Edexcel International A Level Chemistry: Mass spectra and infrared spectroscopy (YCH11) – Revision Notes

These notes condense Topic 10D, Mass spectra and IR, of the Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry (XCH11/YCH11) specification, Issue 1, September 2017, namely outcomes 10.21, 10.22 and 10.23 (Core Practical 8). It is assessed in the IAS written examination for Unit 2: Energetics, Group Chemistry, Halogenoalkanes and Alcohols. Each idea is explained in full, with five worked examples, in the Topic 10D study guide. When these notes feel familiar, move on to the spectra practice set.

Also useful: the Edexcel IAL Chemistry course page, the outcome checklist, these notes on how the mass spectrometer works, and the free diagnostics to find gaps quickly.

10.21 Mass spectra: key definitions

• Molecular ion, M⁺: the ion formed when a whole molecule loses one electron. M(g) → M⁺(g) + e⁻.
• m/z: mass-to-charge ratio. For a 1+ ion, m/z is simply the ion’s mass.
• Molecular ion peak: found at the largest m/z value, if you ignore the tiny M+1 peak from carbon-13. Its m/z gives Mr.
• Fragmentation: M⁺ splits into a positive ion and a radical. M⁺ → X⁺ + Y•. Only X⁺ is detected.
• Base peak: the tallest peak, given 100% relative abundance. It is often a fragment.

Fragment ions to recognise

m/z	Ion (or ions)
15	CH₃⁺
29	C₂H₅⁺ or CHO⁺
31	CH₂OH⁺
43	C₃H₇⁺ or CH₃CO⁺
45	COOH⁺, or CH₃CHOH⁺ from a secondary alcohol
57	C₄H₉⁺ or C₂H₅CO⁺

Gaps between peaks

Gap from M⁺	Radical lost
15	CH₃•
17	OH•
29	C₂H₅• or CHO•
45	COOH•

Halogen patterns

Halogen	Isotopes	M⁺ pattern
one Cl	³⁵Cl and ³⁷Cl, about 3 : 1	two peaks 2 apart, heights about 3 : 1
one Br	⁷⁹Br and ⁸¹Br, about 1 : 1	two peaks 2 apart, heights about 1 : 1

Any fragment that keeps the halogen also shows the pair. A fragment that has lost it is a single peak.

Method in steps: suggesting a structure from a mass spectrum

1. Find M⁺ (highest m/z, ignoring M+1). Write down Mr.
2. Check for a pair of peaks two units apart (Cl or Br).
3. Use any functional group information (from IR or the question) to write a general formula. Solve for the number of carbons.
4. List the main fragment peaks. For each, work out the ion and the mass lost.
5. Draw the isomers that fit the formula. Ask which bonds each one can break and which ions that would give.
6. Pick the isomer whose expected fragments match. Write “suggest” and give the reason.

Small worked reminder. M⁺ = 86 with a peak at 57: 86 − 57 = 29, so C₂H₅• or CHO• has been lost.

10.22 Infrared spectra: key ideas

• Bonds absorb IR radiation at the frequency at which they stretch. Each bond type has a typical range.
• The x-axis is wavenumber in cm⁻¹; absorptions are dips in transmittance.
• Hydrogen bonding makes O–H absorptions broad.
• Below about 1500 cm⁻¹ lies the fingerprint region, used to match a whole spectrum to a known compound.

IR ranges used in these notes

Questions supply wavenumber data; use the values given there. These are the typical values used across this unit’s pages:

Bond and compound type	Range (cm⁻¹)	Look for
C–H, alkanes	2962–2853	present in almost everything
=C–H, alkenes	3095–3010	just above 3000
C–H, aldehydes	2900–2820 and 2775–2700	the 2775–2700 band
C=C, alkenes	1669–1645	weak to medium
O–H, alcohols	3550–3230	broad
O–H, carboxylic acids	3300–2500	very broad
C=O, aldehydes	1740–1720	strong
C=O, ketones	1720–1700	strong
C=O, carboxylic acids	1725–1700	strong
C–Cl	800–600	low end
C–Br	600–500	low end
C–I	about 500	low end
N–H, amines	3500–3300	sharper than O–H

Method in steps: reading an IR spectrum

1. Strong peak at 1750–1680 cm⁻¹? Then C=O is present.
2. Very broad 3300–2500 band with C=O? Carboxylic acid.
3. C=O, no O–H, but a peak at 2775–2700? Aldehyde. No such peak? Ketone.
4. Broad 3550–3230 band with no C=O? Alcohol.
5. Peaks at 1669–1645 and 3095–3010? Alkene.
6. Sharper peak at 3500–3300 with no C=O? Consider an amine.
7. Peaks below 800 cm⁻¹ that fit the C–X ranges? Possible halogenoalkane; confirm with a mass spectrum or a silver nitrate test.

