How would you differentiate between monosubstituted and disubstituted alkenes?

1 Answer
Feb 11, 2016

I would use infrared spectroscopy to differentiate the alkenes.

Explanation:

The infrared "=C-H"=C-H and "C=C"C=C stretching vibrations of the alkenes all occur at about "3050 cm"^(-1)3050 cm1 and "1660 cm"^(-1)1660 cm1, 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)1000 cm1. These peaks are usually within "25 cm"^(-1)25 cm1 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).

Hex-1-eneHex-1-ene
(from chemistry.umeche.maine.edu)

cis-Disubstituted

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

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trans-Disubstituted

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

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gem-Disubstituted

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

2MePent12MePent1
(from webbook.nist.gov)