Question

A student took the proton NMR spectrum of phenylalanine in D₂Osolution, and had the instrument suppress the DOHsolvent peak. The spectrum is shown below. The integrated relative areas of the peaks are 5:1:1:1.

(a) Draw the structure of phenylalanine as it exists in D₂Osolution. (There is a large excess of D₂O, and any exchangeable protons in phenylalanine will exchange with the solvent.)

(b) Assign the peaks in the spectrum to the protons in the structure.

(c) Why don’t we see the -NH₂or -COOHprotons in the spectrum?

(d) What is the relationship between the two protons that generate nearly mirror-image multiplets at 3.1 and 3.3?

Short answer

(a)

(b)

(c) In D₂O, there is rapid exchange between protons on carboxylic group and amino group, thus, when H is replaced with D, no signal gets generated in NMR.

(d) The relation between two protons that generate nearly mirror-image multiplets is diastereotopic in nature.

Step-by-step solution

Step-1. Explanation of part (a):

In D₂O solution, protons in phenylalanine exchange with D₂O solvent. The exchangeable protons will exchange with the deuterium in solvent and disappear from the spectrum.

Phenylalanine Phenylalanine in D₂Osolvent

Step-2. Explanation of part (b):

Protons which are exchangeable with solvent, will not show peaks in NMR spectrum. Hydrogen which is attached to carbon to which amino group of phenylalanine is attached, will show higher chemical shift due to presence of electronegative atom that is nitrogen adjacent to it. Benzene nucleus of phenylalanine will show chemical shift around 7.3 to 7.4. Hydrogens next to benzene will show chemical shift around 3.1 to 3.3.

Assignment of peaks to protons in NMR spectrum

Step-3. Explanation of part (c):

In D₂O solvent, protons on COOH and NH₂exchange rapidly. When H is replaced with D, signal in the proton NMR spectrum is not generated. Amide protons exchange very slowly in contrast to amino group protons.

Step-4. Explanation of part (d):

In compounds with chiral centers as in amino acids, neighbouring protons are not present in the identical environment. These protons are diastereotopic in nature and are distinguishable in NMR. These protons exhibit different chemical shifts. Relation between protons that generate nearly mirror image multiplets at 3.1 and 3.3 is disatereotopic.