Question 11.4: Propose a structure for a compound, C5H12O, that fits the fo...

Organic Chemistry with Biological Applications

Propose a structure for a compound, $C_{5} H_{12} O$ that fits the following $^{1} H$ NMR data: 0.92 δ (3 H, triplet, J = 7 Hz), 1.20 δ (6 H, singlet), 1.50 δ (2 H, quartet, J = 7 Hz), 1.64 δ (1 H, broad singlet).

Strategy

As noted in Worked Example 11.2, it’s best to begin solving structural problems by calculating a molecule’s degree of unsaturation. In the present instance, a formula of $C_{5} H_{12} O$ corresponds to a saturated, open-chain molecule, either an alcohol or an ether.

To interpret the NMR information, let’s look at each absorption individually. The three-proton absorption at 0.92 δ is due to a methyl group in an alkane-like environment, and the triplet splitting pattern implies that the $CH_{3}$ is next to a $CH_{2}$ Thus, our molecule contains an ethyl group, $CH_{3} CH_{2}$ -. The six-proton singlet at 1.20 δ is due to two equivalent alkane-like methyl groups attached to a carbon with no hydrogens, $(CH_{3} )_{2} C$ and the two-proton quartet at 1.50 δ is due to the $CH_{2}$ of the ethyl group. All 5 carbons and 11 of the 12 hydrogens in the molecule are now accounted for. The remaining hydrogen, which appears as a broad one-proton singlet at 1.64 δ is probably due to an OH group, since there is no other way to account for it. Putting the
pieces together gives the structure: 2-methylbutan-2-ol.

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