Differentiation of isomeric allylic alkenyl methyl ethers by Raman spectroscopy

The Raman spectra of several pairs of alkenyl methyl ethers of general structure R1R2CCR5C(R3R4)OCH3 and R1R2C(OCH3)C(R5)CR3R4 (R1, R2, R3, R4, R5 = H or CnH2n+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(sp2CH) and ν(CC) stretching modes are found to be particularly useful in this connection: R1R2CCHCH2OCH3 and R1R2C(OCH3)CHCH2 ethers (R1, R2 = CH3, C2H5) are easily distinguished on this basis. Differentiation of their lower homologues, R1CHCHCH2OCH3 and R1CH(OCH3)CHCH2 (R1 = 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 CH3CHC(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 δ(sp2CH) deformation vibrations. Even C2H5CHCHCH(C3H7)OCH3 and C3H7CHCHCH(C2H5)OCH3 are found to have distinctive Raman spectra, but differentiation of these closely related isomers requires additional consideration of the low wavenumber region.