Functional group tests, infrared and mass spectrometry

Test-tube tests you must know:

• Alkene — decolourises bromine water (orange → colourless).
• Carboxylic acid — adds to a carbonate to give effervescence (CO₂). Alcohols and phenols do not.
• Aldehyde — gives a silver mirror with Tollens’ reagent, and a brick-red precipitate with Fehling’s. A ketone gives neither.
• Primary/secondary alcohol — turns acidified dichromate from orange to green. A tertiary alcohol does not.
• Halogenoalkane — warm with NaOH(aq), acidify with HNO₃, then add AgNO₃: white (Cl), cream (Br), yellow (I).

Infrared spectroscopy — bonds absorb IR radiation and vibrate. Each bond absorbs at a characteristic wavenumber (cm⁻¹). Use the data sheet, but recognise:

• C=O: 1680–1750 cm⁻¹ — a strong, sharp peak.
• O–H (alcohol): 3230–3550 cm⁻¹ — broad. O–H (carboxylic acid): 2500–3000 cm⁻¹ — very broad.
• N–H (amine): 3300–3500 cm⁻¹.
• The complex region below 1500 cm⁻¹ is the fingerprint region, unique to each compound — it is matched against a database to make an exact identification.

Mass spectrometry — the peak at the highest m/z is the molecular ion (M⁺) peak, and it gives the M_r directly. The molecular ion also fragments: a peak at m/z 15 is CH₃⁺, 29 is C₂H₅⁺ or CHO⁺, and 43 is C₃H₇⁺ or CH₃CO⁺.

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NMR: ¹³C and ¹H

Nuclei with an odd mass number (¹H, ¹³C) behave like tiny magnets. In a strong magnetic field they can align with or against it; a radio-frequency photon of exactly the right energy flips them. The frequency required depends on the electron environment of the nucleus — so chemically different atoms absorb at different frequencies.

Chemical shift, δ, is measured in parts per million (ppm) relative to a standard. The more electronegative the neighbouring atoms, the more the nucleus is deshielded and the larger the shift.

¹³C NMR — the easy one. The number of peaks = the number of different carbon environments. That is essentially all you need (there is no splitting to worry about).

¹H NMR — three pieces of information from one spectrum:

• Number of peaks → the number of different proton environments.
• Integration (the relative area under each peak) → the relative number of hydrogens in each environment.
• Splitting pattern → how many hydrogens are on the adjacent carbon(s).

Why TMS ((CH₃)₄Si) is the standard, defined as δ = 0: all 12 of its hydrogens are equivalent, so it gives a single sharp peak; it is chemically inert and non-toxic; it is volatile, so it is easily removed from the sample; and it absorbs upfield of virtually every other proton, so its peak does not overlap with the sample.

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Sort each proton by its chemical shift

Tap a proton environment, then tap the chemical shift range it appears in.

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TLC, column and gas chromatography

All chromatography separates a mixture by the balance between two phases:

• the stationary phase (the silica or alumina on a TLC plate; the packing in a column; the liquid coating in a GC column) — a component that adsorbs strongly to it moves slowly;
• the mobile phase (the solvent, or the carrier gas) — a component that is more soluble in it moves quickly.

Thin-layer chromatography (TLC) — measure the retention factor:

Colourless spots are visualised under UV light (using a fluorescent plate) or with a locating agent such as ninhydrin for amino acids.

Gas chromatography (GC) — the sample is vaporised and carried by an inert gas through a long column. Each component has a characteristic retention time. The area under each peak is proportional to the amount of that component, so GC is quantitative.

The limitation, and the fix: two different compounds can have the same R_f or the same retention time, so chromatography alone can never prove an identity. That is why it is coupled to a second technique — GC–MS, where each separated component goes straight into a mass spectrometer for a definitive identification.

Match the technique to what it tells you

Tap a technique on the left, then what it tells you on the right.

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Reading a spectrum without losing marks

• The M⁺ peak is the one at the HIGHEST m/z (ignore the tiny M+1 peak from ¹³C). It gives the M_r directly.
• The n + 1 rule counts the hydrogens on the ADJACENT carbon(s) — not the hydrogens in the peak itself. A CH₃ next to a CH₂ is a triplet (2 + 1), even though it has three hydrogens.
• Integration gives RATIOS, not absolute numbers. A 2 : 3 ratio could be 2 : 3 or 4 : 6 — use the M_r to decide.
• Equivalent protons do not split each other. The three hydrogens of a CH₃ are in the same environment, so they give one peak.

The four reasons TMS is the standard — a single sharp peak (all 12 H equivalent), non-toxic and inert, volatile (easily removed from the sample), and it absorbs upfield of virtually everything else, so its peak does not overlap. Quote at least two.

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The big ideas to know

That is AQA 3.3.6, 3.3.15 and 3.3.16 — and the last of the 21 topics. Press Finish.