Question

4. Examine the IR below and classify the compound. A. Alcohol B. Aldehyde C. Amine D. Ketone

forget the previous one please answer me this one.

1. Indicate how you would use spectroscopy in the IR to distinguish between the following pairs of constitutional isomers. in the second column of the table indicate Band position (cm-1) and Explanation

Answer 1

So in each pair of isomers, I'll call the first molecule I second II

1) The former (I) have just sp3 C-H bonds whereas the latter (II) has an sp C-H bond. So molecule II will have a characteristic strong band at around 3300 cm-1  whereas I doesn't have such a band.

2) In I, the carbonyl group is conjugated with the adjacent double bond, whereas in II it is not. Hence II will have the characteristic C=O stretching frequency of around 1700-1725 cm-1 whereas I will have a frequency 25-45 cm-1 lower than this value.

3) I is an ether and II is an aldehyde. I gives a strong band around 1300–1000 cm-1 and II gives a band around

1720-1740 cm-1.

4) I is an ether and II is an alcohol. I gives a strong band around 1300–1000 cm-1 and II gives a broad and large band around 3600 cm-1.

5) In I, the carbonyl group is that of an amide and in II it is a ketone, I will have a lower C=O stretching frequency than II, as the Nitrogen in amide donate lone pair to C=O group, making it weaker, and hence lowering C=O stretching frequency. I will have a band in the range 16400-1700 cm-1 and II around 1700-1725 cm-1.