Question

A compound has an M+ ion at 158 amu. The intensity of the M+1 peak is 9.9% of the M+ peak. There is no significant M+2 peak. The only IR absorbances in the interpretable region are at 2850 and 1740 cm−1. Draw a structure consistent with this information.

Answer 1

From mass data, there is no significant M+2 peak means compound does not has any Cl or Br atom. Further, No peak above 3000 cm-1 indicates that molecule is saturated (no aromatic ring or alkene or alkyne).  IR absorbances in the interpretable region at 2850 and 1740 cm−1 corresponds to the presence of aliphatic C-H and ester carbonyl group. By this way it is clear that molecule must has two oxygen atoms (required for ester functionality).

The intensity of the M+1 peak is 9.9% of the M+ peak and this will provide the number of C atoms in the molecule as:

number of carbon atoms = intensity of [M+1] peak / 1.1

= 9.9 / 1.1

= 9 carbon atoms

Now, the remaning mass = 158 - (12 x 9)

= 158 - 108

= 50 amu

After substrcting the mass of two oxygen atoms, remaining mass = 50 - (2 x 16) = 18 amu

This remaining mass will belong to H atoms as number of H atom = 18 / 1.01 = 17.8

or can say 18 H atoms

Now, the chemical formula can be given as: C9H18O2

The possible structure would be: