Question

How many kinds of electronically nonequivalent protons are present in each of the following compounds, and thus how many NMR absorptions might you expect in each? (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br}\) (b) \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NO}_{2}\) (d) Toluene (e) 2 -Methylbut-1-ene (f) cis-Hex-3-ene

Short answer

(a) 2, (b) 4, (c) 3, (d) 3, (e) 4, (f) 4

Step-by-step solution

Identify Chemical Environment of Protons

For each molecule, identify the unique environments in which protons (hydrogens) are located. Consider symmetry and connectivity to differentiate environments.

Analyze Compound (a) CH3CH2Br

In \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br} \), there are two types of protons. The methyl group (\( \mathrm{CH}_3 \)) has three protons in one environment, and the methylene group (\( \mathrm{CH}_2 \)) has two protons in a different environment.

Analyze Compound (b) CH3OCH2CH(CH3)2

For \( \mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{CH} \left( \mathrm{CH}_{3} \right)_{2} \), there are four types of protons: (1) the methoxy group (\( \mathrm{CH}_3 \)), (2) the methylene group (\( \mathrm{CH}_2 \)), (3) the methine group (\( \mathrm{CH} \)), and (4) the equivalent methyl groups (\( \mathrm{CH}_3 \)).

Analyze Compound (c) CH3CH2CH2NO2

In \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NO}_{2} \), there are three types of protons: (1) the terminal methyl group (\( \mathrm{CH}_3 \)), (2) the middle methylene group (\( \mathrm{CH}_2 \)), and (3) the methylene group attached to the nitro group (\( \mathrm{CH}_2 \)).

Analyze Compound (d) Toluene

Toluene has one kind of methyl proton (\( \mathrm{CH}_3 \)) and two types of aromatic protons due to the symmetry: ortho/para and meta. Thus, there are three signals expected.

Analyze Compound (e) 2-Methylbut-1-ene

For 2-Methylbut-1-ene, there are four types of protons: (1) the terminal vinyl proton, (2) the methylene protons adjacent to the double bond, (3) the methyl group attached to the double bond, and (4) the terminal methyl group.

Analyze Compound (f) cis-Hex-3-ene

In cis-Hex-3-ene, there are four types of protons: (1) the vinyl protons on each carbon of the double bond, (2) the protons on each methylene group adjacent to the double bond, and (3) the terminal methyl group.

Key concepts

Chemical Environment of Protons

When we talk about the chemical environment of protons in NMR spectroscopy, we refer to the unique electronic surroundings of hydrogen atoms in a molecule. These environments are influenced by nearby atoms, bonds, and the molecule's overall structure. To determine these environments, consider factors like:

• Symmetry: Identical atoms connected in the same way have the same chemical environment.
• Nearby electronegative atoms: They can deshield protons, making them resonate at different frequencies.
• Functional groups: Different groups can create distinct environments for the protons attached to them.

Identifying these different environments helps in understanding how many unique signals (peaks) you might see in an NMR spectrum. Each unique environment typically corresponds to a distinct NMR absorption.

NMR Absorptions

NMR absorptions or signals are the peaks you see in an NMR spectrum. They result from protons in a molecule absorbing radiofrequency radiation at distinct resonant frequencies. These frequencies are specific to the electronic environment around each proton. Factors affecting these absorptions include:

• Chemical shifts: Determined by the chemical environment, shifts indicate how shielded or deshielded the protons are.
• The number of equivalent protons: More protons resonating at the same frequency result in stronger peaks.
• Splitting patterns: Interactions between non-equivalent protons can split signals into multiplets.

Each distinct chemical environment usually results in one signal, but complex molecules might show more intricate patterns due to proton-proton coupling.

Equivalent Protons

Equivalent protons are those that share the same electronic environment in a molecule. They resonate at the same frequency in an NMR spectrum, giving rise to a single signal.These protons are:

• Often in symmetric parts of the molecule, such as the protons in a methyl group (\( \mathrm{CH}_3 \)).
• Experiencing similar magnetic environments due to equivalent bonding and spatial arrangement.
• Counted collectively when determining the intensity of an NMR peak.

Understanding equivalent protons helps simplify the analysis of an NMR spectrum, as it reduces the number of unique signals to consider.

Electronically Nonequivalent Protons

Electronically nonequivalent protons are those within a molecule that differ in their electronic surroundings. They are affected differently by local electronic fields, causing them to resonate at different frequencies on an NMR spectrum.

• Diversity in nearby atoms and bonds can make protons electronically distinct.
• These protons contribute to the complexity of the NMR spectrum.
• Understanding these protons helps predict the number and type of signals in an NMR.

When analyzing compounds, identifying these protons allows for the determination of the expected number of absorptions. For example, understanding how chemical groups such as methine, methylene, and methyl have distinct environments can shed light on expected NMR patterns. Recognizing these subtleties is crucial for interpreting and predicting NMR spectra correctly.