Stereochemical Nonequivalence of Protons in NMR Spectroscopy

– In the this topic we talk about Stereochemical Nonequivalence of Protons in NMR Spectroscopy.

Stereochemical Nonequivalence of Protons

– Stereochemical differences often result in different chemical shifts for protons on the same carbon atom.

– For example, the two protons on C₁ of allyl bromide (3-bromopropene) are not equivalent.

– H^a is cis to the -CH₂Br group, and H^b is trans.

– H^a absorbs at δ 5.3; H^b absorbs at δ 5.1.

– There are four different (by NMR) types of protons in allyl bromide, as shown in the structure at right in the margin.

– To determine whether similar-appearing protons are equivalent, mentally substitute another atom for each of the protons in question.

– If the same product is formed by imaginary replacement of either of two protons, those protons are chemically equivalent.

– For example, replacing any of the three methyl protons in ethanol by an imaginary Z atom gives the same compound.

– These three hydrogens are chemically equivalent, and they will all appear at the same chemical shift.

– Freely rotating CH₃ groups always have chemically equivalent protons.

– When this imaginary replacement test is applied to the protons on C1 of allyl bromide, the imaginary products are different.

– Replacing the cis hydrogen gives the cis diastereomer, and replacing the trans hydrogen gives the trans diastereomer.

– Because the two imaginary products are diastereomers, these protons on C1 are called diastereotopic protons.

– Diastereotopic protons appear in the NMR at different chemical shifts, and they can split each other.

– Cyclobutanol shows these stereochemical relationships in a cyclic system.

– The hydroxyl proton H^a is clearly unique; it absorbs between δ 3 and δ 5, depending on the solvent and concentration.

– H^b is also unique, absorbing between δ 3 and δ 4.

– Protons and H^e are H^f diastereotopic (and absorb at different fields) because H^e is cis to the hydroxyl group, while H^f is trans.

– To distinguish among the other four protons, notice that cyclobutanol has an internal mirror
plane of symmetry.

– Protons H^c are cis to the hydroxyl group, while protons H^d are trans. Therefore, protons H^c are diastereotopic to protons H^d and the two sets of protons absorb at different magnetic fields and are capable of splitting each other.

– Diastereomerism also occurs in saturated, acyclic compounds; for example, 1,2-dichloropropane is a simple compound that contains diastereotopic protons.

– The two protons on the -CH₂Cl group are diastereotopic; their imaginary replacement gives diastereomers.

– The most stable conformation of 1,2-dichloropropane (in the margin) shows that the diastereotopic protons on C1 exist in different chemical environments.

– They experience different magnetic fields and are nonequivalent by NMR.

– The NMR spectrum of 1,2-dichloropropane is shown in The following Figure.

– The methyl protons appear as a doublet at δ 1.6, and the single proton on C2 appears as a complex multiplet at δ 4.15 .

– The two C1 protons on appear as distinct absorptions at δ 3.6 and δ 3.77 .They are split by the proton on C2 and they also split each other.

– The presence of an asymmetric carbon atom adjacent to the CH₂Cl group gives rise to the different chemical environments of these diastereotopic protons.

– When a molecule contains an asymmetric carbon atom, the protons on any methylene group are usually diastereotopic.

– They may or may not be resolved in the NMR, however, depending on the differences in their environments.

References:

• Organic chemistry / L.G. Wade, Jr / 8th ed, 2013 / Pearson Education, Inc. USA.
• Fundamental of Organic Chemistry / John McMurry, Cornell University/ 8th ed, 2016 / Cengage Learningm, Inc. USA.
• Organic Chemistry / T.W. Graham Solomons, Craig B. Fryhle , Scott A. Snyder / 11 ed, 2014/ John Wiley & Sons, Inc. USA.