Question

Which set of underlined hydrogen has its ¹H NMR signal at higher frequency?

1. CH₃CH₂CH₃ or CH₃OCH₂CH₃
2. CH₃CHCH₂ or CH₃OCHCH₂

Short answer

a. CH₃OCH₂CH₃

b.CH₃CHCH₂

Step-by-step solution

Comparison of frequencies between propane and methoxyethane (a)

The most deshielded protons have the highest frequency, i.e. the ones which have lowest electron density.

In methoxyethane, there is an oxygen atom. Oxygen has very high electronegativity, so that oxygen withdraws electrons from , which is underlined, towards itself. So it has high frequency. Since propane has no oxygen it has lower frequency than methoxyethane.

Comparison between frequencies of propene and methoxyethene (b)

In methoxyethene, there is lone pair of electrons in oxygen atom. So it has the ability to donate these electrons through resonance. The resonance of methoxyethene is shown below:

Resonance in methoxyethene

Therefore, it will have lower frequency than propene. Since there is no oxygen atom in propene, this will not occur in propene. So propene has high frequency.