Chemistry · Organic chemistry · Grade 10

Identifying organic substances

Six unlabelled test tubes: which reagent “gives away” which organic substance?

The situation

In the chemistry room a lab technician found six bottles whose labels had peeled off. They are known to contain glycerol, glucose, phenol, ethanol, ethanoic acid and starch paste, but which is in which bottle is unknown. Smelling or tasting is out of the question: phenol is poisonous and burns the skin. The detective chemist has seven reagents. Which reagent “gives away” which substance, and how can all of them be identified with the fewest tests?

Topic

Identifying organic substances by qualitative reactions. Chemist-detective: glycerol, glucose, phenol, ethanol, ethanoic acid and starch in unlabelled test tubes — which reagent reveals which

Key ideas

Qualitative tests
Functional groups
Formula from combustion data

Formulas

C_{6} H_{12} O_{6} + 2 [Ag (NH_{3})_{2}] OH \overset{}{\to} C_{6} H_{11} O_{7} NH_{4} + 2 Ag ↓ + 3 NH_{3} + H_{2} O

Silver mirror: the aldehyde group (in glucose) reduces silver; schematically: $C_{6} H_{12} O_{6} + Ag_{2} O \overset{}{\to} C_{6} H_{12} O_{7} + 2 Ag ↓$ .

C_{6} H_{12} O_{6} + 2 Cu (OH)_{2} \overset{}{\to} C_{6} H_{12} O_{7} + Cu_{2} O ↓ + 2 H_{2} O

Glucose with $Cu (OH)_{2}$ gives a deep blue solution in the cold (polyhydric alcohol) and red $Cu_{2} O$ when heated (aldehyde).

C_{6} H_{5} OH + 3 Br_{2} \overset{}{\to} C_{6} H_{2} Br_{3} OH ↓ + 3 HBr

Phenol decolourises bromine water and gives a white precipitate of 2,4,6-tribromophenol.

C_{x} H_{y} O_{z} : n (C) = n (CO_{2}), n (H) = 2 n (H_{2} O)

Composition from combustion: the mass of oxygen is found by difference, $m (O) = m - m (C) - m (H)$ .

M = 2 D_{H_{2}}

Molar mass from the density of a gas or vapour relative to hydrogen (relative to air: $M = 29 D_{air}$ ).

Theory

Qualitative reaction (practical 3.23)

A qualitative reaction is a reaction that identifies a substance or its functional group by a visible sign: a colour change, the formation of a precipitate or the evolution of a gas. A good reagent is selective: it responds to only one group. Procedure: pour 1–2 mL of the unknown substance into a clean test tube (never add reagent to the stock bottle!), add a few drops of reagent, and if necessary heat the tube in a water bath. Record every observation in the table. In the table of reagents, the green frame marks the characteristic sign of a substance, that is, a result that none of the other five gives.

What the reagents “reveal”

Bromine water — a double bond and phenol: phenol gives a white precipitate (2,4,6-tribromophenol); an aldehyde group also decolourises it slowly (glucose → gluconic acid). KMnO₄ (acidified with sulfuric acid, heated) decolourises almost every easily oxidised substance, so it is not selective; ethanoic acid, however, resists it. Silver mirror (Tollens' reagent $[Ag (NH_{3})_{2}] OH$ ) — an aldehyde group. Freshly precipitated Cu(OH)₂ ( $CuSO_{4} + 2 NaOH \overset{}{\to} Cu (OH)_{2} ↓ + Na_{2} SO_{4}$ ) gives a deep blue solution with polyhydric alcohols (glycerol, glucose); on heating, an aldehyde group reduces it to red $Cu_{2} O$ ; an acid turns the precipitate into light blue copper(II) ethanoate. FeCl₃ — purple colour with phenol. Iodine — deep blue colour with starch. Soda ( $NaHCO_{3}$ ) — $CO_{2}$ bubbles with a carboxylic acid.

Detective: fewest tests

Testing every tube with seven reagents means 42 tests. The clever way: first take the reagent that divides the substances into groups. If one reagent has several different results, it gives a lot of information in a single test. Within a group, the characteristic reagents are used. Elimination also replaces a test: once five substances are identified, the sixth is known automatically — there is no need to test the last tube. In detective mode the number of tests is counted.

