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 IMATH_16

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

Common traps

Treating bromine water and permanganate as equivalent unsaturation tests. Bromine water is more specific; permanganate also reacts with alcohols and aldehydes.

Forgetting to do the carbonate test before the sodium test. A sample of acid plus alcohol would give hydrogen with sodium and confuse the identification. The carbonate test discriminates first.

Using wet sodium. Sodium reacts with water more vigorously than with most alcohols, and any water in the sample will mask the alcohol test. Dry the sample first.

Claiming tertiary alcohols give an orange-to-green dichromate result. They do not. Tertiary alcohols are inert to dichromate.

Saying "an organic acid effervesces with sodium hydroxide". Hydroxide gives a neutralisation, not effervescence. Carbonate gives effervescence because $CO_2$ is released.

In one sentence

Test for C=C with cold bromine water (orange to colourless), for hydroxyl with sodium (hydrogen) or acidified dichromate (orange to green, primary and secondary only), and for carboxyl with sodium carbonate ($CO_2$ effervescence), running the tests in that order so each functional group is unambiguously assigned.