Identification of functional groups: tests, spectroscopy

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Cambridge International AS & A Level Chemistry (9701) – Analytical Techniques: Identification of Functional Groups (Topic 22)

1. Scope of the Topic (Syllabus 22)

  • Qualitative analysis – classical tests for organic and inorganic functional groups.
  • Quantitative analysis – gravimetry, titrimetry (acid‑base, redox, complexometric), electro‑chemical methods (potentiometry, coulometry).
  • Instrumental methods – IR, ¹H NMR, UV‑Vis, Mass Spectrometry, Flame/Atomic‑Absorption Spectroscopy, Chromatography (TLC, GC, HPLC).
  • Data handling – calibration curves, Beer‑Lambert law, error propagation, standard deviation, % RSD, limits of detection (LOD) and quantification (LOQ).

2. Safety Overview (selected points)

  • All reactions must be performed in a fume‑hood with gloves, goggles and a lab coat.
  • Lucas reagent – conc. HCl, corrosive.
  • Tollens’ reagent – strong oxidiser; keep away from organic vapours.
  • Iodoform test – NaOH and elemental I₂, handle with gloves.
  • Silver nitrate – photosensitive; store in amber bottle.
  • Dispose of waste according to local regulations.

3. Classical Qualitative Tests

3.1 Organic Functional‑Group Tests

Functional Group Test (reagents & conditions) Observed Result Key Balanced Reaction
Primary & secondary alcohols Lucas test – conc. HCl + ZnCl₂, 25 °C Cloudy solution or separate layer of alkyl chloride (secondary > primary, tertiary instantaneous) R‑CH₂‑OH + HCl → R‑CH₂‑Cl + H₂O
Phenol Ferric chloride test – FeCl₃ (aq), neutral pH Deep violet, blue or green colour (depends on substituents) Ph‑OH + FeCl₃ → [Fe(OPh)₃] (coloured complex)
Aldehyde Tollens’ test – Ag(NH₃)₂⁺ (aq) + NH₃, warm Silver mirror on inner wall of test tube R‑CHO + 2[Ag(NH₃)₂]⁺ + 3 OH⁻ → R‑COO⁻ + 2 Ag(s) + 4 NH₃ + 2 H₂O
Ketone (methyl‑ketone) Iodoform test – I₂ + NaOH, 25 °C Yellow precipitate of iodoform (CHI₃) RC(O)CH₃ + 3 I₂ + 4 NaOH → RCOONa + CHI₃ + 3 NaI + 3 H₂O
Carboxylic acid Sodium bicarbonate test – NaHCO₃ (aq) Effervescence (CO₂ gas) R‑COOH + NaHCO₃ → R‑COONa + CO₂(g) + H₂O
Primary amine Hinsberg test – 0.1 M HCl (soluble salt) then 0.1 M H₂SO₄ (insoluble salt) Soluble chloride salt; addition of H₂SO₄ gives a white precipitate (secondary amine) R‑NH₂ + HCl → R‑NH₃⁺Cl⁻
Ester Acidic hydrolysis – H⁺, reflux; test liberated alcohol (e.g., FeCl₃ for phenolic ester) Alcohol detected (characteristic odour or colour test) R‑COOR' + H₂O ⟶ R‑COOH + R'‑OH (catalysed by H⁺)
Alkyl halide (Cl, Br, I) Silver nitrate test – AgNO₃ (aq) in ethanol, dark White (Cl⁻), yellow (Br⁻) or brown (I⁻) precipitate of AgX R‑X + AgNO₃ → R‑NO₃ + AgX(s)
Unsaturation (C=C) Bromine water test – Br₂ in CH₂Cl₂ De‑colourisation of orange bromine solution R‑CH=CH‑R' + Br₂ → R‑CHBr‑CHBr‑R'

