Question

The ¹H NMR spectra of three isomers with molecular formula C₇H₁₄O are shown here. Which isomer produces which spectrum?

a.

b.

c.

Short answer

NMR stands for Nuclear Magnetic Resonance Spectroscopy. This is the technique used for determining the structure of organic compounds. It is a non-destructive technique. It is of two types—¹H NMR and¹³C NMR.

Proton nuclear magnetic resonance (¹H NMR) is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecule of substance in order to determine the structure of its molecules.

Step-by-step solution

Step-by-step Solution Step 1: NMR spectroscopy

NMR stands for Nuclear Magnetic Resonance Spectroscopy. This is the technique used for determining the structure of organic compounds. It is a non-destructive technique. It is of two types—¹H NMR and¹³C NMR.

Proton nuclear magnetic resonance (¹H NMR) is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecule of substance in order to determine the structure of its molecules.

Number of signals

The number of signals present in NMR reflects the number of magnetically different protons. Thus the number of signal reflects the number of chemically different protons or sets of protons. For example, methanol has two different set of protons (methyl and hydroxyl).

Double Bond Equivalent (DBE)

Degree of unsaturation is also known as double bond equivalent (DBE). If the molecular formula is given, plug in the number into this formula.

$\mathrm{DBE}=\frac{2\mathrm{C}+2+\mathrm{N}-\mathrm{X}-\mathrm{H}}{2}$

Where, C is the number of Carbon.

N is the number of nitrogen.

H is the number of hydrogen.

X is the number of halogen.

For ${\mathrm{C}}_7{\mathrm{H}}_{14}\mathrm{O}$,

$\begin{array}{l}\mathrm{DBE}=\frac{2\left(7\right)+2+0-0-14}{2}\\ \underset{}{}\underset{}{}\underset{}{}\underset{}{}=\frac{2}{2}\\ \underset{}{}\underset{}{}\underset{}{}\underset{}{}=1\end{array}$

The actual formula, ${\mathrm{C}}_7{\mathrm{H}}_{14}\mathrm{O}$corresponds to one degree of unsaturation that is due to Carbonyl group (C=O) present in the compound.

Identify the isomer produces which spectrum

(a) The ¹H NMR spectrum of 4, 4-dimethylpentan-2-one shows three signals.

(b) The ¹H NMR spectrum of 2-methylhexan-3-one shows five signals.

(c) The ¹H NMR spectrum of 2,4-dimethylpentan-3-one shows two signals.