Q.(a) An organic compound (A) of molecular formula $C_6H_6O$, gives violet colour with neutral ferric chloride. Compound (A) when refluxed with $CHCl_3$ and NaOH gives two isomers (B) and (C). Compound (A) when added to diazomethane in alkaline medium gives an ether (D). Identify (A), (B), (C) and (D). Explain the reactions.

(a) Phenol identification via the Reimer–Tiemann reaction (dichlorocarbene) and O-methylation by diazomethane. (b) Potassium dichromate identification via the chromyl-chloride test and its chromate/dichromate interconversion with alkali. (c) Acetaldehyde prepared from acetonitrile by Stephen reduction, identified by the Tollens' test and trimerisation to paraldehyde. (d) A Kohlrausch's-law calculation of equivalent and molar conductance of $Al_2(SO_4)_3$ at infinite dilution from the given ionic equivalent conductances.

(a) Identification of A, B, C, D

(A): molecular formula $C_6H_6O$ and a violet colouration with neutral ferric chloride is the classic identification test for a phenol — so (A) = phenol, $C_6H_5OH$.

(A) + $CHCl_3$/NaOH, reflux $\to$ (B) and (C): this is the Reimer–Tiemann reaction. NaOH first deprotonates phenol to the phenoxide ion. Separately, NaOH abstracts a proton from $CHCl_3$ and the resulting trichloromethyl anion undergoes $\alpha$-elimination of $Cl^-$ to generate the reactive electrophile dichlorocarbene, $:CCl_2$. This carbene attacks the electron-rich phenoxide ring (predominantly at the ortho position, with some para attack), giving a dichloromethyl-substituted intermediate that is hydrolysed under the basic reaction conditions (and on subsequent acidification) to the aldehyde. This gives a mixture of two isomeric hydroxybenzaldehydes:

• (B) = salicylaldehyde (2-hydroxybenzaldehyde) — the major product (ortho attack is favoured, assisted by intramolecular coordination/direction of the phenoxide oxygen).
• (C) = 4-hydroxybenzaldehyde (p-hydroxybenzaldehyde) — the minor product (para attack).

$C_6H_5OH \xrightarrow[\text{(ii) H}_3O^+]{\text{(i) CHCl}_3,\ NaOH} \text{o-HOC}_6H_4CHO\ (\text{major}) + \text{p-HOC}_6H_4CHO\ (\text{minor})$

(A) + diazomethane (in alkaline medium) $\to$ (D): phenol's acidic $-OH$ proton is removed and the phenoxide oxygen is methylated by diazomethane ($CH_2N_2$), which decomposes releasing $N_2$ gas, to give the methyl ether:

$C_6H_5OH + CH_2N_2 \rightarrow C_6H_5OCH_3\ (\textbf{D, anisole}) + N_2\uparrow$

(b) Identification of A, B, C

(A): an orange-red crystalline solid that is a powerful oxidising agent is potassium dichromate, $K_2Cr_2O_7$.

(A) + KCl + conc. $H_2SO_4$, heated $\to$ (B): this is the standard chromyl chloride test. Potassium dichromate reacts with a chloride salt and concentrated sulphuric acid to liberate chromyl chloride, $CrO_2Cl_2$, a deep orange-red fuming liquid/vapour:

$K_2Cr_2O_7 + 4KCl + 6H_2SO_4 \xrightarrow{\Delta} 2CrO_2Cl_2\ (\textbf{B}) + 6KHSO_4 + 3H_2O$

(A) + KOH $\to$ (C): dichromate ($Cr_2O_7^{2-}$, orange, stable in acidic medium) is converted by alkali to chromate ($CrO_4^{2-}$, yellow, stable in basic medium) — the well-known pH-dependent chromate–dichromate equilibrium:

$K_2Cr_2O_7 + 2KOH \rightarrow 2K_2CrO_4\ (\textbf{C, yellow}) + H_2O$

(c) Identification of A, B, C

(B): molecular formula $C_2H_3N$ = acetonitrile, $CH_3CN$.

(B) + $SnCl_2$/HCl (dry ether) $\to$ (A): this is the Stephen reduction. Acetonitrile is reduced by $SnCl_2$/HCl in dry ether to an imine (aldimine) hydrochloride intermediate, which hydrolyses on treatment with water to the aldehyde:

$CH_3CN \xrightarrow{SnCl_2/HCl} CH_3CH{=}NH{\cdot}HCl \xrightarrow{H_2O} CH_3CHO\ (\textbf{A, acetaldehyde}, C_2H_4O) + NH_4Cl$

(A) reduces Tollens' reagent: acetaldehyde, being an aldehyde, reduces ammoniacal silver nitrate (Tollens' reagent) to metallic silver (silver-mirror test), while itself being oxidised to acetate:

$CH_3CHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow CH_3COO^- + 2Ag\downarrow + 4NH_3 + 2H_2O$

(A) + a drop of conc. $H_2SO_4$ $\to$ (C): acetaldehyde undergoes acid-catalysed cyclic trimerisation to give paraldehyde (2,4,6-trimethyl-1,3,5-trioxane), a stable six-membered cyclic triether:

$3\,CH_3CHO \xrightarrow{H_2SO_4\ (cold)} (CH_3CHO)_3\ (\textbf{C, paraldehyde})$ …