Polymerization of HCHO and CH₃CHO occurs in the presence of?

Explanation

Aldehydes like formaldehyde (HCHO) and acetaldehyde (CH₃CHO) undergo polymerization in the presence of acidic catalysts.

For the options provided, the correct catalyst is Conc. H₂SO₄. 

Polymerization process for these two aldehydes:

Formaldehyde (HCHO)

With acid catalysts:

• In the presence of an acid such as concentrated sulfuric acid, formaldehyde can polymerize to form cyclic trimers and tetramers.
• The most common product is 1,3,5-trioxane, a cyclic trimer.

General polymerization:

• Formaldehyde readily polymerizes even in neutral or basic conditions to form paraformaldehyde, a straight-chain polymer. 
• Acetaldehyde (CH₃CHO)

With acid catalysts:

• Acetaldehyde, when treated with strong acids like concentrated sulfuric acid, trimerizes to form paraldehyde, a cyclic trimer.
• At lower temperatures, it can also form the cyclic tetramer, metaldehyde.

With acid/base catalysts:

• Acetaldehyde also undergoes aldol polymerization, which can be initiated by both acidic and basic catalysts to form different products. 

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