Theory — Functional Groups and Characteristic Tests
A functional group is a specific arrangement of atoms within an organic molecule that gives the molecule its characteristic chemical behaviour. Two molecules that share the same functional group undergo the same kinds of reactions, regardless of the rest of the carbon skeleton. Classical qualitative tests exploit this: each reagent reacts selectively with one (or a few) functional groups, producing an observable change — a colour shift, a precipitate, a gas, heat, or a characteristic smell.
Functional groups covered in this lab
Reagents used and what they detect
| Test | Reagent | Detects | Positive observation |
|---|---|---|---|
| Bromine water | Br₂ in H₂O (orange) | C=C, C≡C, phenol | Orange colour decolorises (alkene/alkyne); phenol gives white ppt as well |
| Baeyer (KMnO₄) | Cold dilute KMnO₄ (purple) | C=C, C≡C, aldehyde, 1°/2° alcohol | Purple → brown MnO₂ suspension |
| Tollens' | Ag(NH₃)₂⁺ in NH₃ | Aldehyde (and terminal alkyne) | Silver mirror on inner wall of tube |
| Fehling's | Cu²⁺/tartrate (deep blue) | Aliphatic aldehyde | Blue → brick-red Cu₂O ppt |
| 2,4-DNP (Brady's) | 2,4-dinitrophenylhydrazine | Aldehyde or ketone | Yellow → orange → red ppt |
| Iodoform | I₂ in NaOH | CH₃–CO–R or CH₃–CH(OH)–R | Pale yellow CHI₃ ppt + antiseptic smell |
| Lucas | ZnCl₂ in conc. HCl | Distinguishes 1° / 2° / 3° alcohols | 3°: cloudy in seconds · 2°: cloudy in 5–10 min · 1°: no change at RT |
| Jones | CrO₃ / H₂SO₄ (orange) | 1°/2° alcohols, aldehydes | Orange → blue-green Cr³⁺ |
| NaHCO₃ | Saturated aqueous | Carboxylic acid | Vigorous effervescence (CO₂) |
| Esterification | ROH + conc. H₂SO₄, warm | Carboxylic acid (and ester already) | Sweet / fruity smell |
| FeCl₃ (neutral) | Dilute ferric chloride | Phenol (and enol) | Deep violet / purple colour |
| Hinsberg | PhSO₂Cl / NaOH | Distinguishes 1° / 2° / 3° amines | 1°: clear → ppt on acidification · 2°: ppt remains in base · 3°: no reaction |
Reading the results
A single positive test is rarely definitive — a combination of positive and negative tests is what pins down a functional group. For example, a sample that gives a positive 2,4-DNP (ppt) and a positive Tollens' (silver mirror) must be an aldehyde. A sample that gives a positive 2,4-DNP but a negative Tollens' must be a ketone. Recording every observation carefully — even the negative ones — is what makes qualitative analysis reliable.
Material Safety Data Sheets (MSDS / SDS)
Before using any chemical reagent, you must consult its Safety Data Sheet (SDS) — the modern term for what was historically called an MSDS. An SDS is a standardised 16-section document that summarises everything you need to handle a chemical safely. The SDS is required by OSHA's Hazard Communication Standard for every laboratory chemical and must be readily accessible to all lab users.
