Question

You take \(\mathrm{a}^{1} \mathrm{H}\) -NMR spectrum of a sample that comes from a bottle of \(1-\) bromopropane. However, you suspect that the bottle might be contaminated with 2 bromopropane. The NMR spectrum shows the following peaks: $$ \begin{array}{|l|l|l|} \hline \boldsymbol{\delta}(\mathbf{p p m}) & \text { splitting } & \text { Integration } \\ \hline 4.3 & \text { septet } & 0.0735 \\ \hline 3.4 & \mathrm{t} & 0.661 \\ \hline 1.9 & \text { sextet } & 0.665 \\ \hline 1.7 & \mathrm{~d} & 0.441 \\ \hline 1.0 & \mathrm{t} & 1.00 \\ \hline \end{array} $$ How badly is the bottle contaminated? Specifically, what percent of the molecules in the bottle are 2 -bromopropane?

Short answer

Answer: The percentage of 2-bromopropane in the mixture is approximately 14.3%.

Step-by-step solution

Identify the characteristic peaks for 1-bromopropane and 2-bromopropane.

The peaks in the given NMR spectrum can be assigned as follows: 1-bromopropane: - 3.4 ppm (triplet, integration: 0.661) - CH_3 group - 1.7 ppm (doublet, integration: 0.441) - CH_2 group - 1.0 ppm (triplet, integration: 1.00) - CH_2 group 2-bromopropane: - 4.3 ppm (septet, integration: 0.0735) - CH_2 group - 1.9 ppm (sextet, integration: 0.665) - CH_2 group

Determine the concentrations ratio of 1-bromopropane and 2-bromopropane.

Based on the peak integrations, we can determine the ratio of 1-bromopropane and 2-bromopropane in the mixture. For that, we can use the integration values of the unique peaks for each compound. For 1-bromopropane, we can use the peak at 1.7 ppm with an integration of 0.441. For 2-bromopropane, we can use the peak at 4.3 ppm with an integration of 0.0735. The ratio of 1-bromopropane:2-bromopropane is therefore 0.441:0.0735.

Calculate the percentage of 2-bromopropane in the mixture.

To calculate the percentage of 2-bromopropane in the mixture, we can use the following formula: Percentage of 2-bromopropane = (integration of 2-bromopropane / (integration of 1-bromopropane + integration of 2-bromopropane)) × 100 Percentage of 2-bromopropane = (0.0735 / (0.441 + 0.0735)) × 100 ≈ 14.3% So, about 14.3% of the molecules in the bottle are 2-bromopropane.

Key concepts

1H NMR

Nuclear magnetic resonance (NMR) spectroscopy is an innovative analytical technique. It's used to understand the structure of organic molecules. The ¹H NMR is a specific type that focuses on hydrogen atoms, or protons, in these molecules.
When a sample is placed in a magnetic field, the nuclei of the hydrogen atoms resonate at specific frequencies. These frequencies depend on their chemical environment. This resonance is picked up and displayed as a spectrum.

• Each peak in the ¹H NMR spectrum corresponds to a different type of hydrogen atom in the molecule.
• The position of the peak (measured in ppm) indicates the chemical environment of those hydrogens, often revealing the proximity or presence of electronegative atoms or conjugated systems.
• The splitting patterns in a peak, like singlets, doublets, or triplets, can tell us how many neighboring hydrogens are nearby.

Understanding these peaks' positions and patterns, coupled with peak integration, unlocks the molecule's layout and structure.

bromopropane isomers

Bromopropane has two primary isomers - 1-bromopropane and 2-bromopropane. Isomers are molecules with the same molecular formula but differ in the arrangement of atoms. These differences in structure lead to variations in physical and chemical properties.
For 1-bromopropane, the bromine atom is connected to the first carbon in the propane chain. This arrangement gives the molecule one configuration.
On the other hand, 2-bromopropane has the bromine atom attached to the second carbon, creating an alternative form of the molecule. This difference in attachment site significantly influences their behavior, including how they appear on an ¹H NMR spectrum.

• In 1-bromopropane, one can expect a peak corresponding to the methyl group (CH₃) and the different configurations of the CH₂ groups.
• 2-bromopropane, meanwhile, will show peaks reflecting its molecular structure, including the bromine's impact on the chemical shifts of the neighboring hydrogens.

Both isomers provide unique NMR signals that allow chemists to identify them distinctly, particularly essential when assessing potential contamination or purity issues in a mixture.

peak integration analysis

In ¹H NMR spectroscopy, peak integration is a critical feature for quantifying the number of hydrogen atoms contributing to a particular peak. It refers to the area under the peaks in the NMR spectrum.
Integration values represent the number of hydrogen atoms in the molecule's different environments, directly correlating to the height and area of each peak.

• The ratios of these integration values give us a quantitative view of these protons' relative amounts.
• This analysis is key in mixtures where multiple compounds are present, such as our bromopropane case.

By comparing integration values from peaks unique to 1-bromopropane and 2-bromopropane, one can deduce the relative concentrations of each isomer in a mixture. In the example you're working on, we see that the presence of a peak at 4.3 ppm with an integration value of 0.0735, combined with the peak from 1-bromopropane at 1.7 ppm, allows us to calculate the percentage of each component.
This result gives us a clear understanding of how much contamination in terms of percentage exists in the mixture.