To obtain a pleasant smell of ester when heated with ethanol and a small quantity of sulphuric acid, we need to consider the reaction known as esterification. Esterification is the process of combining an alcohol (in this case, ethanol) with an acid (in this case, acetic acid) to form an ester.
Let's evaluate each option and determine which one will give a pleasant smell of ester:
A: CH₃COOH (acetic acid)
- Acetic acid reacts with ethanol to form ethyl acetate, which has a pleasant fruity smell. This option is correct.
B: CH₃CH₂OH (ethanol)
- Ethanol is one of the reactants in the esterification process, but it does not produce a pleasant smell of ester on its own.
C: CH₃OH (methanol)
- Methanol is not involved in the esterification reaction and will not produce a pleasant smell of ester.
D: CH₃CHO (acetaldehyde)
- Acetaldehyde can undergo reactions to form esters, but it does not directly react with ethanol to produce a pleasant smell of ester.
Therefore, the correct answer is option A: CH₃COOH (acetic acid).

Answer:

When ethanol and a small quantity of sulphuric acid are heated together with a compound, it undergoes an esterification reaction to produce an ester. Esters are organic compounds with a pleasant fruity smell and are commonly used as flavorings and fragrances.

Out of the given options, the compound that will give a pleasant smell of ester when heated with ethanol and sulphuric acid is acetic acid (CH3COOH).

Reason:
When acetic acid is heated with ethanol and a small quantity of sulphuric acid, it undergoes an esterification reaction to produce ethyl acetate (CH3COOCH2CH3), which is a pleasant-smelling ester. The reaction can be represented as follows:

CH3COOH + CH3CH2OH → CH3COOCH2CH3 + H2O

The reaction is catalyzed by sulphuric acid, which acts as a dehydrating agent, removing water from the reaction mixture and driving the esterification reaction forward.

Explanation:
- Acetic acid (CH3COOH) is a carboxylic acid with a sour smell.
- Ethanol (CH3CH2OH) is an alcohol with a characteristic smell.
- Sulphuric acid (H2SO4) is a strong acid that acts as a catalyst in the esterification reaction.
- When acetic acid is heated with ethanol and sulphuric acid, the carboxylic acid reacts with the alcohol to form an ester, ethyl acetate (CH3COOCH2CH3), along with the formation of water (H2O).
- Ethyl acetate is a colorless liquid with a sweet, fruity smell, reminiscent of pears or bananas. It is widely used as a solvent and as a flavoring or fragrance ingredient in various products.
- The esterification reaction between acetic acid and ethanol is an example of a condensation reaction, where a molecule of water is eliminated during the formation of the ester.
- The presence of sulphuric acid as a catalyst helps in the removal of water, thereby shifting the equilibrium towards the formation of the ester.
- Hence, when acetic acid is heated with ethanol and sulphuric acid, it produces ethyl acetate, which gives a pleasant smell of ester.