Question

The molecular formula is C9H12. i need help detrermining its structure and labeling the spikes (chemical shifts)

I UIT CDC13 QE-300

Answer 1

The molecular formula of the unknown molecule is C9H12

Therefore, double bond equivalent is

= {(2 * 9) + 2 -12} / 2 = 4

This value indicates that the unknown molecule contains a benzene ring and this is also proved from 1H and 13 C NMR data. From the 1H NMR peak around 2.9ppm, septet for -CH group which must attached to the two methyl groups and to the benzene ring directly. And the doublet peak at around 1.20ppm for 6H of two methyl groups which is directly attached to a CH group.

Therefore, there is a isopropyl group in the benzene nucleus and the benzene nucleus is mono substituted. That is why five carbon of phenyl ring comes as a multiple in range between 7.1 - 7.3ppm.

Hence the unknown molecule is cumene / isopropyl benzene. Ph-CH(CH3)2

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