Chemical Tests for Functional Groups (HSC SSCE Chemistry): Revision Notes

Chemical Tests for Functional Groups

Introduction

Organic compounds are essential to every aspect of our lives. They form the basis of the food we eat, the medicines we use to treat diseases, the fuels that power our world, and even make up the structure of our own bodies. With millions of naturally occurring and synthetic organic compounds in existence, chemists need reliable methods to identify and characterise these substances.

Understanding the structure of organic molecules at the atomic level is crucial for several reasons. It allows chemists to predict how molecules will react, to isolate new compounds from natural sources, and to synthesise new substances with specific properties. This knowledge has revolutionised the development of pharmaceuticals, materials, and countless other applications of organic chemistry.

Key organic compound classes

Organic compounds are classified into different groups based on their functional groups. This note focuses on three important classes that you need to be able to identify through chemical testing:

Class	Suffix	Functional Group	Example	Structure
Alkene	-ene	Carbon-carbon double bond ($\text{C}=\text{C}$)	Ethene	$\text{C}_2\text{H}_4$
Alcohol	-ol	Hydroxyl group ($-\text{OH}$)	Ethanol	$\text{C}_2\text{H}_5\text{OH}$
Carboxylic acid	-oic acid	Carboxyl group ($-\text{COOH}$)	Ethanoic acid	$\text{CH}_3\text{COOH}$

Alkenes are hydrocarbons containing at least one carbon-carbon double bond. This double bond makes alkenes more reactive than alkanes (which have only single bonds). The general formula for alkenes is $\text{C}_n\text{H}_{2n}$.

Alcohols are organic compounds containing one or more hydroxyl groups ($-\text{OH}$) attached to a carbon atom. The hydroxyl group is polar, making alcohols more soluble in water than hydrocarbons. Alcohols can be classified as primary, secondary, or tertiary depending on how many carbon atoms are attached to the carbon bearing the hydroxyl group.

Carboxylic acids contain the carboxyl functional group ($-\text{COOH}$), which consists of a carbonyl group ($\text{C}=\text{O}$) and a hydroxyl group on the same carbon atom. This functional group is acidic, releasing $\text{H}^+$ ions in solution.

Chemical tests for identifying functional groups

The chemical properties of different organic compound classes can be exploited to distinguish between them. The following sections describe the key tests used to identify carboxylic acids, alkenes, and alcohols. These tests are based on the characteristic reactions of each functional group.

Tests for carboxylic acids

Carboxylic acids are the most easily identified of the three classes because they exhibit clear acidic properties. Two main tests confirm the presence of a carboxylic acid:

Litmus test

The simplest initial test is to use blue litmus paper. Carboxylic acids will turn blue litmus paper red, indicating that the solution is acidic. This occurs because the carboxyl group donates a proton ($\text{H}^+$) to form the carboxylate ion:

$\text{RCOOH} \rightleftharpoons \text{RCOO}^- + \text{H}^+$

However, this test alone is not conclusive, as other acids would also turn litmus red. You need a confirmatory test to specifically identify carboxylic acids.

Sodium carbonate test

The definitive test for carboxylic acids involves adding sodium carbonate solution ($\text{Na}_2\text{CO}_3$) to the sample. If a carboxylic acid is present, you will observe the formation of colourless gas bubbles. This gas is carbon dioxide ($\text{CO}_2$), which can be confirmed by the limewater test—limewater turns cloudy in the presence of $\text{CO}_2$.

The reaction between a carboxylic acid and sodium carbonate is:

$2\text{RCOOH}(aq) + \text{Na}_2\text{CO}_3(aq) \rightarrow 2\text{RCOONa}(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)$

The production of $\text{CO}_2$ bubbles is a reliable indicator that distinguishes carboxylic acids from other acidic substances.

Tests for alkenes

Alkenes are characterised by their carbon-carbon double bond, which is a region of high electron density. This makes alkenes more reactive than alkanes and allows them to undergo addition reactions readily.

Bromine water test

The key test for alkenes involves adding bromine water (or bromine dissolved in an organic solvent) to the sample. Bromine water has a distinctive brown colour. When an alkene is present, the bromine undergoes an addition reaction across the double bond, and the solution loses its colour—it becomes decolourised.