3.2 Inorganic (Metal‑Ion) Functional‑Group Tests

Ion / Group Test (reagents & conditions) Observed Result Key Reaction (example)
Alkali‑metal cations (Na⁺, K⁺) Flame test – dip nichrome wire in sample, place in Bunsen‑flame Na⁺ – intense yellow; K⁺ – lilac (may need cobalt glass) Na⁺ → Na⁺ (excited) → Na⁺ + hν (589 nm)
Calcium ion (Ca²⁺) Ammonium oxalate test – (NH₄)₂C₂O₄, warm White precipitate of calcium oxalate, insoluble in dilute HCl Ca²⁺ + C₂O₄²⁻ → CaC₂O₄(s)
Sulfate ion (SO₄²⁻) Barium chloride test – BaCl₂ (aq), acidified White precipitate of BaSO₄, insoluble in HCl SO₄²⁻ + Ba²⁺ → BaSO₄(s)
Carbonate / bicarbonate (CO₃²⁻ / HCO₃⁻) Acid test – dilute HCl Effervescence of CO₂ gas CO₃²⁻ + 2 H⁺ → CO₂(g) + H₂O
Ammonium ion (NH₄⁺) Nessler’s reagent – K₂HgI₄ in alkaline solution Yellow to brown colour (intensity proportional to NH₄⁺) NH₄⁺ + 2 [HgI₄]²⁻ + OH⁻ → HgNH₂I + HgI₂ + 2 I⁻ + H₂O

4. Quantitative Analytical Techniques

4.1 Gravimetric Analysis

  • Principle: Convert the analyte into a sparingly soluble, pure solid of known composition and weigh it.
  • Typical organic example: Determination of chloride as AgCl.
  • Typical inorganic example: Determination of calcium as CaCO₃ (precipitation with (NH₄)₂C₂O₄, filtration, ignition to CaO, back‑calcculation).
  • Key steps: precipitation → ageing (if required) → filtration → washing → drying/ignition → weighing.
  • Result expression: moles = mass / Mₘ; % w/w = (moles × Mₘ / sample mass) × 100.
  • Sources of error: incomplete precipitation, co‑precipitation, loss of precipitate during transfer, moisture retained in the solid.

4.2 Titrimetric (Volumetric) Methods

Type Representative Reaction Indicator / End‑point detection Typical syllabus application
Acid‑base HCl + NaOH → NaCl + H₂O Phenolphthalein (colourless → pink at pH ≈ 8.3) Acidity of carboxylic acids, neutralisation of bases.
Redox KMnO₄ + 5 Fe²⁺ + 8 H⁺ → Mn²⁺ + 5 Fe³⁺ + 4 H₂O Self‑indicator (purple MnO₄⁻ disappears) Quantification of aldehydes, Fe²⁺, H₂O₂.
Complexometric (EDTA) Ca²⁺ + EDTA⁴⁻ → [Ca‑EDTA]²⁻ Eriochrome Black T (wine‑red → blue) Hard‑water analysis (Ca²⁺, Mg²⁺).

General procedure for each titration:

  1. Standardise the titrant (known concentration) using a primary standard.
  2. Measure a known volume (or mass) of the sample.
  3. Add indicator (if required) and titrate to the end‑point.
  4. Record the volume of titrant used (V₁).
  5. Calculate the analyte concentration using C₁V₁ = C₂V₂ (or the appropriate stoichiometric factor).
  6. Repeat at least three times; compute mean, standard deviation (σ) and % RSD = (σ / mean) × 100 %.

4.3 Electro‑chemical Methods

  • Potentiometry – measurement of cell potential. Example: pH meter (glass electrode) using the Nernst equation
    E = E⁰ − (0.0591 / n) log [H⁺].
  • Coulometry – amount of electricity (Q = I t) required for complete conversion of the analyte. Used for trace halides, metals, and for standardising solutions.
  • Key concepts for all electro‑chemical methods:
    • Calibration with standards.
    • Limit of detection (LOD = 3σ / m) and limit of quantification (LOQ = 10σ / m), where σ is the standard deviation of a series of blank measurements and m is the slope of the calibration curve.
    • Error propagation: for a calculated quantity Q that depends on measured variables x, y, …, the combined standard uncertainty is
      σ_Q = √[(∂Q/∂x)²σ_x² + (∂Q/∂y)²σ_y² + …].

