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Consider the following reaction sequence: CaC2 + N2 ---(1100 deg C)---> C + M M + H2SO4 ---> white precipitate + N N (at pH 7 to 9) forms a dimer, which on pyrolysis gives an aromatic compound O. If 'a' is the number of sigma bonds in O and 'b' is the number of lone pairs on all nitrogen atoms in O, find the value of a/b.

1. 1.5
2. 2.0
3. 2.5
4. 3.0

Correct answer: 2.5

Solution

The aromatic product O is melamine (C3H6N6). It has 15 sigma bonds (6 ring C-N, 3 exocyclic C-N, 6 N-H) and 6 lone pairs (one on each of the 6 nitrogen atoms), giving a/b = 15/6 = 2.5.

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