See image — Alcohols Phenols and Ethers Chemistry Question

💡 Solution & Explanation

Concept: An aldehyde reacts with an alcohol in the presence of an acid catalyst to form an acetal (or hemiacetal, then acetal). The transformation shown converts butanal (an aldehyde, RCHO) into its dimethyl acetal (RCH(OCH3)2), where both hydrogen atoms on the carbonyl oxygen are replaced by two methoxy groups. Step 1 - Identify the transformation: The starting material is butanal (H3C-CH2-CH2-CHO) and the product is the corresponding dimethyl acetal (H3C-CH2-CH2-CH(OCH3)2). This is an acetal formation reaction. Step 2 - Mechanism of acetal formation: Under acid catalysis (H+), the aldehyde is protonated to form an oxocarbenium ion. Methanol (CH3OH) acts as the nucleophile, attacking the electrophilic carbonyl carbon. After loss of water, a hemiacetal is formed, and a second methanol molecule attacks to give the dimethyl acetal after loss of another proton. The reaction is typically carried out at room temperature or mild conditions; 'heat' in option (a) refers to the acid-catalyzed conditions promoting the reaction (though excess alcohol and removal of water drives equilibrium toward acetal). Step 3 - Evaluate option (a): CH3OH with H+ (cat.) is precisely the reagent system required for acetal formation from an aldehyde. This is the standard, well-known acetal synthesis condition. This is correct. Step 4 - Evaluate option (b): H2O with H+ (cat.) and heat would hydrolyze an acetal back to an aldehyde (the reverse reaction), or simply protonate the aldehyde without forming an acetal. Water does not install OCH3 groups. This is incorrect. Step 5 - Evaluate option (c): Mg in ether with CH3OH is used for Grignard-related reactions or dissolving metal reductions, not for acetal formation. This does not produce a dimethyl acetal. This is incorrect. Step 6 - Evaluate option (d): SOCl2 with CH3OH converts carboxylic acids to acid chlorides or esters, not aldehydes to acetals. This is incorrect. Therefore, the correct answer is A.