= Solution
\b[A: <aldehyde> <oxygen> is the best acceptor; there is no conventional strong O–H or N–H donor.] The N-methylpyrrole <nitrogen> contributes its <lone pair> to aromatic bonding and is not an ordinary <amine> acceptor. A formyl C–H or ring C–H can participate in weak contacts, but is not comparable to the donors in B and C.
\b[B: <pyridine> <nitrogen> is the best acceptor and the phenolic OH is the best donor.] Its <oxygen> can accept more weakly, but <nitrogen> provides the primary basic site in the depicted hydroxypyridine form.
\b[C: lactam <carbonyl> <oxygen> is the best acceptor and the <amide> N–H is the donor.] <amide> <nitrogen> is not a useful acceptor because its <lone pair> is delocalized toward the <carbonyl>.
\b[D: the secondary <amine> <nitrogen> is the best acceptor, and its N–H is the only conventional donor, but a weak one here.] Its <ketone> <oxygen> is also an acceptor, although weaker than the <amine> site in this comparison. These assignments distinguish functional-group identity and lone-pair availability, rather than counting heteroatoms.
Back to article page
