Question

The \(^{1} \mathrm{H}\) NMR spectrum of bromoethane, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br},\) consists of signals at \(\delta 1.7\) and 3.4 ppm. Assign the signals and predict their coupling patterns.

Short answer

The \( \text{CH}_3 \) corresponds to \( \delta 1.7 \) ppm (triplet), and \( \text{CH}_2 \) corresponds to \( \delta 3.4 \) ppm (quartet).

Step-by-step solution

Identify the Hydrogen Types in Bromoethane

Bromoethane, \( \text{CH}_3\text{CH}_2\text{Br} \), contains two types of hydrogen atoms: the \( \text{CH}_3 \) (methyl) group hydrogen atoms and the \( \text{CH}_2 \) (methylene) group hydrogen atoms. This means there are two different environments for the hydrogen atoms, which will produce distinct signals in the \(^1\text{H}\) NMR spectrum.

Analyze the Chemical Shift Values

The \( \delta 1.7 \) ppm signal corresponds to the methyl group (\( \text{CH}_3 \)). Methyl groups are typically found in the region of \( 0.9 \) to \( 2.0 \) ppm. The \( \delta 3.4 \) ppm signal corresponds to the methylene group (\( \text{CH}_2 \)). This shift is downfield due to the electron-withdrawing effect of the bromine atom nearby, which deshields the methylene protons.

Predict Coupling Patterns for Methyl Group

The \( \text{CH}_3 \) group at \( \delta 1.7 \) ppm is adjacent to a \( \text{CH}_2 \) group. According to the \( n+1 \) rule (where \( n \) is the number of neighboring protons), the \( \text{CH}_3 \) signal will experience splitting into a triplet. This is because the \( \text{CH}_3 \) group has two neighboring protons (\( n=2 \)), resulting in \( n+1 = 3 \) peaks.

Predict Coupling Patterns for Methylene Group

The \( \text{CH}_2 \) group at \( \delta 3.4 \) ppm is adjacent to a \( \text{CH}_3 \) group. Thus, it will experience splitting into a quartet. The \( \text{CH}_2 \) group has three neighboring protons (\( n=3 \)), leading to \( n+1 = 4 \) peaks.

Key concepts

Understanding Chemical Shift

The chemical shift in proton NMR provides essential clues about the electronic environment around hydrogen atoms in a molecule. Chemical shift values are influenced by factors such as electronegativity and the hybridization state of neighboring atoms. For instance, in bromoethane, the presence of an electron-withdrawing bromine atom makes the nearby methylene group protons deshielded. Deshielded protons appear downfield, leading to higher chemical shift values, like the \( \delta 3.4 \) ppm observed for methylene in bromoethane.
In contrast, the methyl group protons are more shielded and appear in a different range, \( \delta 1.7 \) ppm, typical for methyl groups. This variance in chemical shift helps differentiate between these hydrogen environments.
Remember, the more electronegative atoms pull electron density away, increasing the chemical shift number, while less electronegative environments show smaller shifts. Understanding these factors helps assign peaks to specific groups.

Exploring Coupling Patterns

In proton NMR, the coupling patterns arise because of interactions between neighboring protons. This interaction is quantitatively described by the \( n+1 \) rule. This rule states that a set of n equivalent protons adjacent to a given hydrogen atom will split the NMR signal of that hydrogen into \( n+1 \) peaks.
For bromoethane:

• The methyl group \(\text{CH}_3\), neighbors a \(\text{CH}_2\) group (2 protons), thereby splitting its signal into a triplet: \( n+1=3 \) peaks.
• Similarly, the methylene group \(\text{CH}_2\), neighbors a \(\text{CH}_3\) group (3 protons), which splits into a quartet: \( n+1=4 \) peaks.

These patterns, from singlets to multiplets, give information on how many hydrogen atoms are nearby, thus helping to elucidate molecular structure.

Differentiating Methyl and Methylene Groups

In organic chemistry, understanding the distinct characteristics of methyl and methylene groups is crucial. Both groups contain hydrogen atoms, yet they differ in the number of hydrogens and their chemical environment.
A methyl group \(\text{CH}_3\) consists of three hydrogen atoms bonded to one carbon. It typically appears in the upfield region of an NMR spectrum since it is less affected by nearby electronegative elements.
On the other hand, a methylene group \(\text{CH}_2\) has two hydrogen atoms bonded to one carbon. Its chemical shift tends to occur further downfield, as seen with bromoethane’s methylene group at \( \delta 3.4 \) ppm, due to deshielding from the bromine atom.
Differentiating these groups not only involves recognizing their chemical shift, but also inspecting their coupling patterns for a complete picture of their surroundings.