Question

The following partial IR spectra correspond to a primary amine, a secondary amine, and an alcohol. Give the functional group for each spectrum.

Short answer

1^st spectrum = Secondary amine

2^nd structure = Primary amine

3^rd structure = Alcohol

Step-by-step solution

Theory of Infrared (IR) spectrum

Infrared or IR Spectrum refers to a plot of Percentage Transmittance versus the stretching frequency of the bonds (in cm^-1) . The vibrational stretching frequency for any polar bond depends on the Hooke’s Law. The equation for Hooke’s Law is as follows:

Detailed Explanation for answer of  the 1st Spectrum 

The IR stretching frequency value for N-H bond is sharp peak seen at around 3300 cm^-1 . The general formula for secondary amine is R₂NH. Since there is one N-H bond in secondary amine, sharp singlet is seen at 3300 cm^-1.

Hence, the functional group is secondary amine

Detailed Explanation for answer of  the 2nd Spectrum 

The IR stretching frequency value for N-H bond is sharp peak seen at around 3300 cm^-1. The general formula for primary amine is RNH₂ . Since there are two N-H bonds in primary amine, sharp doublet is seen at 3300 cm^-1.

Hence, the functional group is primary amine.

 Detailed Explanation for answer of  the 3rd Spectrum

The IR stretching frequency value for O-H bond is broad peak seen at around 3300 cm^-1. The broad peak is seen in the spectrum at 3300 cm^-1 .

Hence, the functional group is alcohol.