Small worked reminder. Propan-1-amine, CH₃CH₂CH₂NH₂, should show N–H at 3500–3300 and C–H at 2962–2853 cm⁻¹, with no C=O and no broad O–H.

Must-know distinctions

• Molecular ion peak vs base peak. M⁺ is at the highest m/z and gives Mr. The base peak is the tallest and is often a fragment.
• Alcohol O–H vs acid O–H. Alcohol: broad, 3550–3230. Acid: very broad, 3300–2500, running into the C–H peaks, and always with C=O.
• Aldehyde vs ketone. Both have C=O near 1700–1740. Only the aldehyde has C–H at 2775–2700. In a mass spectrum, CHO⁺ at 29 fits an aldehyde and CH₃CO⁺ at 43 fits a methyl ketone.
• Chlorine vs bromine. Both give peaks two apart. Chlorine is about 3 : 1; bromine is about 1 : 1.
• Primary vs secondary alcohol in a mass spectrum. CH₂OH⁺ at 31 suggests a primary alcohol. CH₃CHOH⁺ at 45 suggests a secondary alcohol with an OH on carbon 2.
• IR vs MS. IR identifies bonds and so functional groups. MS gives Mr and the carbon skeleton. Isomers with the same groups often need MS; isomers with different groups are easier by IR.

10.23 Core Practical 8 in one paragraph

This practical is the analysis of some inorganic and organic unknowns. Recall the Unit 1 and Unit 2 tests: bromine water (C=C), PCl₅ (–OH, misty fumes), acidified dichromate(VI) (orange to green with primary and secondary alcohols and aldehydes), Benedict’s or Fehling’s (red precipitate with aldehydes), sodium hydrogencarbonate (fizzing with acids), silver nitrate (halides), flame tests, and the tests for carbonate, sulfate and ammonium ions. Link every observation to a conclusion, and use spectra to confirm it.

Quick self-test

1. Chloromethane, CH₃Cl, is analysed. Give the m/z values of its two molecular ion peaks and their approximate height ratio.

2. Give two different ions that could cause a peak at m/z 29.

3. An IR spectrum shows a strong peak at 1715 cm⁻¹ and a very broad band from 3300 to 2500 cm⁻¹. Name the type of compound.

4. A compound has M⁺ at 86 and a large peak at 57. State the mass lost and suggest what it was.

5. Which bond absorbs at 3095–3010 cm⁻¹?

6. Explain why the O–H absorption of an alcohol is broad.

7. Bromoethane, C₂H₅Br, is analysed. Give the m/z values of its molecular ion peaks and describe their relative heights.

8. An IR spectrum shows peaks at 1730 and 2720 cm⁻¹ but nothing broad above 3000 cm⁻¹. Name the type of compound.

9. Hexan-1-ol and hexan-2-ol give similar IR spectra. Suggest one mass spectrum peak that would distinguish them.

10. Write an equation for the formation of the molecular ion of methanol.

Answers

1. m/z 50 (CH₃³⁵Cl) and m/z 52 (CH₃³⁷Cl), in a ratio of about 3 : 1.

2. C₂H₅⁺ and CHO⁺.

3. A carboxylic acid (C=O plus the very broad acid O–H).

4. 86 − 57 = 29, so C₂H₅• or CHO• was lost.

5. The C–H bond in an alkene (=C–H).

6. Hydrogen bonding between alcohol molecules means O–H bonds in slightly different surroundings absorb at a spread of wavenumbers.

7. m/z 108 (C₂H₅⁷⁹Br) and m/z 110 (C₂H₅⁸¹Br), of almost equal height.

8. An aldehyde: C=O at 1730 and aldehyde C–H at 2720, with no O–H.

9. Hexan-1-ol can give CH₂OH⁺ at m/z 31; hexan-2-ol can give CH₃CHOH⁺ at m/z 45.

10. CH₃OH(g) → CH₃OH⁺(g) + e⁻

Where marks are usually lost

• Calling the tallest peak the molecular ion when it is the base peak.
• Giving a fragment with no positive charge, or giving the radical a charge.
• Writing a fragmentation equation that does not balance in mass. The ion plus the radical must add up to M⁺.
• Stating “a peak at 1700 shows a ketone” without ruling out an aldehyde (2775–2700) and an acid (3300–2500).
• Confusing the alcohol O–H range with the acid O–H range.
• Quoting a wavenumber without naming the bond responsible, or naming a functional group without quoting the evidence.
• Missing the two-unit pair that shows Cl or Br, or swapping the 3 : 1 and 1 : 1 ratios.
• Giving one structure when the data fit more than one isomer, instead of saying which isomers fit and what would separate them.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry (XCH11/YCH11) specification, Issue 1, September 2017 (Pearson Education Limited). Mass spectra and IR is part 10D of Topic 10, outcomes 10.21 to 10.23, in Unit 2.