The 16 sections of an SDS (in order) are:
- Identification — chemical name, supplier, recommended use
- Hazard identification — GHS hazard class, signal word (Danger / Warning), pictograms, hazard statements (H-codes)
- Composition — chemical components, CAS numbers, concentrations
- First aid measures — what to do for skin, eyes, inhalation, ingestion exposure
- Firefighting measures — suitable extinguishing media, special hazards from combustion
- Accidental release measures — spill cleanup procedures
- Handling and storage — safe handling, ventilation, incompatible storage
- Exposure controls / PPE — exposure limits (PEL, TLV), required gloves, goggles, lab coat, fume hood
- Physical and chemical properties — molecular formula, melting point, boiling point, density, solubility, pH, refractive index, flash point, vapour pressure
- Stability and reactivity — incompatible chemicals, conditions to avoid, hazardous decomposition products
- Toxicological information — LD₅₀, target organ toxicity, carcinogenicity
- Ecological information — environmental impact, aquatic toxicity
- Disposal considerations — proper waste category, hazardous waste profile
- Transport information — UN number, DOT/IATA classification
- Regulatory information — TSCA, RCRA, state right-to-know lists
- Other information — preparation date, revisions
For the reagents in this lab, Sections 1, 2, 4, 8, 9, 10, and 13 are the most important to read before each experiment. Of these, students typically extract:
- Molecular formula from Section 3 or Section 9 (used for theoretical-yield calculations)
- Physical properties from Section 9 (m.p., b.p., density, solubility, refractive index — used to verify product identity)
- Hazards and PPE from Sections 2 and 8 (what gloves to wear, whether the fume hood is required)
- First-aid procedure from Section 4 (what to do if the reagent contacts skin or eyes)
- Disposal route from Section 13 (which waste container the residue must go into)
Section 8 — PPE: Nitrile or neoprene gloves; safety goggles; lab coat; fume hood required.
Section 9 — Properties: Yellow-orange crystalline solid. M.W. 198.14 g/mol. M.p. 198–202 °C (decomposes). Practically insoluble in water; soluble in DMF and pyridine.
Section 13 — Disposal: Halogenated organic waste (because of the nitro groups). Do NOT pour down the drain.
Microscale Operations
Modern organic teaching laboratories use microscale equipment — apparatus designed to handle reactions on a small scale (typically 0.1–1 mmol of starting material, or 25–250 mg). Microscale work uses less reagent, generates less waste, costs less per experiment, and reduces hazard exposure. The trade-off is that small quantities require more careful technique and specialised tools (1-mL conical vials, microspatulas, capillary melting-point tubes, etc.).
Four microscale techniques are routine for verifying product identity and purity:
Melting point determination
A capillary tube packed with 2–3 mm of dry, finely ground sample is heated in a Mel-Temp or Thomas-Hoover apparatus at 1–2 °C/min through the expected range. The melting range is recorded as the temperature at which the first drop of liquid appears to the temperature at which the entire sample is liquid. A pure compound melts within 1–2 °C; a wider range (≥4 °C) indicates impurity. Comparing the observed m.p. to the literature value (from Section 9 of the SDS) confirms identity.
Boiling point determination
For volatile liquids, a microscale b.p. apparatus uses a small inverted bell (a sealed capillary in a Thiele tube of mineral oil) over the sample. As the bath heats, gas bubbles emerge from the capillary; when bubbling stops at a steady rate as the bath cools, the temperature is the boiling point. Pressure correction is applied if the lab altitude differs significantly from sea level.
Refractive index
Measured on an Abbé refractometer at 20 °C with sodium-D-line illumination (denoted nD20). One drop of liquid sample is placed between the prisms and the boundary line is brought into focus on the cross-hairs. The refractive index is a physical constant (typically 4 decimal places, e.g., 1.3614 for ethanol). Comparing measured nD to the literature value (Section 9 of the SDS) confirms purity within ±0.001 for a clean sample.
Recrystallisation
Solid product is dissolved in minimum hot solvent, hot-filtered to remove insoluble impurities, then allowed to cool slowly so that pure crystals form. Soluble impurities remain in solution. The choice of solvent depends on the rule "the compound is soluble hot but only slightly soluble cold." Common recrystallisation solvents: water, ethanol, ethanol-water mixtures, ethyl acetate, hexane. Yield is typically 60–80%; m.p. of the recrystallised product is sharper than the crude.
In this virtual lab, melting points and boiling points appear as reference data (looked up from the SDS for each reagent and product) rather than measured directly. The goal is to understand what each measurement tells you and how it would be performed in a wet lab — so that when you do this in person, you know how to interpret the values you obtain and how they confirm or disconfirm a proposed identification.
Lab Notebook and ACS-Style Reports
A laboratory notebook is the primary record of what you did, what you observed, and what you concluded. It must be sufficient for someone else (a co-worker, a supervisor, a regulator) to reproduce your work. Notebook entries are contemporaneous — written as the work is performed, not reconstructed afterwards.
