Q19P - Organic Chemistry (Mcmurry)

Question

When the  ${}^{\mathbf{1}}\mathbf{H}$NMR spectrum of an alcohol is run in dimethyl sulfoxide (DMSO) solvent rather than in chloroform, exchange of the O-H proton is slow and spin-spin splitting is seen between the O-H proton and C-H protons on the adjacent carbon. What spin multiplicities would you expect for the hydroxyl protons in the following alcohols?

1. 2-Methyl-2-propanol
2. Cyclohexanol
3. Ethanol
4. 2-Propanol
5. Cholesterol
6. 1-Methylcyclohexanol

Step-by-Step Solution

Answer

The spin multiplicities of the hydroxyl proton in the given alcohols are

1. Unsplit
2. Doublet
3. Triplet
4. Doublet
5. Unsplit

Step-by-Step Solution Step 1: Spin multiplicities of alcohols in H 1 NMR spectrum

In the ${}^1\mathrm{H}$ NMR spectrum, the spin multiplicities of alcohol under slow exchange conditions are different for primary, secondary, and tertiary alcohols. These are: unsplit for tertiary, doublet for secondary, and triplet for primary alcohol.

Step 2: The spin multiplicity for the hydroxyl proton in 2-Methyl-2-propanol

a. Here, 2-Methyl-2-propanol is a tertiary alcohol ( ${\text{R}}_{\text{3}}\text{COH}$). We know the –OH signal of tertiary alcohol is unsplit.

2-Methyl-2-propanol

Step 3: The spin multiplicity for the hydroxyl proton in Cyclohexanol

b. Here, Cyclohexanol is a secondary alcohol (${\text{R}}_2\text{CHOH}$ ). We know the signal of secondary alcohol gets splits into a doublet.

Cyclohexanol

Step 4: The spin multiplicity for the hydroxyl proton in Ethanol

C. Here, Ethanol is a primary alcohol (${\text{RCH}}_2\text{OH}$ ). We know the signal of primary alcohol gets splits into a triplet. So, the -OH signal appears as a triplet.

Ethanol

Step 5: The spin multiplicities for the hydroxyl proton in 2-Propanol

d. Here, 2-Propanol is a secondary alcohol ( ${\text{R}}_2\text{CHOH}$). We know the signal of secondary alcohol gets splits up into a doublet. So, the -OH absorption splits into a doublet.

2-Propanol

Step 6: The spin multiplicities for the hydroxyl proton in Cholesterol

e. From the structure of Cholesterol, it is a secondary alcohol (${\text{R}}_2\text{CHOH}$ ). We know the signal of secondary alcohol gets splits up into a doublet. So, -OH absorption splits into a doublet.

Cholesterol

Step 7: The spin multiplicities for the hydroxyl proton in 1-Methylcyclohexanol

f. The structure of 1-Methylcyclohexanol has a tertiary alcohol ( ${\text{R}}_3\text{COH}$).

We know the signal of –OH in tertiary alcohol is unsplit.

1-Methylcyclohexanol