Answer to Question #114489 in Organic Chemistry for Pawan

Question

Differentiate between HC≡CH and CH3CH2CH2CH2CH≡CH using IR spectroscopy. Give reason also.

Answer 1

The IR spectrum for acetylene is

C-H stretch has two peaks:

symmetric and asymmetric stretching (3300 and 3330 cm^-1)

The other two lower peaks are C-H bending.

Note the the stretch $C\equiv C$ (at 2126 cm^-1) is not seen in ethyne (IR inactive).

The IR spectrum for heptyne-1

The C-H stretch of the C-H bond adjacent to the $C\equiv C$ bond occurs at (3314 cm^-1), the $C\equiv C$ bond stretch (2126 cm^-1) and the C-H bend of the C-H bond adjacent to the $C\equiv C$ bond (637 cm^-1).