The general reaction is:

$\text{R-CH}=\text{CH-R'} + \text{Br}_2 \rightarrow \text{R-CHBr-CHBr-R'}$

This decolourisation occurs rapidly at room temperature and requires no UV light (unlike the substitution reaction between bromine and alkanes, which requires UV light). If the brown colour of bromine disappears quickly when added to your sample, it indicates the presence of a carbon-carbon double bond.

Tests for alcohols

Alcohols can be identified through several different chemical tests. Because multiple tests are available, it is often useful to use more than one to confirm the presence of an alcohol.

Sodium metal test

The most definitive test for alcohols involves adding a small piece of sodium metal to the sample. This is a teacher demonstration only due to the reactivity of sodium.

When sodium is added to an alcohol, colourless gas bubbles form. This gas is hydrogen ($\text{H}_2$), which can be confirmed using the "pop test"—the gas burns with a popping sound when ignited.

The general reaction is:

$2\text{ROH} + 2\text{Na} \rightarrow 2\text{RO}^-\text{Na}^+ + \text{H}_2$

The product is an alkoxide ion ($\text{RO}^-$), which is the conjugate base of the alcohol. The alkoxide is a strong base and readily reacts with water to regenerate the alcohol:

$2\text{CH}_3\text{CH}_2\text{O}^-(aq) + \text{H}_2\text{O}(l) \rightarrow 2\text{CH}_3\text{CH}_2\text{OH}(aq) + \text{OH}^-(aq)$

The rate at which hydrogen gas is produced can also provide additional information about the type of alcohol present:

• Primary alcohols react fastest
• Secondary alcohols react at a moderate rate
• Tertiary alcohols react most slowly

This difference in reactivity is due to the increasing steric hindrance around the hydroxyl group as more carbon atoms are attached to the carbon bearing the $-\text{OH}$ group.

Bromine water test (negative result)

Unlike alkenes, alcohols do not decolourise bromine water at room temperature. This negative result helps distinguish alcohols from alkenes. If a sample does not react with bromine water but does produce hydrogen with sodium metal, it is likely to be an alcohol.

Esterification test

Alcohols can be identified through their ability to form esters when reacted with carboxylic acids. Add a small amount of glacial acetic acid (concentrated ethanoic acid) and $2-3$ drops of concentrated sulfuric acid to the alcohol sample. Warm the mixture in a water bath for approximately 10 minutes, then pour the mixture into cold water.

If an alcohol is present, an ester will form. Esters have characteristic fruity or sweet smells, so you will detect a pleasant, fruity odour. This smell is distinctive and easily recognised.

Acidified potassium permanganate test

Add drops of acidified potassium permanganate ($\text{KMnO}_4$) solution to the sample. Acidified permanganate has a characteristic pink-purple colour.

If a primary or secondary alcohol is present, the permanganate will be reduced and the pink colour will disappear—the solution becomes decolourised. This occurs because primary and secondary alcohols can be oxidised by permanganate.

Primary alcohols are oxidised first to aldehydes and then to carboxylic acids:

$\text{Primary alcohol} \xrightarrow{[\text{O}]} \text{Aldehyde} \xrightarrow{[\text{O}]} \text{Carboxylic acid}$

Secondary alcohols are oxidised to ketones:

$\text{Secondary alcohol} \xrightarrow{[\text{O}]} \text{Ketone}$

However, tertiary alcohols do not react with permanganate because they cannot be easily oxidised. The pink colour remains unchanged if a tertiary alcohol is present.

Practical application: identifying unknown compounds

By systematically applying these tests, you can identify unknown organic compounds. Here is an example showing how to use test results:

Summary of key differences

To successfully identify organic compounds, remember these key distinguishing features:

Carboxylic acids are the only compounds that:

• Turn blue litmus red
• Produce $\text{CO}_2$ bubbles with sodium carbonate

Alkenes are the only compounds that:

• Decolourise bromine water rapidly at room temperature without UV light

Alcohols are compounds that:

• Produce $\text{H}_2$ gas with sodium metal
• Do NOT decolourise bromine water
• Can form esters with fruity smells
• Decolourise acidified permanganate (primary and secondary only)

By applying these tests systematically and carefully observing the results, you can confidently identify whether an unknown organic compound is a carboxylic acid, an alkene, or an alcohol.