The main mechanism of aldehydes and ketones is:

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The question asks about the main mechanism of aldehydes and ketones and provides several candidate mechanistic categories to assess. 
Option 1: nucleophilic acyl substitution. This is a mechanism typical of carboxylic acid derivatives like acid chlorides or esters reacting with nucleophiles under base or acid catalysis. Aldehydes and ketones do not generally undergo nucleophilic acyl substitution because they lack a good leaving group on the carbonyl carbon. Thus this is not the primary pathway for aldehydes/ketones. 
Option 2: electrophilic aromatic substitution. This mechanism is characteristic of reactions that activate an aromatic ring toward attack by an electrophile (like nitration, sulfonation). Aldehydes and ketones themselves are not functioning as electrophilic aromatic substitution partners in their common carbonyl chemistry, so this option mislabels their typical reactivity. 
Option 3: electrophilic acyl substitution. This would imply the carbonyl carbon is attacked in a way that leads to substitution at the acyl group, often seen with activated acyl derivatives undergoing substitution. Again, aldehydes and ketones do not undergo acyl substitution in their standard chemistry because there is no suitable leaving group attached to the carbonyl carbon to be displaced in a single step; their reactivity is not described by acyl substitution. 
Option 4: electrophilic acyl addition. This describes the classic addition to a carbonyl group, such as nucleophiles adding to the electrophilic carbonyl carbon to form a tetrahedral, covalent intermediate. This is indeed a fundamental mechanism for many carbonyl reactions (e.g., nucleophilic additions to aldehydes/ketones). However, the phrasing here as “electrophilic acyl addition” is unusual; aldehydes and ketones are not themselves performing electrophilic addition as the primary mechanism in isolation—rather, nucleophiles add to the electrophilic carbonyl carbon. In standard nomenclature, this is typically described as nucleophilic addition to a carbonyl, not as an electrophilic acyl addition. This makes the option linguistically and conceptually awkward, and not the canonical description of aldehyde/ketone chemistry. 
Option 5: bimolecular nucleophilic substitution (S_N2). This mechanism involves backside attack with displacement and is characteristic of certain alkyl halides or sulfonates, not carbonyl chemistry of aldehydes and ketones. Carbonyl compounds do not undergo SN2 at the carbonyl carbon. 
Option 6: none of these. Given the above, the standard primary mechanisms for aldehydes and ketones include nucleophilic addition to the carbonyl and related tautomerization or oxidation/reduction chemistry, not the listed substitutions or aromatic substitutions. None of Options 1–5 accurately captures the canonical main mechanism in a standard, general description of aldehyde/ketone carbonyl chemistry. 
Therefore, none of the presented mechanisms correctly describe the main pathway for aldehydes and ketones in typical contexts, and the option that would reflect that reality is the final “none of these” choice.

CHE 008B A01 SS1 2025 Lecture 7 - Carbonyls as Electrophiles

The main mechanism of aldehydes and ketones is:

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