Differentiation of isomeric allylic alkenyl methyl ethers by Raman spectroscopy

Abstract

The Raman spectra of several pairs of alkenyl methyl ethers of general structure R 1R 2C CR 5C(R 3R 4)OCH 3 and R 1R 2C(OCH 3)C(R 5) CR 3R 4 (R 1, R 2, R 3, R 4, R 5 = H or C n H 2 n+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 ν(sp 2C H) and ν(C C) stretching modes are found to be particularly useful in this connection: R 1R 2C CHCH 2OCH 3 and R 1R 2C(OCH 3)CH CH 2 ethers (R 1, R 2 = CH 3, C 2H 5) are easily distinguished on this basis. Differentiation of their lower homologues, R 1CH CHCH 2OCH 3 and R 1CH(OCH 3)CH CH 2 (R 1 = CH 3, C 2H 5, C 3H 7), by similar means is also quite straightforward, even in cases where cis and trans isomers are possible. Pairs of isomeric ethers, such as CH 3CH C(CH 3)CH 2OCH 3 and CH 3CH(OCH 3)C(CH 3) CH 2, 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 δ(sp 2C H) deformation vibrations. Even C 2H 5CH CHCH(C 3H 7)OCH 3 and C 3H 7CH CHCH(C 2H 5)OCH 3 are found to have distinctive Raman spectra, but differentiation of these closely related isomers requires additional consideration of the low wavenumber region.