How would you differentiate between monosubstituted and disubstituted alkenes?

Question

Ruby Allen · Accepted Answer

I would use infrared spectroscopy to differentiate the alkenes.

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)#.

.

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)#.

Addison Armstrong · Answer

undefined Monosubstituted alkenes have one substituent group attached to the double bond, while disubstituted alkenes have two substituent groups attached to the double bond. This difference can be observed in the structural formula of the molecule or by analyzing the NMR spectrum, where the number of peaks corresponding to the hydrogen atoms attached to the double bond reflects the degree of substitution.