Analytical techniques

• Infrared shows bonds; mass spectra show masses. Find M⁺ by position, never by height — and a chlorine compound’s M⁺ is never at 64.5.
• In an answer, quote the row: “O–H, hydroxy, 3200–3650 cm⁻¹, observed at 3350.” The range is part of the evidence.

• 22.1.1 I can analyse — analyse an infrared spectrum of a simple molecule to identify functional groups (see the Data section for the functional groups required)
• 22.2.1 I can analyse — analyse mass spectra in terms of m/e values and isotopic abundances (knowledge of the working of the mass spectrometer is not required)
• 22.2.2 I can calculate — calculate the relative atomic mass of an element given the relative abundances of its isotopes, or its mass spectrum
• 22.2.3 I can deduce — deduce the molecular mass of an organic molecule from the molecular ion peak in a mass spectrum
• 22.2.4 I can suggest — suggest the identity of molecules formed by simple fragmentation in a given mass spectrum
• 22.2.5 I can deduce — deduce the number of carbon atoms, n, in a compound using the [M + 1]⁺ peak and the formula n = (100 × abundance of [M + 1]⁺ ion) ÷ (1.1 × abundance of M⁺ ion)
• 22.2.6 I can deduce — deduce the presence of bromine and chlorine atoms in a compound using the [M + 2]⁺ peak

Units and significant figures are marked. The syllabus states that failure to quote units, the inclusion of units in quantities defined as ratios, and answers given to an inappropriate number of significant figures are all liable to be penalised. Give a calculated answer to the same number of significant figures as the least precise data, or one more; keep full precision in the working and round only at the end. A fifth of the qualification is experimental: Papers 3 and 5 test AO3 only, and their questions may be set in contexts outside the syllabus content, so this chapter’s practical focus is set out as data handling and evaluation; topic 22 has no laboratory procedure of its own.

