Which of the underlined protons in each pair absorbs farther downfield in NMR spectrum #CH_3CH_2CH_3# or #CH_3OCH_3#?

1 Answer
Ernest Z.
Jul 31, 2015

The #"CH"_3# protons in #"CH"_3"OCH"_3# absorbs further downfield.

Explanation:

The #"CH"_3# group in #"CH"_3"OCH"_3# is deshielded by the electronegative #"O"# atom.

Deshielding shifts the absorption downfield.

So we predict that the methyl protons in dimethyl ether should absorb further downfield than the methyl protons in propane.

This prediction is confirmed by their NMR spectra.

The chemical shift of the methyl groups in propane is δ = 0.9, while for dimethyl ether the chemical shift is δ = 3.2.

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