2.9Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Organic Chemistry: Techniques & Spectra — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
10 min read 19 parts Lesson 9 of 53 Unit quiz
Curriculum Equation Key Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Lesson summary
2.9 Organic Chemistry: Techniques & Spectra — mass spectra and infrared spectroscopy — IAL EDEXCEL LINEAR (CHEMISTRY) notes: Interpret mass spectra, including molecular ion, isotope and fragment peaks, and use infrared absorptions to identify functional groups.A small sample is b…
- Relative abundance
- Molecular ion peak
- Fragmentation
- Wavenumber
- Fingerprint region
- Distillation
Part 1 of 9Principle of mass spectrometry
A small sample is bombarded with high-energy electrons, which remove an electron to form a positive molecular ion with one unpaired electron.
The molecular ion can fragment into new ions, molecules and radicals: M+ → X+ + Y•.
- Only the positive fragments are detected.
The fragment ions are accelerated by an electric field and deflected into the detector according to their mass-to-charge ratio, m/z.
Most ions have a 1+ charge, so m/z equals the mass.
- An ion of mass 16 with a 2+ charge has m/z = 8.
Smaller and more highly charged ions are deflected more, so a small m/z can mean a small fragment, a high charge or both.
Base peak
- Base peak the peak of the most abundant ion.
Part 2 of 9Isotopes and relative abundance
Isotopes
- Isotopes atoms of the same element with the same number of protons but different numbers of neutrons.
Relative abundance
- Relative abundance the proportion of each isotope in a natural mixture, shown by peak heights.
Chlorine is 75% 35Cl and 25% 37Cl; boron is 19.9% 10B and 80.1% 11B.
Part 3 of 9Molecular ion and M+1 peak
Molecular ion and M+1 peak
The peak with the highest m/z (apart from M+1) is the molecular ion, giving the relative molecular mass.
The M+1 peak is due to 13C, which has about 1.1% natural abundance.
- The more carbon atoms in the molecule, the taller the M+1 peak.
- Hexane has a taller M+1 peak than ethane.
The M+1 peak is not the molecular ion; for example, pent-1-ene (C5H10) has M = 70 and but-1-ene (C4H8) has M = 56.
Part 4 of 9Isotope patterns of halogen compounds
Isotope patterns of halogen compounds
Halogenoalkanes show several peaks around the molecular ion because of halogen isotopes.
Chlorine compounds show M and M+2 peaks in the ratio 3:1.
Bromine compounds show M and M+2 peaks in the ratio 1:1.
- CH3Br shows equal peaks at m/z 94 and 96.
Part 5 of 9Fragmentation
1
Different compounds can have the same molecular mass, so fragments are used to work out the structure.
2
The difference between M+ and a fragment shows the neutral group lost, e.g. 15 = CH3.
3
Loss of small molecules gives peaks at M−18 (H2O), M−28 (CO) and M−44 (CO2).
4
Stable cations (acylium ions, tertiary carbocations) give tall peaks.
| m/z | Fragment ion |
|---|---|
| 15 | CH3+ |
| 29 | C2H5+ or CHO+ |
| 31 | CH2OH+ |
| 43 | C3H7+ or CH3CO+ |
| 45 | COOH+ |
Fragmentation
Alkanes fragment by breaking C-C bonds.
Alcohols often lose water, giving a peak at M−18, and a peak at 31 from CH2OH+.
- Propan-1-ol fragments to m/z 59 (loss of H•), 42 (loss of H2O), 31 (loss of •C2H5) and 29 (loss of •CH2OH).
Isomers can be distinguished by fragments: propanone gives a strong peak at 43 (CH3CO+) and 15, whereas propanal gives m/z 29.
Part 6 of 9Infrared spectroscopy
Infrared spectroscopy
Infrared radiation is absorbed by covalent bonds, which vibrate by stretching and bending at specific frequencies.
