Differentiation of isomeric allylic alkenyl methyl ethers by Raman spectroscopy.
The Raman spectra of several pairs of alkenyl methyl ethers of general structure R(1)R(2)C=CR(5)C(R(3)R(4))OCH3 and R(1)R(2)C(OCH3)C(R(5))=CR(3)R(4) (R(1), R(2), R(3), R(4), R(5) = H or C(n)H(2n+1), n = 1-3) are reported and discussed, with a view to establishing whether Raman spectroscopy offers a viable means of distinguishing between these isomeric unsaturated species. Key bands associated with the nu(sp2C-H) and nu(C=C) stretching modes are found to be particularly useful in this connection: R(1)R(2)C=CHCH2OCH3 and R(1)R(2)C(OCH3)CH=CH2 ethers (R(1), R(2) = CH3, C2H5) are easily distinguished on this basis. Differentiation of their lower homologues, R(1)CH=CHCH2OCH3 and R(1)CH(OCH3)CH=CH2 (R(1) = CH3, C2H5, C3H7), by similar means is also quite straightforward, even in cases where cis and trans isomers are possible. Pairs of isomeric ethers, such as CH3CH=C(CH3)CH2OCH3 and CH3CH(OCH3)C(CH3)=CH2, in which the structural differences are more subtle, may also be distinguished with care. Deductions based on bands ascribed to the stretching vibrations are usually confirmed by consideration of the signals associated with the corresponding delta(sp2C-H) deformation vibrations. Even C2H5CH=CHCH(C3H7)OCH3 and C3H7CH=CHCH(C2H5)OCH3 are found to have distinctive Raman spectra, but differentiation of these closely related isomers requires additional consideration of the low wavenumber region.
Clifford
Paul
P
Division of Chemical and Forensic Sciences, School of Life Sciences, University of Bradford, Bradford, West Yorkshire BD7 1DP, United Kingdom.
Bowen
Richard D
RD
Edwards
Howell G M
HG
Farwell
Dennis W
DW
D000498:Allyl Compounds / Q000737:chemistry* / Q000145:classification; D007536:Isomerism; D008738:Methyl Ethers / Q000737:chemistry* / Q000145:classification; D015394:Molecular Structure; D013059:Spectrum Analysis, Raman
D000498:Allyl Compounds; D008738:Methyl Ethers