Oxidative cleavage of β-carotene's conjugated double bonds yields specific carbonyl products

Steps:

• β-Carotene has 11 conjugated C=C bonds in its symmetric structure with two ionone rings and a polyene chain.
• Oxidation (e.g., ozonolysis) cleaves all C=C bonds, producing small carbonyl fragments: aldehydes from unsubstituted carbons and ketones from methyl-substituted carbons.
• The chain's 9 double bonds include 6 methyl-substituted ones (-C(CH3)=CH- or -CH=C(CH3)-), each yielding one ketone (e.g., acetone, methyl ketones).
• Ring double bonds contribute 2 more ketone-containing fragments (e.g., from β-ionone cleavage to levulinic acid derivatives), totaling 6 distinct keto products; others are aldehydes like formaldehyde or glyoxal.

Why B is correct:

• β-Carotene's structure has exactly 6 methyl-bearing double bonds in the chain plus ring contributions, per standard organic chemistry cleavage rules, yielding 6 unique ketones.

Why the others are wrong:

• A: Underestimates chain methyl substitutions (only 4 would ignore ring and central fragments).
• C: Overcounts by including multi-cleavage variants or non-keto carbonyls.
• D: Assumes all 11 cleavages produce ketones, ignoring unsubstituted bonds yielding aldehydes.

Final answer: B