Question

How could you use \({ }^{1}\) H NMR to distinguish between the following pairs of isomers? (a) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{2} \mathrm{CH}_{3}\) and \(\quad \mathrm{CH}_{2}\) \(\mathrm{H}_{2} \mathrm{C}-\mathrm{CHCH}_{2} \mathrm{CH}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3}\) and \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (d) \(\quad \stackrel{0}{\|}\) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{C}\left(\mathrm{CH}_{3}\right) \mathrm{CCH}_{3} \quad\) and \(\quad \mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCCH}_{3}$$\quad \mathrm{O}\) \(\|\)

Short answer

NMR distinguishes isomers by different chemical shifts and coupling patterns for hydrogen atoms in unique environments.

Step-by-step solution

Understanding the Structure

For each pair of isomers, first identify the position and environment of the hydrogen atoms in the molecules. This will help predict the number and type of signals each distinct hydrogen environment will produce in the NMR spectrum.

Analyze the Alkene Signals - Part (a)

The first pair of isomers is 1-pentene (\(\text{CH}_3 \text{CH}=\text{CHCH}_2 \text{CH}_3\)) and 2-pentene (\(\text{CH}_2 \text{H}_2 \text{C}-\text{CHCH}_2 \text{CH}_3\)). For 1-pentene, there will be a signal corresponding to the terminal vinyl group hydrogens at around 4.6 - 5.0 ppm. In 2-pentene, this signal shifts downfield slightly to around 5.2 - 5.7 ppm because of the different molecular environment.

Inspect the Ether Signals - Part (b)

For the ethers, the first structure is ethyl methyl ether (\(\text{CH}_3 \text{CH}_2 \text{OCH}_2 \text{CH}_3\)), whereas the second is propoxymethane (\(\text{CH}_3 \text{OCH}_2 \text{CH}_2 \text{CH}_3\)). In ethyl methyl ether, the hydrogens on the \(\text{CH}_2 \text{O}\) group will appear around 3.3-3.7 ppm for both isomers, but the integration will help differentiate them. Ethyl methyl ether will show three distinct hydrogen groups in its spectrum, whereas propoxymethane will have a characteristic triplet and quartet from the ethyl group.

Consider the Carbonyl and Alkene Signals - Part (d)

For the last pair, isobutylene (\(\stackrel{0}{\|} \text{H}_2 \text{C}=\text{C}(\text{CH}_3)\text{CCH}_3\)) and methyl vinyl ketone (\(\text{CH}_3 \text{CH}=\text{CHCCH}_3 \quad \text{O}\) \(\|\)), it's key to look for carbonyl signals and alkene shifts. Isobutylene will show signals from vinyl hydrogens likely between 4.6-5.0 ppm without any downfield aldehyde peaks. For methyl vinyl ketone, a downfield carbonyl-associated multiplet signal around 5.5–6.5 ppm and a strong carbonyl peak around 9-10 ppm will be noticeable.

Key concepts

Isomer Differentiation

Isomer differentiation in NMR spectroscopy primarily involves analyzing the differences in the arrangement of atoms within molecules that have identical molecular formulas. Each type of isomer will have unique hydrogen environments that create distinct signals in the proton NMR (\({ }^{1}\) H NMR) spectrum. These signals depend on factors like chemical shift, integration, and splitting patterns.

**Chemical Shifts:** These shifts relate to the environment around specific hydrogen atoms and influence where on the NMR spectrum the signals appear. For example, hydrogen atoms near electronegative atoms or unsaturated systems usually show downfield chemical shifts (higher ppm values).

**Integration:** This measures the area under each signal and corresponds directly to the number of hydrogens contributing to that signal. Integration helps distinguish isomers by providing a count of hydrogens in different environments.

**Splitting Patterns:** Splitting occurs because protons are influenced by adjacent protons’ magnetic fields. Each pattern aids in determining the number of adjacent hydrogen atoms, contributing to further differentiating isomers.

When employing NMR to distinguish between isomers, detailed attention to these aspects allows identification of differences in molecular connectivity and environment, providing a robust means of isomer differentiation.

Alkene Signals

Alkene signals in NMR are essential for identifying unsaturated hydrocarbons like alkenes due to how the double bond affects the hydrogen atoms attached to it. This characteristic makes alkene regions significant when analyzing NMR spectra.

**Chemical Shifts for Alkenes:** Typically, protons attached to sp² hybridized carbon atoms in alkenes appear downfield between 4.6 - 6.5 ppm. For 1-pentene and 2-pentene, the different positions of the double bond change the hydrogen environments, slightly altering their chemical shifts. For instance, terminal alkene groups (1-pentene) appear slightly upfield than internal alkenes (such as 2-pentene).

**Splitting Patterns and Couplings:** Involves coupling constants which are unique for protons on carbon-carbon double bonds due to the alignment and electronic environment differences. Protons on a double-bonded carbon can exhibit complex splitting patterns, influenced by adjacent hydrogens' positions and numbers.

By assessing these signals and their peculiarities, scientists and researchers can effectively distinguish alkenes within isomeric mixtures or within intricate organic structures.

Ether Signals

Ether signals in NMR spectroscopy are vital for recognizing ethers as distinct chemical environments showcase different spectral cues. In ethers, oxygen atoms create particular conditions for the protons attached to neighboring carbon atoms, affecting their NMR signals.

**Typical Chemical Shifts for Ethers:** Protons bound to carbon atoms next to oxygen usually appear downfield, commonly between 3.3 - 4.0 ppm. This is due to the deshielding effect imparted by the highly electronegative oxygen.

**Integration and Distinction in Peaks:** When distinguishing ethers like ethyl methyl ether and propoxymethane, noting the integration of signals is crucial. Ethyl methyl ether's CH 3 O and CH 2 groups will show distinct areas that correlate to the count of these groups differently compared to propoxymethane, which might show both a triplet and a quartet typical of an ethyl chain.

Using integration as a tool, alongside predicted chemical shifts for ether protons, allows for confident differentiation of ethers in complex NMR spectra. Researchers can then piece together the molecular structure more adeptly through these spectral traits.