Question

What spectral features including mass spectra, IR spectra, proton spectra and carbon spectra allow you to differentiate the product (Benzonitrile) from the starting material (Benzoic Acid)?

Answer 1

Benzoic acid is a carboxylic acid because of the presence of a carboxyl group. Benzonitrile is a hydrogen cyanide compound but the hydrogen atom is replaced by a phenyl group.

In NMR spectra, the OH band should be analyzed on the 1H NMR. In 13C NMR, double bond and triple bond should be analyzed.

The fragmentation is occurred between a phenyl ring and the carbon. The mass spectra fragmentation for benzonitrile is 26 and for benzoic acid, it is 45.

In IR spectra, the range should be 2230 – 2250 cm^-1 due to the presence of a triple bond between carbon and nitrogen in benzonitrile. The value for carboxyl group of benzoic acid is 1700 cm^-1 but this spectra is destroyed by the C-C bonds in the ring.

In IR spectra, the O-H peak for benzoic acid is around 3000 cm^-1. The absence of O-H peak in benzonitrile is required.

The above mentioned methods are used to differentiate the benzonitrile and benzoic acid.