Question

Following is a \({ }^{1} \mathrm{H}\)-NMR spectrum of 2 -butanol. Explain why the \(\mathrm{CH}_{2}\) protons appear as a complex multiplet rather than as a simple quintet.

Short answer

Answer: The \(\mathrm{CH}_{2}\) protons in the \({ }^{1} \mathrm{H}\)-NMR spectrum of 2-butanol appear as a complex multiplet rather than a simple quintet because the coupling constants between the \(\mathrm{CH}_{2}\) protons and their neighboring groups (-CH(OH)- and -CH3) are different. This difference in coupling constants causes the expected peaks to overlap and create a complex multiplet pattern.

Step-by-step solution

Understand the structure of 2-butanol

In 2-butanol, the structure is given as -CH3-CH(OH)-CH2-CH3. To understand the NMR spectrum, we need to see the neighboring groups of \(\mathrm{CH}_{2}\). In this case, it has -CH(OH)- on one side and -CH3 on the other side.

Determine the expected coupling pattern

Using the N+1 rule, we can predict the coupling pattern for the \(\mathrm{CH}_{2}\) protons. The \(\mathrm{CH}_{2}\) group is not symmetrical, with the two neighboring groups having a different number of hydrogen atoms. On one side, there is one hydrogen atom attached to the oxygen (-CH(OH)-), and on the other side, there are three hydrogen atoms (-CH3). So, we would expect a quartet (N+1 = 2+1) from coupling with -CH(OH)-, and a doublet (N+1 = 1+1) from coupling with -CH3.

Understand the complex multiplet

In the given \({ }^{1} \mathrm{H}\)-NMR spectrum, the \(\mathrm{CH}_{2}\) protons appear as a complex multiplet, which means that the expected simple quintet (N+1 = 4+1) doesn't occur. This is because the coupling constant (J) between the \(\mathrm{CH}_{2}\) and -CH(OH)- protons is not the same as the coupling constant between the \(\mathrm{CH}_{2}\) and -CH3 protons, due to the different chemical environments. When coupling constants are not the same, the peaks can overlap, leading to a complex multiplet pattern instead of the simple expected quintet.

Conclusion

In the \({ }^{1} \mathrm{H}\)-NMR spectrum of 2-butanol, the \(\mathrm{CH}_{2}\) protons appear as a complex multiplet rather than a simple quintet because the coupling constants between the \(\mathrm{CH}_{2}\) protons and their neighboring groups (-CH(OH)- and -CH3) are different. This difference in coupling constants causes the expected peaks to overlap and create a complex multiplet pattern.