4.4 Data‑Handling & Quantitative Evaluation

  • Calibration curve – plot of measured response (e.g., absorbance, peak area) versus known concentration. Linear regression gives slope (m) and intercept (b).
  • Beer‑Lambert law (spectroscopic methods): A = ε c l (ε = molar absorptivity, c = concentration, l = path length).
  • Standard deviation (σ) for n replicate measurements:
    σ = √[ Σ(x_i − x̄)² / (n − 1) ].
  • % RSD (relative standard deviation): % RSD = (σ / x̄) × 100 %.
  • LOD & LOQ – calculated from the blank:
    • Measure the blank (or a series of low‑concentration standards) to obtain σ_blank.
    • LOD = 3 σ_blank / m, LOQ = 10 σ_blank / m.

Worked example (UV‑Vis LOD)

  1. Blank absorbance measured 5 times: 0.002, 0.001, 0.003, 0.002, 0.001 → mean = 0.002, σ_blank = 0.00071.
  2. Calibration curve for a coloured ion: A = 0.125 c + 0.001 (c in mmol L⁻¹, slope m = 0.125 L mmol⁻¹ cm⁻¹).
  3. LOD = 3 × 0.00071 / 0.125 ≈ 0.017 mmol L⁻¹.
  4. LOQ = 10 × 0.00071 / 0.125 ≈ 0.057 mmol L⁻¹.

5. Instrumental Methods

5.1 Infrared (IR) Spectroscopy

  • Principle: Absorption of IR radiation causes vibrational transitions; frequency ν ∝ √(k/μ) (k = bond force constant, μ = reduced mass).
  • Sample preparation: neat liquid, KBr pellet, or ATR (attenuated total reflectance).
  • Interpretation cues: position (cm⁻¹), intensity (strong/medium/weak), shape (sharp/broad), and presence of overtones.
  • Quantitative use: Beer‑Lambert law for thin films; absorbance proportional to concentration.

5.2 Nuclear Magnetic Resonance (¹H NMR) Spectroscopy

  • Resonance condition: Δν = γ B₀ / 2π (γ = gyromagnetic ratio, B₀ = magnetic field strength).
  • Key parameters:
    • Chemical shift (δ, ppm) – shielding/deshielding.
    • Integration – area ∝ number of equivalent protons.
    • Multiplicity & J‑values (Hz) – spin‑spin coupling; n + 1 rule for simple systems.
    • Coupling patterns – doublet, triplet, quartet, multiplet, etc.
  • Quantitative NMR (qNMR): Add an internal standard of known mass and purity; calculate analyte amount from the ratio of integrated areas.

5.3 UV‑Visible Spectroscopy

  • Electronic transitions:
    • σ → σ* (high energy, < 200 nm).
    • n → π* (200–400 nm, weak).
    • π → π* (200–300 nm for isolated C=C; > 300 nm for conjugated systems, aromatic rings).
  • Beer‑Lambert law: A = ε c l. Linear calibration (A vs. c) gives concentration of unknowns.
  • Typical λmax values:
    • Isolated alkene – ~170 nm.
    • Conjugated diene – 250–280 nm.
    • Aromatic ring – 260–280 nm (π → π*) and 300–320 nm (n → π*).

5.4 Mass Spectrometry (MS)

  • Ionisation methods (A‑Level relevance): Electron Impact (EI) and Electrospray Ionisation (ESI).
  • Key data: molecular ion (M⁺·), fragment ions, isotopic patterns (e.g., Cl⁺⁻⁺⁻⁺⁻ 3 : 1).
  • Useful for confirming molecular weight, identifying fragments, and distinguishing isomers.

5.5 Flame/Atomic‑Absorption Spectroscopy (FAAS / AAS)

  • Principle: Ground‑state atoms absorb light of a characteristic wavelength; absorbance measured against a blank.
  • Commonly analysed elements: Na, K, Ca, Mg, Fe, Cu, Zn.
  • Calibration: standards of known concentration; LOD typically low ppm to ppb.

