Splitting of a signal in a proton NMR spectrum tells us the number of chemically

Question

Splitting of a signal in a proton NMR spectrum tells us the number of chemically non-equivalent hydrogens in the immediate vicinity of the hydrogen giving the signal. Predict the number of lines exhibited by hydrogens at the labeled positions in a first-order NMR spectrum. (Make the approximation that all coupling constants are equal.) The number of lines exhibited by hydrogen(s) a is The number of lines exhibited by hydrogen(s) b is The number of lines exhibited by hydrogen(s) c is The number of lines exhibited by hydrogen(s) a is The number of lines exhibited by hydrogen(s) b is The number of lines exhibited by hydrogen(s) c is

Explanation / Answer

The splitting of signals in 1H NMR spectrum is governed by n+1 rule. Where n is number of protons on the vicinal carbons.

1.

i. The number of lines exhibited by hydrogen a is two (the signal appears as a doublet due to splitting by the neighboring alkenyl proton b).

ii. The number of lines exhibited by hydrogen b is two (the signal appears as a doublet due to splitting by the neighboring alkenyl proton a).

iii. The number of lines exhibited by hydrogens c is one (the signal appears as a singlet as it has no coupling protons on the vicinal carbon. The vicinal carbon is quaternary).

2.

i. The number of lines exhibited by hydrogen a is one (the signal appears as a singlet as it has no coupling protons on the vicinal carbon. The vicinal carbon is quaternary).

ii. The number of lines exhibited by hydrogen b is two (the signal appears as a doublet due to splitting by the neighboring alkenyl proton c.

iii. The number of lines exhibited by hydrogens c is six (the signal appears as a sextet because of splitting by the methylene group and the methyl group b.

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