Question

For each of the following pairs of compounds, Identify one IR absorption band that could be used to distinguish between them.

Answer 1

1. Ester Carbonyl gives peak at 1745 cm-1 while ketone gives at 1710 cm-1.

2. Three peaks near 3000 corresponds to CH stretch for alkane while in cycloalkane there are two only.

3. Alcohol has OH stretch at 3400 while in ether there is no OH stretch.

4. Amide carbobyl appears at 1630-1660 and ester Carbonyl group appears at 1744 cm-1.

5. Priamary alcohol has C-O stretch at 1085 while secondary alcohol has at 1125 cm-1.

6. Trans alkene is not detected by IR since there is no change in dipole moment of trans alkene. Only cis alkene is appears at 1650 for C=C stretch

7. Ester Carbonyl group appears at 1745 and ketone appears at 1710.

8. Unsaturated ketone appears at 1700 while unconjugated ketone at 1715

Conjugation decreases the absorbance.

9. Alkene C=C appears at 1650 and alkyne triple bond at 2150

10. Aldehyde C=O group appears at 1725 and CH stretch at 2869&2740. While ketone gives at 1705 .

11. Conjugate aldehyde gives peak at 1710 and unconjugated aldehyde gives at 1725 cm-1. Conjugation decreases the IR values

12. First alkene has C=CH stretch at 1660 while second alkene(trisubstituted alkene) has C =C weak stretch at 1630.