Question

$\bf{P}$ (C$_7$H$_{16}$) $\xrightarrow[\text{v) H}_3\text{O}^+]{\text{i) Cr}_2\text{O}_3\text{, 770 K, 20 atm} \\ \text{ii) Cl}_2\text{/ hv (3 eq.)} \\ \text{iii) H}_2\text{O} \\ \text{iv) LiAlH}_4}$ $\bf{Q}$ $\bf{R}$ (major)

Answer 1

Benzaldehyde

Answer 2

The reaction sequence starts with an alkane P with the formula C$_7$H$_{16}$.

Step i) P $\xrightarrow{\text{Cr}_2\text{O}_3\text{, 770 K, 20 atm}}$ Aromatization. This converts an alkane into an aromatic hydrocarbon. For a C$_7$ alkane, the major aromatic product is toluene (methylbenzene), C$_7$H$_8$. This reaction involves cyclization and dehydrogenation. For example, n-heptane undergoes aromatization to toluene.

P (C$_7$H$_{16}$) $\rightarrow$ Toluene (C$_7$H$_8$)

Step ii) Toluene $\xrightarrow{\text{Cl}_2\text{/ hv (3 eq.)}}$ Free radical halogenation at the benzylic position. With 3 equivalents of Cl$_2$ and UV light, all three hydrogen atoms of the methyl group are substituted by chlorine atoms.

Toluene $\rightarrow$ Trichloromethylbenzene (C$_6$H$_5$CCl$_3$)

Step iii) Trichloromethylbenzene $\xrightarrow{\text{H}_2\text{O}}$ Hydrolysis of the geminal trichloride. This yields a carboxylic acid.

C$_6$H$_5$CCl$_3$ + 3 H$_2$O $\rightarrow$ C$_6$H$_5$C(OH)$_3$ + 3 HCl $\rightarrow$ C$_6$H$_5$COOH + H$_2$O

The product is benzoic acid (C$_6$H$_5$COOH).

Step iv) Benzoic acid $\xrightarrow{\text{LiAlH}_4}$ Reduction of the carboxylic acid to a primary alcohol.

C$_6$H$_5$COOH $\xrightarrow{\text{LiAlH}_4}$ C$_6$H$_5$CH$_2$OH

The product is benzyl alcohol (C$_6$H$_5$CH$_2$OH).

Step v) Benzyl alcohol $\xrightarrow{\text{H}_3\text{O}^+}$ Acidic workup after LiAlH$_4$ reduction. This step protonates the alkoxide intermediate to yield the alcohol. So, the product remains benzyl alcohol.

Thus, Q is benzyl alcohol (C$_6$H$_5$CH$_2$OH).

The reaction from Q to R uses pyridinium chlorochromate (PCC), which is shown in the figure. PCC is a mild oxidizing agent that oxidizes primary alcohols to aldehydes and secondary alcohols to ketones. Benzyl alcohol is a primary alcohol.

Q (Benzyl alcohol) $\xrightarrow{\text{PCC}}$ R (major)

C$_6$H$_5$CH$_2$OH $\xrightarrow{\text{PCC}}$ C$_6$H$_5$CHO

The major product R is benzaldehyde (C$_6$H$_5$CHO).

The reaction sequence involves aromatization of an alkane P to toluene, followed by free radical chlorination of toluene to trichloromethylbenzene. Hydrolysis of trichloromethylbenzene yields benzoic acid, which is then reduced by LiAlH$_4$ to benzyl alcohol (Q). Finally, oxidation of benzyl alcohol (Q) with PCC gives benzaldehyde (R).