Question

The proton and carbon NMR spectra for each of three isomeric ketones with the formula C₇H₁₄Oare shown below. Assign a structure to each pair of spectra.

Short answer

A)

B)

Assignment of the NMR spectrum

C)

Step-by-step solution

Carbon NMR spectroscopy

C NMR chemical shifts follow the same principles as those of ¹H, although the typical range of chemical shifts is much larger than for ¹H. The chemical shift reference standard for ¹³C is the carbons in tetramethylsilane (TMS), whose chemical shift is considered to be 0.0 ppm.

Identification of (a)

The assignment of the ${}^1\mathrm{H}$NMR spectrum of the ketone A is as follows:

Assignment of the NMR spectrum (a)

The assignment of the ¹³C NMR spectrum of the ketone A is as follows:

Assignment of the ¹³C NMR spectrum (a)

The key features of this ¹H NMR spectrum are:

*The methyl protons marked at 1.15 $\mathrm{\delta }$as a 12H, doublet as the molecule under consideration is symmetric and the methyl protons are equivalent. There is one adjacent proton for each of these protons.

*The protons adjacent to the carbonyl group give a septet around $\mathrm{\delta }$2.75. The value is high due to the electron-withdrawing effect of the carbonyl group. As there are 6 neighboring protons the multiplicity is 6+1=7.

(b) The assignment of the ¹H NMR spectrum of the ketone B is as follows:

Assignment of the NMR spectrum (b)

The assignment of the ¹³C NMR spectrum of the ketone B is as follows:

Assignment of the ¹³C NMR spectrum (b)

The key features of this ¹H NMR spectrum are:

*The methyl protons at $\mathrm{\delta }$1.05 as a 9H, singlet as tertiary butyl protons are equivalent. As there is no adjacent proton the peak will come as a singlet.

*The methyl protons adjacent to the carbonyl group give another singlet around $\mathrm{\delta }$2.15. The value is high due to the electron-withdrawing effect of the carbonyl group.

*The methylene protons are marked to give a singlet around 2.35.

(c) The assignment of the ¹H NMR spectrum of the ketone C is as follows:

Assignment of the ¹H NMR spectrum (c)

The assignment of the ¹³C NMR spectrum of the ketone C is as follows:

Assignment of the ¹³C NMR spectrum (c)