Organic analysis involves identifying the functional groups present within a molecule. Before using complex spectroscopic techniques, simple qualitative test-tube reactions can be performed in the laboratory to quickly confirm the presence or absence of specific functional groups.
🔑 Key Principle 1
Qualitative chemical tests rely on distinct visual changes, such as colour changes, precipitate formation, or effervescence, which occur due to the characteristic chemical reactivity of each functional group.
🔑 Key Principle 2
Tollens reagent and Fehlings solution are mild oxidising agents. They are capable of oxidising aldehydes to carboxylic acids, but are not strong enough to oxidise ketones. This difference in chemical reactivity is used to distinguish between these two classes of carbonyl compounds.
Summary of Functional Group Tests
You must know the reagents, conditions, and observations for tests involving alkenes, alcohols, aldehydes, ketones, and carboxylic acids.
A qualitative laboratory procedure used to identify the presence of a functional group or chemical species by observing a distinct visual change.
A mild oxidising agent containing the diamminesilver(I) complex ion, \( [\text{Ag}(\text{NH}_3)_2]^+ \), dissolved in aqueous ammonia. It is reduced to metallic silver when heated with an aldehyde.
A blue alkaline solution containing copper(II) ions complexed with tartrate ions. It is reduced to a brick-red precipitate of copper(I) oxide, \( \text{Cu}_2\text{O} \), when heated with an aldehyde.
The rapid escape of gas bubbles from a liquid mixture as a result of a chemical reaction.
1. Test for Alkenes
Alkenes contain an unsaturated carbon-carbon double bond (\( \text{C}=\text{C} \)), which is an electron-rich area of high electron density. When orange-brown bromine water is added, an electrophilic addition reaction occurs across the double bond. The bromine is consumed, forming a colourless dihalogenoalkane:
\[ \text{CH}_2=\text{CH}_2 + \text{Br}_2\text{(aq)} \rightarrow \text{CH}_2\text{BrCH}_2\text{Br(aq)} \]The visual change is orange-brown to colourless. Saturated hydrocarbons (alkanes) do not react with bromine water under standard laboratory conditions, so the mixture remains orange-brown.
2. Test for Alcohols
Alcohols can be classified as primary (1°), secondary (2°), or tertiary (3°). We can distinguish them using acidified potassium dichromate(VI), \( \text{K}_2\text{Cr}_2\text{O}_7 \), acidified with dilute sulfuric acid, \( \text{H}_2\text{SO}_4 \):
- Primary and Secondary Alcohols: Easily oxidised. The orange dichromate(VI) ions, \( \text{Cr}_2\text{O}_7^{2-} \), are reduced to green chromium(III) ions, \( \text{Cr}^{3+} \). \[ \text{R-CH}_2\text{OH} + [\text{O}] \rightarrow \text{R-CHO} + \text{H}_2\text{O} \]
- Tertiary Alcohols: Do not react because there is no hydrogen atom attached to the carbon bonded to the hydroxyl (\( \text{-OH} \)) group. The solution remains orange.
In exams, do not write "dichromate oxidises alcohols". Be specific: acidified potassium dichromate(VI) turns green with primary and secondary alcohols, but stays orange with tertiary alcohols. Make sure to specify the condition: heated under reflux or distillation.
3. Distinguishing Aldehydes from Ketones
Both aldehydes and ketones contain the carbonyl group (\( \text{C}=\text{O} \)). However, aldehydes can be oxidised to carboxylic acids, while ketones are resistant to further oxidation. This difference in chemical properties is exploited using mild oxidising agents:
Tollens Reagent (Silver Mirror Test)
Tollens reagent contains the diamminesilver(I) complex ion, \( [\text{Ag}(\text{NH}_3)_2]^+ \). When heated in a water bath with an aldehyde, the aldehyde is oxidised to a carboxylic acid, and the silver(I) ions are reduced to metallic silver:
\[ [\text{Ag}(\text{NH}_3)_2]^+ + \text{e}^- \rightarrow \text{Ag(s)} + 2\text{NH}_3 \]This forms a silver mirror coating on the inside of the test tube. Ketones show no reaction, so the solution remains colourless.