For each experiment, the notebook entry should include:
- Title and date at the top of the page
- Purpose — one sentence stating what you intend to determine
- Balanced chemical equation(s) for every reaction performed, with structural formulas drawn in addition to molecular formulas
- Reagent table — for each reagent: name, molecular formula, molecular weight, density (if liquid), mass or volume used, moles, equivalents, m.p. or b.p. (from SDS), hazard summary
- Procedure — written in past tense or imperative, in sufficient detail that another chemist could follow it. Reference the lab manual or modify it as actually performed.
- Observations — exactly what you saw at each step (colour changes, gas evolution, temperature changes, exotherm, time required)
- Calculations — theoretical yield, percent yield, m.p. range obtained vs literature, and any corrections (pressure, water content)
- Spectra and data — taped or stapled into the notebook; or a clear cross-reference to where the data are stored
- Conclusions — what was learned, with assessment of confidence
- Signature and date at the end of the entry; if you make a correction later, single-line cross-out and re-sign with date
A formal lab report follows the ACS (American Chemical Society) style: Title, Abstract (≤200 words), Introduction, Experimental Section (Materials and Methods), Results, Discussion, Conclusion, References, and (where appropriate) Supporting Information. The Experimental Section uses past tense and describes what was done in sufficient detail to allow reproduction. The Results section presents data without interpretation. The Discussion section interprets the data, compares to the literature, addresses anomalies, and admits limitations. References are formatted in ACS Style: numbered superscripts in the text, with full citations in the order of first appearance at the end.
Section I — Known compounds
Pick a known sample from the rack (labelled by functional group) and mix it with each test reagent. Observe the result and record it in the log. This calibrates your understanding of what each positive and negative result looks like.
Section II — Four unknowns
Four lettered samples (A, B, C, D) are provided. Use whichever tests you think will distinguish them. Make your identification for each, then click "Reveal Unknowns" at the end to see the actual compounds.
Section III — SDS interpretation and microscale data
Six SDS extracts are presented. For each, answer questions about hazard, PPE, disposal, and physical properties needed for the experiment. Then complete a microscale-data exercise: match observed m.p., b.p., or nD values to candidate compounds.
Instructions — Running the Virtual Experiment
Section I — Learning the positive tests
Section II — Identifying four unknowns
Section III — SDS interpretation and microscale data
Simulation — Virtual Qualitative Bench
Samples (knowns)
Click a sample to add it
Test reagents
Observations Log — Section I
| Sample | Reagent | Observation | Result |
|---|---|---|---|
| No tests logged yet. Run a test and click "+ Log Observation". | |||
Unknowns
Click an unknown to add it
Test reagents
Observations Log — Section II
| Unknown | Reagent | Observation | Result |
|---|---|---|---|
| No tests logged yet. | |||
Your identifications
Round 1 — SDS interpretation
Each card shows an extract from a real Safety Data Sheet for a reagent commonly used in qualitative tests. Read the extract, then answer the four questions about hazards, PPE, disposal, and first aid.
Round 2 — Match microscale data to compounds
Five microscale measurement records are shown. For each, decide which candidate compound the data fit best. Use SDS Section 9 reference values (m.p., b.p., nD20) — or your own knowledge of expected ranges — to make the call.
Candidate compounds (each used exactly once):
- Benzoic acid — solid, m.p. 122 °C, sublimes; recrystallised from hot water
- Acetone — colourless liquid, b.p. 56 °C, nD20 = 1.3588, miscible with water
- Ethanol — colourless liquid, b.p. 78 °C, nD20 = 1.3614, miscible with water
- Cyclohexanone — colourless liquid, b.p. 156 °C, nD20 = 1.4502, slightly soluble in water
- Salicylic acid — solid, m.p. 158–160 °C, recrystallised from hot ethanol-water
Round 3 — Theoretical and percent yield
For each scenario, calculate the theoretical yield and percent yield. Type your answer in the box and press Check. The system accepts a tolerance of ±5%.
Team Questions
Example Lab Report
Sample report demonstrating the expected format and level of detail. Use as a guide for your own submission.
