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4.8Unit 4. Rates, Equilibria and Further Organic Chemistry

Organic Analytical Techniques — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

15 min read 24 parts Lesson 8 of 56 1 quiz

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4.8 Organic Analytical Techniques — high resolution MS, carbon-13 NMR and low resolution proton NMR — IAL EDEXCEL LINEAR (CHEMISTRY) notes: Use accurate masses to deduce molecular formulae, count carbon and proton environments from 13C and low-resolution 1H NMR, and use TMS, chem…

  • High resolution mass spectrometry
  • Chemical shift
  • Chemical environment
  • Integration
  • Deuterated solvent
  • Chromatography

Part 1 of 7High resolution mass spectrometry

High resolution mass spectrometry
At AS level mass spectrometry gave relative atomic mass from isotopic abundance, molecular formulae from molecular ion peaks and structures from fragments.
High resolution instruments measure m/z to four or more decimal places.
Atomic masses are not whole numbers (except 12C), so each molecular formula has a unique accurate mass.
ElementAccurate Ar
Hydrogen1.0078
Carbon12.0000
Nitrogen14.0031
Oxygen15.9949
High resolution mass spectrometry
Calculate the accurate mass of each candidate formula and match it to the measured M+.
  • C3H8 (44.0626), CO2 (43.9898) and C2H4O (44.0262) all have nominal Mr 44 but different accurate masses.
  • A peak at 44.0262 fits C2H4O.
It gives a molecular formula but not the structure; propanal and propanone have the same formula, so fragments are needed for the structure.
If no formula matches exactly, check the calculation and choose the closest.

Part 2 of 7Principles of NMR spectroscopy

1
Nuclei with spin placed in a strong magnetic field absorb radio-frequency radiation; atoms with odd mass numbers,
  • such as 1H and 13C, show NMR signals.
2
Samples are measured against tetramethylsilane (TMS), Si(CH3)4, the reference defined as 0 ppm.
  • It gives one strong, sharp peak because its 12 hydrogens (and 4 carbons) are equivalent.
  • It is non-toxic, does not react with the sample and is easily removed because of its low boiling point.
3
The position of a signal relative to TMS is the chemical shift, measured in ppm.
4
Samples are dissolved in a solvent without hydrogen, such as CDCl3 (deuterated) or CCl4, so the solvent gives no proton signal.
5
Adding D2O removes an O-H or N-H signal from a proton spectrum.

Part 3 of 7Carbon-13 NMR

1
About 1% of carbon is 13C, which shows signals; 12C does not.
2
Each peak represents a different carbon environment, so the number of peaks equals the number of non-equivalent carbons.
3
Features
  • Sharp single signals with no complicated splitting.
  • Peak height is not proportional to the number of carbons in that environment.
  • Chemical shift depends on neighbouring atoms; electronegative atoms increase δ.
4
Symmetry reduces the number of peaks.
  • Propanone has 2 peaks and propanal has 3.
  • Counting the peaks is the first step;
    • it separates isomers such as the three dihydroxybenzenes (1,3-dihydroxybenzene gives 4 peaks).
Carbon typeApproximate shift / ppm
Alkyl C-C5-40
C-O50-90
C=C and aromatic100-160
C=O in acids and esters160-185
C=O in aldehydes and ketones190-220

Part 4 of 7Low resolution proton NMR

1
Hydrogens in different chemical environments (different neighbouring atoms) absorb at slightly different field strengths,
  • giving different chemical shifts.
2
The number of peaks equals the number of different hydrogen environments.
3
Equivalent hydrogens give one peak.
  • All four hydrogens in ClCH2CH2Cl are equivalent, giving one peak.
  • 2-methylpropane has two environments: three equivalent CH3 groups and one CH.
4
Ethanol, CH3CH2OH, has three environments: CH3 (about 1.2 ppm), CH2 (about 3.7 ppm) and OH (about 5.4 ppm).
EnvironmentApproximate shift / ppm
Alkyl0.5-2
Next to C=O2-3
Next to O3.3-4.5
Aromaticabout 7
Aldehyde9-10

Part 5 of 7Integration

1
The area under each peak is proportional to the number of equivalent hydrogens causing it.
2
The integration trace gives the ratio of hydrogens in each environment.
3
Ethanol gives CH3 : CH2 : OH = 3 : 2 : 1.
4
ClCH2COOCH3 gives CH2 : CH3 = 2 : 3.
5
The height of a peak shows intensity of absorption, but area gives the number of protons.

Part 6 of 7Combining techniques to identify a compound

1
High resolution MS gives the accurate M+ and molecular formula.
2
IR identifies functional groups (C=O, O-H, N-H).
3
13C NMR counts carbon environments and gives shifts.
4
1H NMR gives environments and the ratio of hydrogens.
5
Splitting patterns give neighbouring hydrogens.
6
Assemble fragments into a structure and check against all the data.

Part 7 of 7More from the specification

1
Gas chromatographic retention times
  • Gas chromatographic retention times comparing peak elution durations against standard references relies on characteristic retention times to identify organic components.
2
Mobile phase
  • The solvent or gas that moves through the stationary phase.
3
Column chromatography
  • Separation using a column of solid stationary phase and liquid mobile phase.
4
Retention time
  • The time for a component to pass through the column.
5
Chromatography
  • Method separating substances by their different solubilities in a solvent.
6
Compound
  • Two or more elements chemically combined in a fixed ratio.
7
Retention
  • Extent to which a component sticks to the stationary phase.
8
Mixture
  • Two or more substances mixed but not chemically combined.
9
Stationary phase
  • The solid or liquid phase fixed in place in chromatography.

Key terms

TermMeaning
High resolution mass spectrometryMass spectrometry measuring m/z to four or more decimal places so formulae of equal nominal mass can be distinguished.
Chemical shiftThe position of an NMR signal in ppm relative to TMS.
TMSTetramethylsilane, the reference compound defined as 0 ppm.
Chemical environmentA set of equivalent nuclei that experience the same magnetic field.
IntegrationThe relative area under a proton NMR peak, proportional to the number of protons.
Deuterated solventA solvent such as CDCl3 that gives no proton signal.

Avoid these mistakes

1
Counting hydrogen atoms rather than environments as the number of 1H NMR peaks.
2
Reading the height of a 13C peak as the number of carbons — it is not proportional.
3
Using a hydrogen-containing solvent for NMR — it would add a large proton signal.
4
Assuming accurate mass gives the structure — it gives only the molecular formula.

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