Tollens reagent must be prepared freshly in the lab. Add sodium hydroxide to silver nitrate to form a brown precipitate of silver oxide, \( \text{Ag}_2\text{O} \), then add dilute ammonia dropwise until the precipitate just redissolves. Do not heat Tollens reagent directly with a Bunsen burner: always use a warm water bath to avoid the risk of creating explosive mixtures and to control heating.
Fehling Solution
Fehling solution is an alkaline solution containing copper(II) ions complexed with tartrate ions, which gives it a deep blue colour. When heated with an aldehyde, the aldehyde is oxidised and the blue \( \text{Cu}^{2+} \) ions are reduced to a brick-red precipitate of copper(I) oxide, \( \text{Cu}_2\text{O} \):
\[ 2\text{Cu}^{2+} + 2\text{OH}^- + 2\text{e}^- \rightarrow \text{Cu}_2\text{O(s)} + \text{H}_2\text{O} \]The visual change is a blue solution forming a brick-red precipitate. Ketones show no reaction, and the solution remains blue.
4. Test for Carboxylic Acids
Carboxylic acids contain the acidic carboxyl group (\( \text{-COOH} \)). They react with metal carbonates, such as sodium carbonate, \( \text{Na}_2\text{CO}_3 \), in an acid-base reaction to produce a sodium salt, water, and carbon dioxide gas:
\[ 2\text{R-COOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{R-COONa} + \text{H}_2\text{O} + \text{CO}_2 \]This reaction results in rapid effervescence. The carbon dioxide gas produced is bubbled through limewater, which turns cloudy due to the formation of a calcium carbonate precipitate:
\[ \text{Ca(OH)}_2\text{(aq)} + \text{CO}_2\text{(g)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)} \]Always state the starting and final appearance in your observations. Do not just write "red precipitate" or "silver" for Tollens and Fehling's tests. Write "blue solution turns to a brick-red precipitate" and "colourless solution forms a silver mirror on the tube walls". This matches the detail expected by AQA examiners.
Worked Examples
Solution:
1. Add a few drops of Fehling solution to separate test tubes containing propanal and propanone.
2. Warm both test tubes gently in a water bath.
3. Observation with propanal: The blue solution changes to form a brick-red precipitate. This occurs because propanal is an aldehyde and is oxidised to propanoic acid.
4. Observation with propanone: The solution remains blue. This occurs because propanone is a ketone and resists oxidation by mild oxidising agents.
Solution:
1. The reaction with sodium carbonate indicates that compound X is a carboxylic acid, containing the \( \text{-COOH} \) group.
2. Decolourisation of bromine water shows that compound X contains an unsaturated carbon-carbon double bond (\( \text{C}=\text{C} \)).
3. Tollens reagent shows no reaction, confirming the absence of an aldehyde group.
4. Let's analyze the molecular formula \( \text{C}_4\text{H}_6\text{O}_2 \): The carboxyl group (\( \text{-COOH} \)) accounts for 1 carbon, 2 oxygen, and 1 hydrogen atom, plus 1 degree of unsaturation (the \( \text{C}=\text{O} \) bond). This leaves \( \text{C}_3\text{H}_5 \) for the rest of the molecule.
5. The alkene double bond accounts for another degree of unsaturation. Therefore, the remaining part of the molecule is an alkenyl chain. With 4 carbons in total, the structure is butenoic acid. The double bond can be at position 2 or 3. A suitable structure is but-2-enoic acid, \( \text{CH}_3\text{CH}=\text{CHCOOH} \) or but-3-enoic acid, \( \text{CH}_2=\text{CHCH}_2\text{COOH} \).
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