Functional Group Tests: Identification of Four Unknown Organic Compounds
Chemistry 221 | Section: [Your Section] | Date: [Date]
Lab Members: [Names of all members present]
Purpose
To observe the characteristic colour changes, precipitates, effervescence, and odours produced when representative organic compounds are treated with twelve classical qualitative reagents (bromine water, Baeyer's KMnO₄, Tollens', Fehling's, 2,4-DNP, iodoform, Lucas, Jones, NaHCO₃, esterification, FeCl₃, and Hinsberg), and to use the resulting pattern of positive and negative tests to identify four unknown samples.
Theory
Each qualitative reagent reacts with a specific functional group (or a small set of related groups) in a distinctive way. The reactions fall into three categories: oxidation (KMnO₄ and Jones oxidise alcohols and aldehydes; Tollens' and Fehling's oxidise aldehydes), addition or substitution at a π system (Br₂ and KMnO₄ at C=C and C≡C; Br₂ at the activated ring of a phenol), and condensation or acid–base (2,4-DNP hydrazone formation; NaHCO₃ protonation of a carboxylate; esterification; Hinsberg sulphonamide formation). Representative equations are:
R–CHO + 2 [Ag(NH₃)₂]⁺ + 3 OH⁻ → R–COO⁻ + 2 Ag↓ + 4 NH₃ + 2 H₂O (Tollens')
R–CHO + 2 Cu²⁺ + 5 OH⁻ → R–COO⁻ + Cu₂O↓ + 3 H₂O (Fehling's)
R₂C=O + H₂N–NH–Ar → R₂C=N–NH–Ar + H₂O (2,4-DNP)
CH₃–CO–R + 3 I₂ + 4 OH⁻ → CHI₃↓ + R–COO⁻ + 3 I⁻ + 3 H₂O (iodoform)
R–COOH + NaHCO₃ → R–COONa + H₂O + CO₂↑ (effervescence)
R–COOH + R'–OH ⇌ R–CO–O–R' + H₂O (Fischer esterification)
A single positive result is rarely sufficient to identify a functional group: for example, aldehydes and ketones both give 2,4-DNP precipitates, so an aldehyde is distinguished from a ketone only by a positive Tollens' or Fehling's test on top of the 2,4-DNP. Because of this, the identification relies on the combined pattern of positive and negative observations.
Calculations / Worked Identifications — Sample: Unknown A
Observations on Unknown A:
2,4-DNP: orange-yellow crystalline ppt → carbonyl present (aldehyde or ketone)
Tollens': silver mirror on inner wall → aldehyde (not ketone)
Fehling's: deep blue → brick-red Cu₂O ppt → confirms aliphatic aldehyde
Jones: orange Cr(VI) → blue-green Cr(III) → oxidisable group consistent with aldehyde
Iodoform: no yellow ppt → no CH₃–C=O or CH₃–CH(OH)– group at carbon 2
Interpretation: The positive 2,4-DNP locates a C=O. The positive Tollens' and Fehling's narrow this to an aldehyde (RCHO), not a ketone. The positive Jones confirms the oxidisable C–H on the carbonyl carbon. The negative iodoform rules out compounds such as acetaldehyde (which would give a positive iodoform). Unknown A is therefore a simple aliphatic aldehyde with the –CHO group not on C-2 of a methyl chain — consistent with propanal, CH₃CH₂CHO.
CH₃CH₂CHO + 2 [Ag(NH₃)₂]⁺ + 3 OH⁻ → CH₃CH₂COO⁻ + 2 Ag↓ + 4 NH₃ + 2 H₂O
Reagents and Hazards (drawn from SDS)
Before each experiment, the SDS for every reagent was consulted. The most relevant hazard data, PPE requirements, and disposal routes are summarised below. Section 9 (physical and chemical properties) of each SDS provided the literature values used in microscale identification.
