Infrared Spectroscopy 2

Infrared Spectroscopy 2
IR Spectroscopy of Hydrocarbons
Hydrocarbons contain only carbon-carbon bonds and carbon-hydrogen bonds.

Carbon-Carbon Bond Stretching

C - C single bond absorptions (and most other absorptions in the fingerprint region) are not very reliable.

An infrared spectrum does not provide enough information to identify a structure conclusively (unless an authentic spectrum is available to compare "fingerprints").

But, the absorptions of the carbon-carbon and carbon-hydrogen bonds can indicate the presence of double and triple bonds.


Effect of Conjugation:

Most unsymmetrically substituted double bonds produce observable stretching
absorptions in the region of 1600 to 1680 cm - 1 .

Isolated double bonds absorb around 1640 to 1 680 cm- 1, and conjugated double bonds absorb around 1620 to 1 640 cm- 1 .
The aromatic C=C bonds are more like 1.5 bonds than true C=C.

Their reduced ? bonding results in less stiff bonds with lower stretching frequencies, around 1600 cm-1.