What happens when aldehydes are treated with alcohols in the presence of anhydrous acids? 

Acetals are formed 

The correct answer is option 2. Acetals are formed.

When aldehydes are treated with alcohols in the presence of anhydrous acids, the primary products are acetals.

Formation of Hemiacetals: Initially, the aldehyde reacts with one molecule of alcohol to form a hemiacetal. This is a reversible process.

\(\text{Aldehyde} + \text{Alcohol} \overset{\text{H}^+}{\longrightarrow} \text{Hemiacetal}\)

Formation of Acetals: In the presence of anhydrous acid (which acts as a catalyst), the hemiacetal can react with another molecule of alcohol to form an acetal. This involves the loss of water (a dehydration step).

\(\text{Hemiacetal} + \text{Alcohol} \overset{\text{H}^+}{\longrightarrow} \text{Acetal} + \text{H}_2\text{O} \)

The overall reaction can be represented as:

Why Acetals are Formed:

Anhydrous Acid Catalyst: The anhydrous acid (such as HCl, H₂SO₄, or p-toluenesulfonic acid) protonates the oxygen in the aldehyde carbonyl group, making it more electrophilic and facilitating the nucleophilic attack by the alcohol.

Dehydration Step: The removal of water drives the reaction towards the formation of the acetal. This dehydration step is crucial and is promoted by the anhydrous acid catalyst.

Summary:

When aldehydes are treated with alcohols in the presence of anhydrous acids, acetals are the primary products due to the catalytic action of the acid facilitating both the formation of hemiacetals and their subsequent conversion to acetals.