| Reagent | Major hazard | PPE | Key Section 9 data | Disposal |
|---|---|---|---|---|
| 2,4-DNP | Flammable when dry (H228); irritant | Goggles, gloves, lab coat, fume hood | Yellow-orange solid; m.p. 198–202 °C | Halogenated/nitro hazardous waste |
| Tollens' reagent (fresh) | Forms explosive AgCNO on standing; ammonia vapours | Goggles, gloves, fume hood | Colourless; sharp NH₃ odour | Acidify with HCl, collect as silver waste |
| Fehling's solution | Mild irritant; corrosive to eyes | Goggles, gloves | Deep blue; alkaline (pH ~12) | Acidify, collect as copper-bearing aqueous waste |
| Lucas reagent | Highly corrosive (HCl + ZnCl₂) | Goggles, gloves, lab coat, fume hood | Pale yellow viscous liquid | Neutralise with NaHCO₃, collect as Zn-containing aqueous waste |
| Jones reagent (CrO₃/H₂SO₄) | Carcinogenic (Cr(VI), H350); corrosive | Goggles, double gloves, lab coat, fume hood | Orange-red; dense (~1.5 g/mL) | Reduce Cr(VI) to Cr(III) with iron sulfate, collect as heavy-metal hazardous waste |
| Iodoform reagent (I₂/NaOH) | Iodine vapours (irritant); strong base | Goggles, gloves, fume hood | Brown solution; iodine odour | Reduce excess I₂ with thiosulfate, collect as halogenated waste |
| NaHCO₃ (saturated) | Mild irritant only | Goggles, gloves | Clear colourless solution; pH ~8 | Aqueous waste (or drain after dilution per local rules) |
| FeCl₃ (1% aq) | Skin and eye irritant | Goggles, gloves | Pale yellow; pH ~2 (slightly acidic) | Acidic aqueous waste with iron salts |
All quantities used were microscale: typically 0.1–0.3 mL of reagent per test, applied with disposable Pasteur pipettes to 5–10 mg of sample in a test tube. Microscale handling reduces total reagent consumption by ~95% compared with traditional macroscale procedures and minimises hazard exposure.
Microscale Verification of Identity (worked example)
Once Unknown D was tentatively identified as acetone from the test pattern (DNP +, Tollens −, Fehling −, iodoform +), the identity was verified using two independent microscale measurements:
| Measurement | Method | Observed | Literature (SDS §9) | Conclusion |
|---|---|---|---|---|
| Boiling point | Microscale b.p. apparatus (Thiele tube + capillary bell) | 56.0–56.5 °C | 56 °C | Match within ±0.5 °C — pure |
| Refractive index nD20 | Abbé refractometer at 20.0 °C | 1.3585 | 1.3588 | Match within ±0.001 — pure |
| Density | Microscale pipette (50 μL → analytical balance) | 0.789 g/mL | 0.79 g/mL | Match — pure |
All three independent measurements agreed with the literature values within typical microscale precision. Identity confirmed as acetone.
Theoretical and Percent Yield (Unknown D Tollens' silver mirror)
| Quantity | Value |
|---|---|
| Mass of starting carbonyl (acetone, M.W. 58.08) | 0.116 g (2.0 mmol) |
| Reaction stoichiometry | RCHO + 2 [Ag(NH₃)₂]⁺ → 2 Ag (only aldehydes react; not applicable to ketones) |
| Theoretical mass of Ag | 0 g — acetone is a ketone, gives no silver mirror |
| Observed Ag | 0 g (consistent with negative Tollens') |
For Unknown A (propanal, an aldehyde), the parallel calculation gave a theoretical 0.0432 g of silver from 2.00 × 10⁻⁴ mol propanal. The mass collected after suction filtration and drying was 0.0395 g, giving a 91.4 % yield. The slight loss is consistent with mechanical losses on the filter paper during transfer.
