Sum of rings and pi bonds

Sum of number of rings and p-bonds in a molecule

The general formula for an acyclic saturated hydrocarbon is C_NH_2N+2. Hexane is C₆H₁₄. The introduction of a p-bond or formation of a ring requires the loss of a pair of H's (the number of H's decreases by 2; one pairs of H's is lost). Thus 1-hexene and cyclohexane are C₆H₁₂. If a molecule contains only C and H or C, H and O, the general equation for F, the sum of the number of rings and p-bonds, is given by...

for C_NH_MO_R F = (2N + 2 - M)/2 Few examples are:

The presence of O requires no correction because O doesn't alter the maximum number (2N + 2) of H's that can be carried by N C's.

In the case of molecules with halogen or nitrogen, the F equation becomes a bit more complicated. This is because the presence of X or N alters the maximum number of H's the structure can carry from the usual 2N + 2 value. For example, CH₃-Br and CH₃-NH₂ have 3 and 5 H's, respectively, and neither molecule contains a p-bond or ring.

for C_NH_MO_RX_QN_S F = (2N + 2 + S - Q - M)/2 The maximum number of H's this molecule can have is 2N + 2 + S - Q.

A few examples...

C₈H₈NBr
F = (2 x 8 + 2 + 1 - 1 - 8) = 10/2 = 5
F = 5; there are 4 p-bonds and 1 ring

C₆H₁₀N₂O
F = (2 x 6 + 2 + 2 - 10) = 6/2 = 3
F = 3; there obviously are a ring and a p-bond. Where's the other p-bond?

Here's where it is, in the nitro group. So the molecule contains one ring and two p-bonds.

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