IR Spectroscopy

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.
Because C?C bonds are stronger and stiffer than C=C, they absorb IR light at higher frequencies.
Most alkyne C?C bonds have stretching frequencies between 2100 and 2200 cm-1.
Terminal alkynes usually give sharp C?C stretching signals of moderate intensity.
The C?C stretching absorption of an internal alkyne may be weak or absent, however, due to the symmetry of the disubstituted triple bond with a very small or zero dipole moment.