Results Table
Section I — Calibration tests on known samples (representative rows)
| Sample (group) | Reagent | Observation | Result |
|---|---|---|---|
| 1-butanol (1° alcohol) | Lucas | Clear, no change at RT | Negative (consistent with 1°) |
| 2-butanol (2° alcohol) | Lucas | Cloudy emulsion within ~6 min | Positive (2°) |
| t-butanol (3° alcohol) | Lucas | Cloudy within seconds | Positive (3°) |
| Propanal (aldehyde) | Tollens' | Silver mirror on inner wall | Positive |
| Acetone (methyl ketone) | Tollens' | No visible change | Negative |
| Acetone (methyl ketone) | Iodoform | Pale yellow CHI₃ ppt, antiseptic smell | Positive |
| Benzoic acid (COOH) | NaHCO₃ | Vigorous effervescence (CO₂) | Positive |
| Benzoic acid (COOH) | Esterification | Sweet fruity smell on warming | Positive |
| Phenol (ArOH) | FeCl₃ | Deep violet colour | Positive |
| Aniline (1° amine) | Hinsberg | Clear in base, ppt on acidification | Positive — 1° amine |
Section II — Key diagnostic tests on the four unknowns
| Unknown | Decisive tests | Observations | Identification |
|---|---|---|---|
| A | 2,4-DNP, Tollens', Fehling's, iodoform | DNP +, Tollens +, Fehling +, iodoform − | Aliphatic aldehyde (propanal) |
| B | Lucas, Jones, FeCl₃ | Lucas fast cloudy, Jones no reaction, FeCl₃ no colour | Tertiary alcohol (t-butanol) |
| C | NaHCO₃, esterification | NaHCO₃ vigorous fizz, esterification sweet smell | Carboxylic acid (benzoic acid) |
| D | 2,4-DNP, Tollens', iodoform | DNP +, Tollens −, iodoform + | Methyl ketone (acetone) |
Discussion
The calibration runs in Section I matched the textbook expectations for every functional group tested. Lucas's reagent clearly distinguished the three alcohol classes on its own — tertiary cloudy in seconds, secondary cloudy over several minutes, primary unchanged at room temperature — because carbocation stability (3° > 2° > 1°) controls the rate of the SN1 substitution by chloride. Tollens' and Fehling's both gave positive results for propanal (an aliphatic aldehyde) while giving no reaction at all with acetone, confirming that the tests are selective for the aldehyde C–H rather than any carbonyl. The iodoform test was positive for acetone (a methyl ketone) and negative for propanal, showing that the test specifically detects the CH₃–CO– (or CH₃–CH(OH)–) sub-structure rather than carbonyls generally.
The identifications of the four unknowns in Section II illustrate how the pattern of results pins down a structure far more rigorously than any single test. Unknown A was classified as an aldehyde only after three concordant positive tests (2,4-DNP, Tollens', Fehling's) plus a diagnostic negative iodoform that ruled out methyl-substituted aldehydes such as acetaldehyde. Unknown B was placed in the tertiary-alcohol class because the speed of the Lucas test (cloudy within seconds) is distinctive for a 3° alcohol and the Jones reagent could not oxidise it — tertiary alcohols have no α-hydrogen on the carbinol carbon, so CrO₃/H₂SO₄ is unable to form the ketone or acid. Unknown C was identified as a carboxylic acid from the vigorous CO₂ evolution with NaHCO₃ and confirmed by the characteristic fruity ester smell on warming with ethanol and a drop of concentrated sulphuric acid. Unknown D was distinguished from an aldehyde by a negative Tollens' result despite the 2,4-DNP ppt; the additional positive iodoform placed the methyl group immediately adjacent to the carbonyl, consistent with acetone.
The most important lesson from this lab is that qualitative analysis works by elimination. Each test separates the remaining candidates into smaller groups. Recording negative observations is as important as recording positive ones — a negative Tollens' next to a positive 2,4-DNP is itself a piece of structural information.
Conclusion
Twelve classical qualitative tests were used to identify fourteen representative functional groups and, on that basis, to identify four unknown organic samples. The expected colour changes, precipitates, gas evolution, and odours were observed in every calibration run. By combining the results of several diagnostic tests, the four unknowns were assigned unambiguously as a simple aliphatic aldehyde, a tertiary alcohol, a carboxylic acid, and a methyl ketone — identifications consistent with propanal, t-butanol, benzoic acid, and acetone respectively. The experiment confirmed that qualitative analysis is most reliable when the pattern of positive and negative results is used, not any single observation in isolation.
Practice Questions
Answer each fully, showing the reasoning from the observations. Include a balanced equation where asked.