Structure Identification By 1h NMR

STRUCTURE IDENTIFICATION BY ¹H NMR

Generally the NMR spectrum of a compound is used in conjunction with other available information for identification purposes. The reactants and the reagents and reaction conditions can serve as a guide to the types of products that might be expected. Structure identification often merely confirms the structures of products that were predicted from the chemistry employed in the synthesis. In other cases products are obtained whose spectra do not match the predicted products. In such cases more information is usually required to solve the structure. Thus while NMR is an extraordinarily powerful tool, it is not sufficient to solve all structural problems. This latter fact must be kept in mind.

The reaction between 1-phenyl-1-propanol and chromic acid gives a liquid product P₁ with the ¹H NMR spectrum shown in Figure 11.24. The spectrum of the reactant 1-phenyl-1-propanol contains a five-proton broad singlet for the aromatic protons. 

The methine proton H₁ is split into a triplet by H_2,2 and is further split into doublets by the OH proton, which must be exchanging slowly in this sample to give splitting. The OH proton is split by H₁ into the doublet at 2.1δ. Protons H₂ and H₂ are a multiplet rather than a pentet as expected by the n + 1 rule. This is due to the fact that they are diastereotopic. Thus they have different chemical shifts, split each other, and each are split into pentets by the n + 1 rule. The multiplet at 1.75δ is the result.

The NMR spectrum of the product P₁ (Figure 11.25) shows the five-proton aromatic signal at 7.3δ in the reactant has been converted to a three-proton multiplet over the range of 7.35 – 7.6δ and two-proton doublet at 7.94δ (5H total). The product also has an A₂X₃ system (an ethyl group). The chemical shift of the methylene group at 2.99δ in the product is reasonable for a methylene group next to an aromatic carbonyl group. Furthermore the one-proton multiplet at 4.55δ for the methine proton and the one-proton doublet for the OH proton of the starting material are not present in the product. The NMR is consistent with an oxidation of the alcohol to propiophenone as predicted by the chemistry.

When 1-methylcyclohexanol is heated with anhydrous copper sulfate, two products P₂ and P₃ are isolated in a 85 : 15 ratio. The NMR spectra of these products are shown in Figures 11.26 and 11.27. The appearance of signals in the olefinic region of both products indicates that both are elimination products. Furthermore the major product has an intact methyl group (s, 1.63δ, 3 H) whose chemical shift indicates it is likely allylic. The vinyl signal integrates for a single proton. The remaining protons are multiplets that have an integrated area corre-sponding to 8 H. From these data it is clear that the major product P₁ is 1-methyl cyclohexene.

The minor product P₂ does not have a methyl signal, and the vinyl signal is integrated for 2H. The remaining protons have a total integrated area of 10H and there is a 4H multiplet downfield from the remaining 6H multiplet. These data are consistent with exo-methylenecyclohexane as the minor product. The 4H signal is due to the allylic protons of the ring

Treatment of 3-pentanone with isopropenyl acetate is reported to give 3-acetoxy-2-pentene. The product isolated from the reaction has the ¹H NMR spectrum shown in Figure 11.28.

Preliminary examination shows the isolated product to be a mixture; however, the products appear to be similar. The spectrum includes two singlet methyl groups (3H) and two A₂X₃ shown by overlapping triplets at 1.05δ (Can you pick them out?) and two AX₃ groups shown by the allylic methyl doublets at 1.5δ and 1.65δ. Particularly revealing is the vinyl signal at 5.1δ. Its relatively high field results from the fact that enol derivatives are electron rich by resonance interaction of the oxygen lone pairs with the olefinic π system, which causes the vinyl proton β to the oxygen group to be shielded. Furthermore it is not a simple quartet but is actually overlapping quartets due to splitting by a methyl group.

This is indicative of a partial structure.

This partial structure along with the ethyl groups and acetate methyl singlets confirms the structure assignment. The allylic methylene group is at 2.1 – 2.3 and is overlapped by the two acetate singlets. The product is a mixture of the Z and E isomers in the ratio of 2 : 5 as determined by the integrated areas of the methyl doublets. It is not possible to unambiguously assign the isomers from the NMR, but it is likely that the minor isomer is the Z isomer since the allylic methyl group would be sterically deshielded by the acetate group and absorb downfield from the E isomer. The smaller allylic methyl doublet is found downfield from the major isomer

The carbodiimide coupling of N -methylphenylglycine with benzylamine gives a product whose ¹H NMR is shown in Figure 11.29. The expected product is the amino amide. The NMR spectrum shows first that both reactants are incorporated in the product.

The methyl singlet is indicative of the –NHCH₃ group, and the aromatic signal has increased to 10H, indicating that two phenyl rings are present in the product.

The signal at 4.4δ is proper for the benzyl group, but the splitting pattern is problematic until it is recognized that because there is a chiral center at C-2, the benzyl protons are diastereotopic and thus nonequivalent. They are part of an ABX spin system and thus give the complex splitting pattern seen — actually a two-proton multiplet that looks like a doublet or a very close AB quartet.

The two N–H protons in this compound illustrate different exchange behavior. The N–H proton at C-2 comes upfield at 1.74δ as a broadened singlet due to fairly rapid exchange and does not to split either the C-2 proton at 4.07δ or the N-methyl group. Conversely the amide N–H proton is a much broader singlet at 7.55δ and splits the benzylic protons by a small amount because the exchange is slower. It turns out that when protons exchange rapidly, as they do on the NH of the amino group, the spin state of the proton is blurred and coupling information is lost. The neighboring proton cannot actually feel one spin state or the other because the protons with different spin states are exchanging rapidly.

When the proton does not exchange rapidly as on the N–H of the amide group, normal coupling is observed. Since the rates of proton exchange are often critically dependent on the solution conditions, coupling to acidic protons is variable and thus may or may not be observed.

The above examples illustrate how NMR spectra are routinely used to answer questions about reactions and products. Spectra are usually examined in conjunc-tion with other information that permits a broad-based structure identification to be carried out. Outside of structure questions in texts and on exams, one is almost never handed an NMR spectrum and asked to identify the compound in the absence of other supporting information.