• “The tallest peak is the molecular ion, and chloroethane’s is at 64.5.” Correct M⁺ is found by position, not height. It is the peak at the highest m/e apart from the small isotope peaks just above it, and it is often small because molecular ions break up. The tallest peak is the base peak, usually a fragment. And no ion has an averaged mass: chloroethane shows C₂H₅³⁵Cl⁺ at 64 and C₂H₅³⁷Cl⁺ at 66. The Data-section Mr, 64.5, is for mole calculations; there is no peak at 64.5.
• “A trough at 1715 cm⁻¹ means a ketone.” Correct 1715 lies inside both C=O ranges: carbonyl and carboxyl 1670–1740, and ester 1710–1750. Decide from the other troughs: a very broad O–H at 2500–3000 means an acid; a C–O at 1040–1300 with no O–H means an ester; neither means an aldehyde or ketone.
• “The acid’s O–H is at 3300, like an alcohol’s.” Correct The Data section gives two O–H rows: hydroxy 3200–3650 (alcohol, broad) and carboxyl 2500–3000 (acid, very broad, swallowing the C–H). Quote the row that matches the trough.
• “The [M + 1]⁺ peak shows chlorine.” Correct [M + 1]⁺ is ¹³C and is small, a few per cent of M⁺. A halogen gives [M + 2]⁺, and it is large: a third of M⁺ for one Cl, about equal to it for one Br.
• “Ar of chlorine = (35 + 37) ÷ 2 = 36.” Correct A weighted mean: (35 × 75.8 + 37 × 24.2) ÷ 100 = 35.5. The answer must sit between the isotopic masses and nearer the more abundant one.
• “A mass-spectrum answer starts with how the instrument works.” Correct Outcome 22.2.1 says knowledge of the working of the mass spectrometer is not required. Say only that the sample is turned into positive ions which are sorted by m/e, and spend the time reading the spectrum.
• “The tallest peak is the molecular ion.” Repair The tallest peak is the base peak, usually a fragment. M⁺ is the peak at the highest m/e apart from the isotope peaks, and it is often small.
• “Chloroethane’s M⁺ is at 64.5.” Repair No ion has the averaged mass. The spectrum shows 64 (³⁵Cl) and 66 (³⁷Cl) in 3 : 1. Mr 64.5 is for mole calculations only.
• “Ar of chlorine = (35 + 37) ÷ 2 = 36.” Repair A weighted mean: (35 × 75.8 + 37 × 24.2) ÷ 100 = 35.5.
• “Ar = Σ(mass × height) ÷ 100”, for peaks of heights 100, 5.1 and 3.4. Repair Heights scaled to a base peak do not total 100; divide by their sum, 108.5. Dividing by 100 gives an Ar outside the range of the isotopic masses, which is impossible.
• “The [M + 1]⁺ peak shows chlorine.” Repair [M + 1]⁺ is ¹³C and is small; a halogen gives [M + 2]⁺, large: 3 : 1 for Cl, 1 : 1 for Br.
• “n = abundance of [M + 1]⁺ ÷ abundance of M⁺ × 1.1.” Repair n = (100 × abundance of [M + 1]⁺ ion) ÷ (1.1 × abundance of M⁺ ion). The 100 is on top and the 1.1 is in the denominator.
• “M − 18 is loss of OH.” Repair 18 is H₂O; 17 is •OH; 15 is CH₃•; 29 is C₂H₅•.
• “m/e 43 is C₃H₇⁺.” Repair It may be CH₃CO⁺. The infrared spectrum decides: a C=O absorption at 1670–1740 cm⁻¹ points to CH₃CO⁺, and no C=O to C₃H₇⁺. A missing peak at 29 does not decide on its own: chloropropane’s base peak at 43 is C₃H₇⁺, with no peak at 29.
• “When M⁺ breaks up, both pieces give peaks.” Repair Only the ion is detected. The radical is neutral and gives no peak, which is why losses (M − 15 and so on) are read as differences.
• “A trough at 1715 cm⁻¹ means a ketone.” Repair 1670–1740 covers aldehydes, ketones and carboxylic acids, and overlaps the ester range 1710–1750. Look for O–H (acid), C–O with no O–H (ester), or neither (aldehyde or ketone).
• “The carboxylic acid’s O–H is at 3300 cm⁻¹.” Repair The carboxyl O–H is 2500–3000, very broad; 3200–3650 is the hydroxy O–H of an alcohol.
• An infrared axis drawn from 500 on the left to 4000 on the right, and “the infrared spectrum shows the compound is ethanol”. Repair 4000 on the left, decreasing to 500, with absorptions pointing down from 100% transmittance. And infrared shows bonds — an O–H, a C–O, a C–H — not a molecule; the molecular ion and the fragments identify which alcohol.

• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.
• Interleave with the chapters that use this one. Return to spectrum reading from topic 16 (can infrared show that ethanol has been oxidised, and how far?), topic 17 (why infrared cannot stand in for Tollens’ or Fehling’s reagent), topic 21 (which spectra would confirm the product of a two-step synthesis?) and topic 37, the A Level twin, which builds on this whole chapter. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Chemistry 9701 has five components. Topic 22 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E.
• A multiple-choice item (Paper 1) gives a list of absorptions, a few peaks or a pair of heights and asks which compound, which ion or which halogen fits. A structured question (Paper 2) gives a spectrum, or both spectra of one unknown, and asks you to identify the bonds with their Data-section ranges, deduce Mr, the carbon count or the halogen, suggest the fragment ions, and deduce the structure with the reasoning shown.
• Four calculations: Ar from percentages or peak heights (and backwards to an abundance); Mr checked against a proposed formula; n from the [M + 1]⁺ formula, which the syllabus prints; and an M⁺ : [M + 2]⁺ ratio. The infrared table and the Ar values are in the Data section and are supplied; the isotope abundances are given in the question when they are needed.
• Spectrometers are not on the Paper 3 apparatus list, so no Paper 3 question asks you to record a spectrum. What transfers is reading a scale to half its smallest division, the percentage uncertainty of a small peak, significant figures, and, in Paper 5 style, evaluating whether a technique can confirm a product on its own.
• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.