Question

How can I identify one IR absorption band that could be used to distinguish these compounds: cis -2-butene and trans -2-butene?

Answer 1

You look for the C-H out-of-plane bending vibrations that are characteristic of cis and trans alkenes.

Alkenes have similar C=C and =C-H stretching vibrations, but their bending vibrations are characteristic of their substitution.

• cis alkenes absorb strongly near 700 cm⁻¹.
• trans alkenes absorb strongly near 965 cm⁻¹.

I was unable to find spectra of the isomeric butenes.

Here is the spectrum of cis-pent-2-ene.

Note the strong absorption near 700 cm⁻¹.

And here is trans-pent-2-ene.

Note the strong absorption near 965 cm⁻¹.