Question

How would you differentiate between monosubstituted and disubstituted alkenes?

Answer 1

I would use infrared spectroscopy to differentiate the alkenes.

Explanation:

The infrared `"=C-H"` and `"C=C"` stretching vibrations of the alkenes all occur at about `"3050 cm"^(-1)` and `"1660 cm"^(-1)`, so we can't use these vibrations to differentiate among them.

However, alkenes have characteristic, strong out-of-plane bending vibrations in the region below `"1000 cm"^(-1)`. These peaks are usually within `"25 cm"^(-1)` of the values listed below.

Monosubstituted alkenes give two strong peaks in this region; disubstituted alkenes give only one.

`bb"Type"color(white)(mll)bb barν,bb"cm"^(-1)`
`stackrel(———————)("mono"color(white)(ml)980, 905)`
`"cis"color(white)(mmll)705`
`"trans" color(white)(ml)965`
`"gem" color(white)(mm)870`

Compare these values with the spectra below.

Monosubstituted

Hex-1-ene (monosubstituted) has strong peaks at `1000` and `"900 cm"^(-1)`.

(from chemistry.umeche.maine.edu)

cis-Disubstituted

cis-Oct-2-ene has a strong peak at `"701 cm"^(-1)`.

trans-Disubstituted

trans-Oct-2-ene has a strong peak at `"964 cm"^(-1)`.

gem-Disubstituted

2-Methylpent-1-ene has a strong absorption has a strong absorption at `"880 cm"^(-1)`.

(from webbook.nist.gov)