Question

How \[C{H_3}C{H_2}OH\] can be converted to $C{H_3}CHO$ :
A.catalytic hydrogenation
B.treatment with \[LiAl{H_4}\]
C.treatment with pyridinium chlorochromate
D.treatment with $KMn{O_4}$

Answer 1

Hint: Primary alcohols are oxidized to form aldehyde, whereas, secondary alcohols are oxidized to form ketones. The conversion of a hydroxyl group to aldehyde can be done by the removal of hydrogen atoms (Oxidation), looking at the following options; pyridinium chlorochromate and potassium dichromate are both oxidizing agents. However, when pyridinium chlorochromate is used, it is a weaker oxidizing agent compared to potassium permanganate; hence, it reduces ethanol to acetaldehyde (ethanal).

Complete answer:
Oxidation of alcoholic groups leads to the formation of aldehyde. Here, the \[C{H_3}C{H_2}OH\] can be oxidized to lose hydrogens and form $C{H_3}CHO$ by using pyridinium chlorochromate. The pyridinium chlorochromate is a complex of chromium trioxide with pyridine. This reagent is preferred for mild oxidation to form aldehydes and hence the oxidation to carboxylic acid is prevented.
\[C{H_3}C{H_2}OH\]$\xrightarrow{{PCC}}$$C{H_3}CHO$+ ${H_2}$

Thus, the correct option is C. treatment with pyridinium chlorochromate.

Note:
Catalytic hydrogenation is used for the synthesis of the ethanol by the addition of hydrogen to the aldehyde and Lithium aluminum hydride cannot be used as it is a reducing agent. Whereas, when potassium permanganate or potassium dichromate is used, it acts as a strong oxidizing agent, it oxidizes ethanol to aldehyde and ultimately forming ethanoic acid (a carboxylic acid). Other reagents such as collin’s reagent, Corey's reagent, chromium oxide, or copper can also be used in an anhydrous medium to convert alcohol to aldehyde, without converting into carboxylic acids.