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Hydrogen-1 NMR 2 (A2 Only)

Chemistry: AQA A Level, topic 6.9.4

Making Spectra Clear

The choice of solvent before the spectra is recorded makes the process easier. The process of integration after also makes interpretation of the spectra easier.

Solvent choice

Solvent choice

  • The solvent choice in 1H NMR is crucial because many solvents contain 1H's.
  • If a solvent contains 1H's, they will appear in the spectrum and this will cause confusion.
  • This can be solved by using solvents without any 1H's (e.g. CCl4) or deuterated solvents (e.g. CDCl3, D2O).
Deuterated solvents

Deuterated solvents

  • Deuterated solvents are the same as many protonated solvents but each 1H has been replaced by a deuterium (2D) atom.
  • Deuterium atoms have an even nucleon number and so will not interfere with the NMR.
Integration

Integration

  • The area ratios of signals in spectra are hard to evaluate at first glance when signal splitting happens.
  • E.g. It is hard to compare the area ratio under a quartet and a singlet.
  • To make it easier, many NMR machines integrate the peaks to then give an accurate ratio.

Examples of 1H NMR

Below are some examples to demonstrate interpreting 1H NMR spectra.

Example 1: 1,1-dichloroethane

Example 1: 1,1-dichloroethane

  • The spectrum is shown above and the interpretation is on the next slide.
Example 1: 1,1-dichloroethane

Example 1: 1,1-dichloroethane

  • The spectrum shows two peaks representing the two different proton environments.
  • The peak on the left represents the 1H on the -CCl2H because it is deshielded.
  • It is a quartet because the adjacent carbon has three protons attached.
Example 2: Bromo-dimethyl ether

Example 2: Bromo-dimethyl ether

  • The spectrum is shown above and the interpretation is on the next slide.
Example 2: Bromo-dimethyl ether

Example 2: Bromo-dimethyl ether

  • The spectrum shows two peaks representing the two different proton environments.
  • Both peaks are singlets because neither of them are adjacent to any carbons with any protons.
Example 3: Ethanol

Example 3: Ethanol

  • The spectrum is shown above and the interpretation is on the next slide.
Example 3: Ethanol

Example 3: Ethanol

  • The spectrum shows two peaks representing the two different proton environments.
  • The peak on the left represents the 1Hs on the -CH2OH because it is deshielded.
  • The splittings are a triplet and a quartet as expected.

Test yourself on Hydrogen-1 NMR 2 (A2 Only)

  • What is a good alternative to protons to use in the solvent for proton NMR spectroscopy?

  • What is the common goal of both integration and using proton-free solvents?

  • A peak splits into a triplet.
    What does this mean?

  • Where would we expect to find the proton in the group -CHCl2- on a proton NMR spectrum and why?

  • Using the n+1 rule, what would you expect the splitting to be for the middle protons in the environment:
    -CH2-CH2-CH3

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