Question

1. Where do the typical IR absorption come for the following functional groups.

            Alcohol

            Alkyl

            Alkene

            Alkyne

            Aromatics

            Carboxylic acids

2.

Where do the typical IR absorption comes for amines?

Primary amines

Secondary amines

Tertiary amines

Answer 1

Infrared spectroscopy is used to identify the functional groups present in the given compounds.

For a given functional group the IR absorption bands are specific.

For example for the given functional groups

1.Alcohol will show a characteristic broad peak around 3300-3500 cm^-1 in the IR spectrum. This peak is due to the O-H stretching.

2. Alkyl groups main characteristic peak is the presence of two bands in the region 2950-2850 cm^-1 in the IR spectrum. These peaks are due to the C-H stretching.

3.  Alkene functionality main characteristic peak is the presence of a band in the region 3050-3000 cm^-1 in the IR spectrum. These peaks are due to the C=C-H stretching.

4. Alkyne functionality main characteristic peak is the presence of a band in the region 3150-3050 cm^-1 in the IR spectrum. These peaks are due to the C (triple bond) C-H stretching. C (triple bond) C stretching peak is also seen in the region 2200-2300 cm^-1.

5. The presence of Aromatic group can be identified by a peak around 3050 cm^-1 which is due to aromatic C=C-H stretching. Additionally, for aromatic C=C stretching, there will be 4 peaks in the region 1600-1400 cm^-1 which are due to C=C stretching of aromatics.

6. Carboxylic acids absorption of O-H stretching appears as a broad band around 3000-2500 cm^-1. The C=O stretching absorption peak can be seen around 1700-1725 cm^-1.

2) The main characteristic peaks of IR of amine is the presence of broad peaks around 3400 and 3300 cm^-1 due to N-H stretching.

For primary amines there will be two broad peaks in the above said region (3400 and 3300 cm^-1).

For secondary amines only one broad peak will be present in that region, whereas tertiary amines does not show any peak in the region 3400 and 3300 cm^-1 as there is no N-H bond present in tertiary amines.