Question

What would you expect to observe in the \(^{1} \mathrm{H}\) NMR spectrum of triethylamine (14.26)?

Short answer

The NMR spectrum will have a quartet for CH₂ protons and a triplet for CH₃ protons, integrating in a 3:2 ratio.

Step-by-step solution

Identify the Structure

Triethylamine has the chemical structure N(CH₂CH₃)₃. This means it consists of a nitrogen atom bonded to three ethyl groups (CH₃CH₂).

Count the Unique Proton Environments

Examine the triethylamine structure to identify unique proton environments. There are two different types of hydrogen environments: the ethyl (CH₂) group protons and the ethyl (CH₃) group protons.

Analyze the Splitting Patterns

The next step is to determine the splitting pattern for each proton group based on neighboring protons. Ethyl group protons are influenced by each other: - CH₂ protons: They see the CH₃ protons, giving an expected quartet due to the three protons of the CH₃ group (n+1 rule: 3 CH₃ protons + 1 = quartet). - CH₃ protons: They see the CH₂ protons, giving a triplet due to the two protons of the CH₂ group (n+1 rule: 2 CH₂ protons + 1 = triplet).

Predict the Integration Ratios

Determine the relative number of protons in each environment to predict the integration ratios. With three ethyl groups bound to nitrogen, the integration ratio will be three CH₃ groups (9 protons) and three CH₂ groups (6 protons), meaning 3:2 ratio for CH₃ to CH₂.

Key concepts

Proton Environments

In NMR spectroscopy, a key concept is understanding the idea of proton environments. Every hydrogen atom in a molecule, also known as a proton, is in a unique chemical environment based on its position and surrounding atoms. These different environments result in varied magnetic fields, leading to distinct signals in the NMR spectrum. For triethylamine, which has the chemical formula N(CH₂CH₃)₃, there are two main proton environments:

• The CH₂ protons: These protons are directly bonded to both the nitrogen and another carbon atom. They have a different magnetic environment compared to their neighbors.
• The CH₃ protons: These protons, part of the methyl groups, are only bonded to carbon atoms, providing a distinct environment from the CH₂ group.

Recognizing these environments helps predict what the NMR spectrum will look like.

Chemical Structure

The chemical structure of a compound like triethylamine is crucial for predicting its NMR behavior. Triethylamine features a nitrogen atom attached to three ethyl groups, depicted as N(CH₂CH₃)₃. This structure influences several aspects of NMR findings:

• Nitrogen, having a different electronegativity compared to carbon, affects the electron cloud around the neighboring protons.
• The geometry of the molecule determines how protons influence each other, which in turn affects the magnetic environment.
• The symmetry in the molecule is why only two types of proton environments are observed despite having many protons. This symmetry simplifies the analysis of the NMR spectrum.

Understanding the chemical structure is the first step in predicting and analyzing the outcomes of NMR spectroscopy.

Spectral Splitting Patterns

In NMR spectra, proton signals are often split into multiple peaks, known as splitting patterns. These patterns result from interactions with neighboring protons, following the "n+1 rule": a proton is split into (n+1) peaks, where n is the number of neighboring protons.
For triethylamine:

• The CH₂ group protons are adjacent to the CH₃ group. With three neighboring CH₃ protons, the CH₂ protons' signal appears as a quartet (3+1=4 peaks).
• The CH₃ group protons, with two adjacent CH₂ protons, exhibit a triplet (2+1=3 peaks).

This splitting provides valuable information about the number and proximity of neighboring protons, enhancing the clarity of molecular analysis.

Integration Ratios

Integration in NMR helps determine the number of protons contributing to each peak, shown as integrations ratios. In triethylamine, we observe these ratios by:

• Adding the number of protons in each unique environment: there are 9 protons from the CH₃ groups and 6 from the CH₂ groups.
• These add up to a total ratio of 3:2 for CH₃ to CH₂ protons. This ratio appears in the spectrum as different peak sizes, corresponding to the relative number of each type of proton.

Accurate integration reveals the relative amount of protons in each environment, confirming or refining the chemical structure interpretation from the spectrum. This plays a crucial role in quantifying and understanding complex molecular structures.