VERY tricky question. First since you thought it was B lets look at the chemistry involved with carbonyl's for a minute:
when reacted w/ grignards ketones and aldehydes yield alcohols. carboxylic acids and their deravitives (that includes esters) yield carbonyl's. its a question of nucleophilic addition (aldehydes and ketones) versus nucleophilic acyl substitution (carboxylic acids and derivatives).
You need to know the fundamental reaction mechanisms cold, especially for carbonyl compounds . If you have McMurray read the "brief review of organic reactions" and "preview of carbonyl compounds" sections. if not google the following terms for a review or watch chads videos:
ecectrophilic additions
nucleophilic additions
eliminations
nucleophylic acyl substituion
electrophilic aromatic substitution
nucleophilic aromatic substitution
radical reactions (uv or roor initiation)
pericyclic (diels-alder)
specific to carbonyl's:
nucleophilic addn of aldehydes and ketones
nucleophilic acyl substitution of carboxylic acid deravitives
alpha substitution
carbonyl condensations (aldol condensation, malonic ester synth and the like).
you should also know the tetrahedryl intermediates formed cold
So anyway, back to the question you posted. Let's reason it out. Because its a carboxylic acid you would NORMALLY go acyl substitution. the nucleophile attacks the carbonyl carbon, the elections from the double bond feed up to oxygen to form the tetrahedryl intermediate, electrons feed back down to reform the double bond (C=O) and the leaving group (in this case OH) leaves so you should expect that the answer is A.
However, remember that a grignard is basically a magnesium salt of a hydrocarbon acid: R-CH2 MgX, therefore the carbon on the R-C carries an negative charge which carbon really doesnt like. Since the carboxylic acid has a pretty small pKa there are quite a few positively charged H's around in this reaction. negatively chared carbon reacting with a positive charge hydrogen will have a much lower activation energy than the acyl substitution so you get C2H6 and the acetate ion. the addition of H3O+ reprotonates the acetate ion, so the correct answer is C.
Quick way to pick up on this is that addition of the H3O+ reagent. Anytime you see that as a final step in the reagent sequence think of reprotonation. There are two other types of trick question in this vein:
One is the formation of alcohols in addition reactions. They often require H3O+ to protonate the alkoxide ion. The question will leave H3O+ out of the reagent sequence and give the alkoxide ion as one of the choices but we happily chose the alcohol and go on our merry way thinking to ourselves how easy that question was.
The other is pretty much along the same vein but involves the tetrahedryl intermediate. First as it being either the ion or being protonated and second the question asks for the intermediate, we don't reac the question, look at the reaction anc chose the product.
I dont know how many of these "trick" questions show up on the actual DAT (I take it tomorrow) but they are pretty common on the ACS exams.