Problems: composition and combustion (3.24–3.25)

When an organic substance burns, carbon turns into $CO_{2}$ and hydrogen into $H_{2} O$ : $C_{x} H_{y} O_{z} + (x + \frac{y}{4} - \frac{z}{2}) O_{2} \overset{}{\to} x CO_{2} + \frac{y}{2} H_{2} O$ . In elemental analysis (Liebig's method) the sample is burned in a stream of oxygen over heated CuO, and the products pass in turn through two tubes: one containing $CaCl_{2}$ , the other KOH; the tubes are weighed before and after the experiment. Calculation: $n (C) = n (CO_{2})$ , $n (H) = 2 n (H_{2} O)$ , $m (O) = m - m (C) - m (H)$ ; the ratio of the amounts in moles gives the empirical formula. The molar mass comes from the vapour density: $M = 2 D_{H_{2}}$ ; the subscripts are multiplied by the number of times $M$ exceeds the mass of the empirical formula. A calculation from a reaction: for example, 18 g (0.1 mol) of glucose in the silver mirror reaction releases 0.2 mol, that is 21.6 g, of silver.

University: functional analysis (Organic L21–L24)

An unknown organic substance is identified by a scheme: each test responds to one functional group. Combustion test: an aromatic ring gives a yellow, sooty flame. NaHCO₃ — a carboxyl group ( $CO_{2}$ ; phenols do not give it). FeCl₃ — the hydroxyl of a phenol. 2,4-dinitrophenylhydrazine (Brady's reagent) — a carbonyl group: aldehydes and ketones give a yellow-to-red hydrazone precipitate; carboxylic acids, esters and amides do not. Tollens — distinguishes an aldehyde from a ketone. Beilstein test: a copper wire coated with CuO is touched to the substance and put into the flame — a blue-green flame shows the presence of Cl, Br or I (it does not detect fluorine). Dilute HCl — a water-insoluble amine dissolves by forming a salt. Fe(OH)₂ — a nitro group oxidises it to red-brown $Fe (OH)_{3}$ . Sodium — the hydrogen of a hydroxyl group ( $H_{2}$ ; the sample must be anhydrous). Bromine water — a double bond (decolourises), phenol and aniline (white precipitate).

Safety and the model

Safety: wear safety goggles and gloves; phenol and bromine burn the skin — work with them in a fume cupboard; do not taste substances, and if you must smell one, waft the vapour towards you with your hand. When heating a test tube, do not point its mouth at yourself or others. Tollens' reagent cannot be stored — it is prepared fresh each time: as it ages, explosive silver nitride can form; after the work, the residue is destroyed with dilute acid. Dry 2,4-DNPH can explode from shock and friction. Sodium is stored under kerosene: it reacts violently with water and may ignite. Model: the colours and precipitates are typical; reaction times are shortened (in a real experiment the decolourisation of bromine water by glucose and the silver mirror take several minutes); the water bath is always 80 °C; the formula of the phenol–FeCl₃ complex is simplified; in the combustion analysis the sample is assumed to burn completely and the products to be absorbed completely, and the balance precision is 0.001 g.

Did you know?

The silver mirror reagent was described in 1882 by the German chemist Bernhard Tollens. The reagent is not sold or stored: it is prepared fresh each time from silver nitrate and ammonia, because as it ages, explosive silver nitride can form.
The simplest way to detect a halogen in an organic substance — touching a heated copper wire to the substance and putting it into the flame — was proposed in 1872 by Friedrich Beilstein. A green flame shows chlorine, bromine or iodine; this test does not “see” fluorine.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

More experiments in this section

Natural gas, oil and coal — Grade 10
Alcohols and ethers — Grade 10
Phenol, aldehydes and ketones — Grade 10
Acids, esters and soap — Grade 10
Carbohydrates: glucose and starch — Grade 10
Stereochemistry and conformations — Year 2
Radical and electrophilic mechanisms — Year 3
SN1, SN2 and elimination — Year 3
The aromatic ring and azo dyes — Year 3
Carbonyl compounds and synthesis — Year 3