Interpreting the infrared spectrum of E-pent-2-ene and Z-pent-2-ene

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing infrared spectra of E-pent-2-ene and Z-pent-2-ene (comparing the infrared spectra of cis/trans isomers of 2-pentene)

email doc brown

Interpretation of the infrared spectrum of E-pent-2-ene and Z-pent-2-ene

Other than the fingerprint region, although many infrared absorptions are similar, there are some significant differences in the infrared spectra of the stereoisomers of pent-2-ene.

The most prominent infrared absorption lines of E-pent-2-ene (trans):

C-H stretching vibration bands at wavenumbers 3100 to 3040 cm^-1.

The very characteristic C=C stretching band at 1680 to 1620 cm^-1, but relatively weak compared to E-pent-2-ene.

At wavenumbers 980 to 960 cm^-1, there are characteristic C-H stretching and deformation vibration absorptions associated with the RCH=CHR' (R, R' = alkyl) grouping for the E (trans) isomer of pent-2-ene.

The most prominent infrared absorption lines of Z-pent-2-ene (cis):

C-H stretching vibration bands at wavenumbers 3100 to 3040 cm^-1.

The very characteristic C=C stretching band at 1680 to 1620 cm^-1, but a much stronger absorption compared to E-pent-2-ene.

At wavenumbers 730 to 665 cm^-1, there are characteristic C-H stretching and deformation vibration absorptions associated with the RCH=CHR' (R, R' = alkyl) for the Z (cis) isomer of pent-2-ene.

The absence of other specific functional group bands will show that particular functional group is absent from the E-pent-2-ene and Z-pent-2-ene molecular structure.