Absorptions are reported as wavenumber (cm−1), the reciprocal of wavelength, and have characteristic width and intensity.
Hydrogen bonding makes O-H absorptions broad, whereas C=O absorptions are strong and sharp.
| Bond | Wavenumber / cm-1 | Appearance |
|---|---|---|
| O-H (alcohol) | 3200-3550 | Broad |
| O-H (carboxylic acid) | 2500-3300 | Very broad |
| N-H | 3300-3500 | Medium |
| C-H | 2850-3100 | Strong, sharp |
| C=O | 1680-1750 | Strong, sharp |
| C=C | 1620-1680 | Medium |
- The fingerprint region below about 1500 cm−1 is unique to each compound and is compared with a reference spectrum.
Part 7 of 9Using IR spectra to identify functional groups
1
Look first for a broad O-H, a strong C=O near 1700 cm−1 and N-H.
2
An alcohol shows O-H but no C=O.
3
A carboxylic acid shows both a very broad O-H and C=O.
4
An aldehyde, ketone or ester shows C=O only.
5
Propanone shows a sharp absorption near 1710 cm−1; propan-2-ol shows a broad absorption at 3200–3550 cm−1.
6
IR can confirm oxidation of an alcohol: the O-H disappears and a C=O appears.
7
Absorptions can overlap, so IR is combined with mass spectrometry to identify an unknown compound.
Part 8 of 9More from the specification
1Anhydrous chemical drying agents
- Anhydrous chemical drying agents adding anhydrous magnesium sulfate or calcium chloride removes trace water from organic extracts prior to distillation.
2Heating under reflux
- Heating under reflux a condenser returns vapour to the flask so a reaction mixture can boil for a long time without loss.
3Distillation
- Distillation separates a product by boiling point, collecting the fraction at the temperature that corresponds to it.
4Separating funnel
- Separating funnel separates immiscible layers, with the organic layer run off from the aqueous layer.
5Drying
- Drying an anhydrous salt such as magnesium sulfate removes traces of water from the organic product.
6Purity check
- Purity check a sharp boiling or melting point indicates a pure product; spectra confirm structure.
7Separating funnel
- Apparatus for separating two immiscible liquids.
8Drying agent
- An anhydrous salt that removes water from an organic liquid.
9Primary alcohol
- An alcohol in which the C–OH carbon is attached to one other carbon.
10Boiling
- Liquid to gas throughout the liquid at the boiling point.
11Distillation
- Separating a liquid from a solution by boiling and condensing the vapour.
12Condenser
- Apparatus that cools gas back to liquid.
13Reflux
- Heating with a condenser to stop volatile liquids escaping.
14Temperature
- A measure of the average kinetic energy of the particles.
15Fragmentation
- Breaking of the molecular ion into a smaller cation and a radical.
16Liquid
- A state with a fixed volume that flows, particles sliding past each other.
Part 9 of 9More on this topic
- Alcohols oxidise according to their class.
Key terms
| Term | Meaning |
|---|---|
| Molecular ion peak | The peak at highest m/z (ignoring M+1), giving the relative molecular mass. |
| Base peak | The peak of the most abundant ion. |
| Fragmentation | Breaking of the molecular ion into a smaller cation and a radical. |
| m/z | The mass-to-charge ratio of an ion. |
| Wavenumber | The reciprocal of wavelength in cm-1, used to report IR absorptions. |
| Fingerprint region | The IR region below about 1500 cm−1 that is unique to each compound. |
| Isotope | Atoms of an element with the same protons but different neutrons. |
Avoid these mistakes
1
Taking the M+1 peak as the molecular ion — the molecular ion is the highest m/z apart from the M+1 peak.
2
Expecting a 3:1 pair for bromine compounds — chlorine gives 3:1 and bromine gives 1:1.
3
Forgetting that only cations are detected — the radical fragment does not appear.
4
Describing the O-H absorption of a carboxylic acid as sharp — it is very broad.
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