a Grignard reagent - a general class of organomagnesium halides RMgX widely used to form new C-C bonds by reacting with carbonyl compounds, CO2, etc. Other equally valid exampl …

The common name of an alkyl magnesium halide ($\text{RMgX}$) is Grignard reagent.

Because propanoic acid has one more carbon than ethyl chloride, the chain is first extended via a nitrile (using KCN) and the nitrile is then hydrolysed to the carboxylic acid.

Step 1 — Nucleophilic substitution to form the nitrile

Ethyl chloride reacts with $KCN$ (in ethanolic solution) via an $S_N2$ mechanism, in which $CN^-$ displaces $Cl^-$, attaching through carbon (since $KCN$ is ionic and the carbon end of $CN^-$ is the more nucleophilic site):

$CH_3-CH_2-Cl + KCN \xrightarrow{\text{ethanol}} CH_3-CH_2-C\equiv N + KCl$

This step is essential because it is the only step here that adds a carbon atom to the chain — direct hydrolysis of the C–Cl bond alone (giving ethanol) could never yield a 3-carbon acid.

Step 2 — Hydrolysis of the nitrile to the carboxylic acid

The nitrile (propanenitrile) is then hydrolysed under acidic (or alkaline) aqueous conditions with heating; the reaction proceeds through an amide intermediate to the carboxylic acid:

$CH_3CH_2C\equiv N + 2H_2O \xrightarrow{H_3O^+,\ \Delta} CH_3CH_2COOH + NH_4^+$

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Grignard reagents react instantly with the acidic H of water to give the parent hydrocarbon — here, PhMgBr + H₂O gives benzene.

Grignard reagents ($R\text{-}MgX$), prepared by reacting a haloalkane/haloarene with magnesium in dry ether, are extremely reactive towards any source of even mildly acidic hydrogen, because the carbon attached to Mg is highly nucleophilic/basic (behaves like a carbanion, $R^{\delta-}\text{-}Mg^{\delta+}X$).

Water readily protonates this carbanion-like carbon:

$C_6H_5MgBr + H_2O \rightarrow C_6H_6 + Mg(OH)Br$

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A Grignard reagent, R-Mg-X, is formed when magnesium metal inserts into the carbon-halogen bond of an alkyl/aryl halide, creating a carbon-magnesium organometallic bond.

A Grignard reagent is prepared by reacting an alkyl or aryl halide (R-X) with magnesium turnings in dry ether:

R-X + Mg -> R-Mg-X …