Pyrimidine (pKaH = 1.3) is a much weaker base than pyridine (pKaH = 5.2) because

Apyrimidine is not aromatic
Bthe second ring nitrogen withdraws electron density from the nitrogen being protonated
Cpyrimidine has no lone pair on either nitrogen
Dthe second nitrogen donates its lone pair into the ring, increasing electron density

Answer: B

Pyrimidine is 1,3-diazine. Each nitrogen is electronegative and withdraws electron density inductively (and by the ring pi system) from the other, so the lone pair of either N is less available than in pyridine. Both lone pairs are still in-plane sp

^{2}

orbitals.

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Pyrimidine (pKaH = 1.3) is a much weaker base than pyridine (pKaH = 5.2) because

Answer: B

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