How would you use $1^H$ NMR spectroscopy to distinguish between ${\mathrm{CH}}_3\mathrm{CH}{2}_{\mathrm{OH}}$ (ethanol) and ${\mathrm{CH}}_{3}O{\mathrm{CH}}_3$ (dimethyl ether)?

• A. Ethanol and dimethyl ether cannot be distinguished by $1^H$ NMR spectroscopy.
• B. Ethanol shows a triplet and quartet due to the splitting of the methyl and methylene protons, while dimethyl ether shows a singlet for its methyl protons.
• C. Both compounds show only singlets in their $1^H$ NMR spectra.
• D. Dimethyl ether shows a triplet and quartet, while ethanol shows only singlets.

Answer: (B)

Verified step by step guidance

1. Identify the different types of hydrogen environments in ethanol (CH3CH2OH) and dimethyl ether (CH3OCH3). Ethanol has three types of protons: methyl (CH3), methylene (CH2), and hydroxyl (OH), while dimethyl ether has only one type of proton environment: two equivalent methyl groups (CH3).
2. Recall the NMR splitting pattern rules: protons on adjacent carbons split each other's signals according to the n+1 rule, where n is the number of neighboring equivalent protons.
3. For ethanol, the methyl (CH3) protons are adjacent to the methylene (CH2) protons, so the methyl signal will be split into a triplet (because of 2 neighboring protons), and the methylene protons will be split into a quartet (because of 3 neighboring protons). The hydroxyl proton usually appears as a singlet due to rapid exchange and does not cause splitting.
4. For dimethyl ether, both methyl groups are equivalent and have no adjacent protons on carbon atoms (the oxygen does not cause splitting), so the methyl protons appear as a singlet with no splitting.
5. Therefore, by analyzing the splitting patterns in the 1H NMR spectrum—triplet and quartet for ethanol versus a singlet for dimethyl ether—you can distinguish between the two compounds.