Conduct qualitative investigations to test for the presence in organic molecules of carbon-carbon double bonds, hydroxyl groups and carboxylic acids

What this dot point is asking

NESA wants you to know the standard chemical tests that confirm a carbon-carbon double bond (unsaturation), a hydroxyl ($OH$) group on an alcohol and a carboxyl ($COOH$) group on a carboxylic acid, with the reagents, observations, equations and the limits of selectivity for each test.

The answer

Tests for C=C (unsaturation)

Bromine water. Shake the unknown with a few drops of orange bromine water. An alkene rapidly decolourises the orange to colourless by adding $Br_2$ across the double bond:

The reaction is electrophilic addition. Saturated hydrocarbons (alkanes) do not react with bromine water in the dark; they require UV light to undergo radical substitution.

Acidified potassium permanganate (cold dilute, room temperature). Purple $MnO_4^-$ is decolourised by an alkene as the double bond is oxidised to a 1,2-diol. The observation is purple to colourless (or to a brown $MnO_2$ precipitate in neutral conditions):

Under hot acidified conditions the diol oxidises further and the C-C bond is cleaved into two carboxylic acids (or carbon dioxide if a terminal $=CH_2$ is present).

Selectivity caveat. Permanganate also oxidises aldehydes, primary and secondary alcohols, and some aromatic side chains. So a positive permanganate test alone does not prove an alkene. Bromine water at room temperature is more specific.

Tests for hydroxyl $-OH$ (alcohol)

Sodium metal. Drop a small piece of clean sodium into the unknown (dried). An alcohol releases hydrogen gas and forms a sodium alkoxide:

Test the gas with a lit splint (pop). The reaction is calmer than the reaction of sodium with water because alcohols are weaker acids. Tertiary alcohols react more slowly than primary, but all react.

Selectivity caveat. Carboxylic acids also react with sodium (more vigorously). The test confirms an acidic O-H, not specifically an alcohol; combine with the carbonate test (next section) to discriminate.

Acidified potassium dichromate (orange $Cr_2O_7^{2-}/H_2SO_4$). Primary and secondary alcohols reduce dichromate from orange to green:

The orange-to-green colour change is the classical alcohol test. Tertiary alcohols do not react (no $H$ on the OH-bearing carbon to lose).

Ester formation. Heat the unknown with a carboxylic acid (ethanoic acid) and a few drops of concentrated $H_2SO_4$. A sweet, fruity smell indicates ester formation, confirming an alcohol. Reverse the reagents to confirm a carboxylic acid.

Tests for carboxylic acid $-COOH$

Sodium carbonate (or sodium bicarbonate). Add solid $Na_2CO_3$ or $NaHCO_3$ to the unknown dissolved in water (or to the neat liquid). Brisk effervescence of $CO_2$ indicates a carboxylic acid:

Confirm the gas by passing it through limewater (turns milky). Alcohols are too weak an acid to react with carbonate; phenols can react with strong base but not with carbonate. So a positive carbonate test is highly diagnostic of a carboxylic acid.

Litmus or universal indicator. A carboxylic acid solution has pH around 3 to 4 (depending on concentration). Litmus turns red. Alcohols are essentially neutral.

Reactive metal. Magnesium ribbon dissolves in a carboxylic acid solution with hydrogen evolution:

The bubbling is brisker than with an alcohol (the acid is fully ionised in solution; the alcohol's O-H is barely acidic in water).

Esterification. Warm with an alcohol and concentrated $H_2SO_4$; the fruity ester smell confirms the carboxyl group.

A flowchart for an unknown organic liquid

Suppose you suspect an alkane, an alkene, an alcohol or a carboxylic acid:

1. Bromine water. Decolourised rapidly at room temperature? $\Rightarrow$ alkene.
2. If no decolourisation, add sodium carbonate. Effervescence? $\Rightarrow$ carboxylic acid.
3. If no effervescence, add sodium metal. Hydrogen evolved? $\Rightarrow$ alcohol.
4. If no reaction in any of the above, the unknown is the saturated hydrocarbon (alkane), identified by exclusion.

Sequence the tests in this order to avoid false positives: the alkene test goes first because acidified permanganate would also react with the alcohol; carbonate test before sodium because both alcohols and acids react with sodium.

Summary table

Examples in context

Example 1. Margarine quality control by bromine number. Goodman Fielder's margarine plant near Liverpool measures the iodine and bromine number of incoming canola oil to monitor unsaturation, which controls texture and melting point. The bromine value test adds bromine in carbon tetrachloride to a known mass of oil; the volume of bromine consumed gives a quantitative iodine value via stoichiometry. A drop in bromine value flags successful partial hydrogenation. The HSC bromine water test for C=C is the qualitative version of the same chemistry. Students at NSW schools running the test on canola oil watch the bromine colour disappear, exactly the same colour change the QC technician at Goodman Fielder uses.

Example 2. Identifying organic acids in NSW Murray Valley wine. Wineries along the Murray Valley measure tartaric, malic and lactic acid in grape juice by titration, but new juice is first screened qualitatively with sodium hydrogen carbonate to confirm acid presence. A drop of juice on dry $NaHCO_3$ effervesces vigorously if free acid is present, an instant pass-fail screen for spoiled tank deliveries. The acidified dichromate test on the same juice goes green if ethanol contamination from a wild fermentation is present. Both qualitative HSC tests are bench tools winery chemists use before sending samples for HPLC quantification. NESA-style depth study tasks at Riverina and Hunter Valley schools often replicate this winery screening workflow.

Try this

Q1. State the observation for each of: (a) hex-1-ene plus bromine water, (b) ethanol plus acidified dichromate, (c) ethanoic acid plus sodium hydrogen carbonate. [3 marks]

• Cue. (a) Orange to colourless. (b) Orange to green. (c) Effervescence (colourless $CO_2$).

Q2. A 0.225 g sample of an unknown monoprotic carboxylic acid neutralises 30.40 mL of 0.100 mol L$^{-1}$ NaOH. Calculate the molar mass and identify the most likely candidate (methanoic 46, ethanoic 60, propanoic 74, butanoic 88). [3 marks]

• Cue. $n(\text{NaOH}) = 0.100 \times 0.03040 = 3.04 \times 10^{-3}$ mol; 1:1 stoichiometry; $M = 0.225 / 3.04 \times 10^{-3} = 74.0$ g mol$^{-1}$; propanoic acid.

Q3. A student is given three unlabelled liquids X, Y and Z. X decolourises bromine water. Y effervesces with sodium hydrogen carbonate. Z gives an orange-to-green colour change with acidified dichromate but does not effervesce. (a) Identify the functional group present in each. (b) Write a balanced equation for the dichromate test on Z. (c) Predict the IR signature of Y. [3+2+1 marks]

• Cue. (a) X alkene, Y carboxylic acid, Z alcohol. (b) $3RCH_2OH + Cr_2O_7^{2-} + 8H^+ \rightarrow 3RCHO + 2Cr^{3+} + 7H_2O$. (c) Broad O-H 2500 to 3300 plus sharp C=O 1700 to 1720 cm$^{-1}$.