Question

For IR spectroscopy, how do we tell which structure it is?

Alkene 1 or 2800 2600 2400 2200 alte

Answer 1

• C=C stretch from 1680-1640 cm^-1
• =C–H stretch from 3100-3000 cm^-1
• =C–H bend from 1000-650 cm^-1

The IR spectrum of 1-octene is shown below. Note the band greater than 3000 cm^-1 for the =C–H stretch and the several bands lower than 3000 cm^-1 for –C–H stretch (alkanes). The C=C stretch band is at 1644 cm^-1. Bands for C–H scissoring (1465) and methyl rock (1378) are marked on this spectrum; in routine IR analysis, these bands are not specific to an alkene and are generally not noted because they are present in almost all organic molecules (and they are in the fingerprint region). The bands at 917 cm^-1 and 1044cm^{-1 are}attributed to =C&Mac173;H bends.

90- note that this band is greater than 3000 methyl r 1378 80- 004 644 1465CH C-H bend 70 C-H stretch alkene C-H 60- stretch scissoring 50- 917 30- 1-octene 20 -C-H stretch 10 alkane C-H Wavenumbers (cm-1)

It is interesting to overlay a spectrum of 1-octene with octane (shown below). Note how similar they are, except for the bands due to C=C or C=C–H stretches/bends.