UPSC Combined Geo-Scientist, Mains · 2016 · Mains Paper II

Q1(p). NMR distinction of ethers

Question

Predict the proton NMR spectra that distinguish 1,1-dimethoxyethane from 1,2-dimethoxyethane.

1,1-Dimethoxyethane gives a 6H singlet, 3H doublet and 1H quartet; the symmetric 1,2-isomer gives 6H and 4H singlets in the usual averaged spectrum.

In CH₃CH(OCH₃)₂ the two methoxy groups are equivalent (6H singlet).
In CH₃OCH₂CH₂OCH₃ the methoxy groups are equivalent (6H singlet), and the symmetry-related methylene protons give a 4H signal.
The distinguishing evidence is the number of signals, integrations 6:3:1 versus 6:4, and the methyl-doublet/methine-quartet pair. Small long-range couplings are neglected.

The full model answer

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CoversNMR distinction of ethersProton NMR