5.6 Chromatographic Techniques

  • Thin‑layer chromatography (TLC) – qualitative separation; R_f = (distance travelled by compound) / (distance travelled by solvent front).
  • Gas chromatography (GC) – volatile compounds; retention time (t_R) compared with standards.
  • High‑performance liquid chromatography (HPLC) – non‑volatile or thermally labile compounds; pump, injector, column, detector (UV‑Vis or RI).
  • Quantitative use: Peak area (or height) ∝ concentration; construct calibration curve from standards.

6. Spectroscopic Signatures of Common Functional Groups

Functional Group IR (cm⁻¹) – position, intensity, shape ¹H NMR (δ ppm) – shift, multiplicity, integration, J (Hz) UV‑Vis λmax (nm) – typical range
Alcohol (R‑OH) 3200–3600 broad (strong), O–H bend ~ 1400 cm⁻¹ 1–5 ppm, broad singlet, exchangeable with D₂O; integration = 1 H —
Phenol 3200–3500 broad (strong) + aromatic C=C 1500–1600 cm⁻¹; O–H bend ~ 1240 cm⁻¹ 4–7 ppm, broad singlet, D₂O‑exchangeable —
Aldehyde 1720–1740 strong (C=O); C‑H stretch ~ 2720 cm⁻¹ (weak, aldehydic) 9–10 ppm, singlet (CHO), integration = 1 H ≈ 280 nm (n → π*)
Ketone 1710–1725 strong (C=O); no C‑H aldehydic band 2.0–2.5 ppm (α‑CH₂), multiplet depending on neighbours ≈ 280 nm (n → π*)
Carboxylic acid 2500–3300 very broad (O–H); 1710–1730 strong (C=O) 10–12 ppm, broad singlet (COOH), D₂O‑exchangeable ≈ 210 nm (π → π*)
Ester 1735–1750 strong (C=O); 1050–1300 medium (C–O stretch) 3.7–4.2 ppm (OCH₃), 4.0–4.5 ppm (OCH₂), multiplicities as per neighbouring groups ≈ 210 nm (π → π*)
Amine (primary) 3300–3500 broad (N–H); 1600–1650 medium (C=N stretch if present) 0.5–3 ppm (N‑H, exchangeable), 1–4 ppm (alkyl protons) – multiplet patterns follow n + 1 rule —
Alkyl halide (Cl, Br, I) 600–800 strong (C–X stretch) 0.8–1.5 ppm (CH₃), 1.2–1.8 ppm (CH₂); no distinctive NMR feature for halogen —
Alkene (C=C) 1620–1680 strong (C=C stretch); =C–H stretch 3020–3100 weak 4.5–6.5 ppm (vinylic H), multiplicity depends on substitution ≈ 170 nm (π → π*); conjugated systems shift to > 200 nm
Aromatic ring 1600–1580 strong (C=C aromatic); 1500–1400 medium; C–H stretch 3030 weak 6.5–8.0 ppm (aryl H), typically multiplet, integration = number of aromatic protons 260–280 nm (π → π*); 300–320 nm (n → π*)

7. Summary Checklist for the Examiner

  • Can you name at least three qualitative organic tests and two inorganic tests, write the balanced reaction, and describe the observation?
  • Do you understand the principle, steps, and error sources for gravimetric analysis of both an organic (AgCl) and an inorganic (CaCO₃) analyte?
  • Are you able to perform acid‑base, redox, and complexometric titrations, calculate concentration using stoichiometry, and report % RSD?
  • Can you construct and interpret a calibration curve, apply Beer‑Lambert law, and calculate LOD/LOQ with the given formulas?
  • Do you recognise the key IR, ¹H NMR and UV‑Vis signatures for the main functional groups listed in the syllabus?
  • Are you familiar with the basic operation and data interpretation of MS, FAAS, TLC, GC and HPLC?

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