Question

Conversion of ethyl alcohol into acetaldehyde is an example of ?
A. hydrolysis
B. oxidation
C. reduction
D. molecular rearrangement.

Answer 1

Hint: In order to answer this question, you need to think about how aldehydes are prepared from alcohols. Aldehydes have the $CHO$ group, whereas, the alcohols have an $OH$ functional group.

Complete step by step answer:
In order to answer this question, we need to learn how reactions occur in aldehydes and ketones and structure of carbonyl groups. In order to solve this question, we need to learn about aldol condensation reactions. Carbon atom of the carbonyl group is $s{{p}^{2}}$ hybridised having triangular planar geometry. Carbon atoms form three sigma bonds and one n(pi) bond, out of which 3 sigma bonds are located in the same plane whereas the fourth it bond, which is formed by lateral or sideways overlapping, is situated above and below the plane. Bond angles are approximately ${{120}^{0}}$.Carbonyl group is polarised due to difference in electronegativity between carbon and oxygen. Oxygen being more electronegative pulls the shared pair of electrons more towards itself making oxygen as a nucleophilic centre and carbon as an electrophilic centre. Hence carbonyl compounds have substantial dipole moments and their polarity can be expressed on the basis of resonance.
Let us see the mechanism of PCC oxidation, we have used propanol instead of ethanol for better clarity:

The formula of ethanol is $C{{H}_{3}}C{{H}_{2}}OH$ and the formula of acetaldehyde is $C{{H}_{3}}CHO$. From the formula, we can see that ethanol is getting oxidised to form the aldehyde. So, we can represent the reaction as:
\[C{{H}_{3}}C{{H}_{2}}OH\xrightarrow{PCC}C{{H}_{3}}CHO(oxidation)\]
Here, the oxidising agent used in pyridinium chlorochromate. So, as it is getting oxidised,
So, the correct answer is “Option B”.

Note: Aldehydes formed are prone to further oxidation and form carboxylic acid. To prevent this aldehydes are distilled off as soon as possible. Whereas ketones cannot be oxidised further easily, hence their yield is quite high in the reaction.