Question

Describe simple chemical tests that would distinguish between: (a) benzene and toluene; (b) bromobenzene and bromocyclohexane; (c) bromobenzene and 3 -bromo-1-hexene: (d) ethylbenzene and benzyl alcohol \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{OH}\right)\).

Short answer

(a) Benzene and toluene can be distinguished using oxidation test with potassium permanganate (\(KMnO_4\)) in alkaline medium. Benzene shows no reaction, while toluene oxidizes to benzoic acid. (b) Bromobenzene and bromocyclohexane can be distinguished using sodium metal substitution reaction. Bromocyclohexane reacts to give a cyclohexene smell, while bromobenzene doesn't. (c) Bromobenzene and 3-bromo-1-hexene can be distinguished using Baeyer’s test with dilute alkaline potassium permanganate solution. Bromobenzene shows no reaction, whereas 3-bromo-1-hexene decolorizes the solution due to the presence of a carbon-carbon double bond. (d) Ethylbenzene and benzyl alcohol can be distinguished using Lucas test with \(ZnCl_2\) and concentrated HCl. Benzyl alcohol turns white turbid due to benzyl chloride formation, while ethylbenzene does not react.

Step-by-step solution

(a) Distinguishing between benzene and toluene

Benzene and toluene can be distinguished using the oxidation test. The reagent used in this test is potassium permanganate (\(KMnO_4\)) in an alkaline medium. Benzene upon oxidation with \(KMnO_4\) does not give any reaction due to the absence of alkyl group. On the other hand, toluene oxidises to benzoic acid. So, this reaction can be used to distinguish between benzene and toluene.

(b) Distinguishing between bromobenzene and bromocyclohexane

Bromobenzene and bromocyclohexane can be distinguished using a substitution reaction involving sodium (Na). When bromocyclohexane is heated with sodium metal, a characteristic smell of cyclohexene is observed due to the formation of cyclohexene. Bromobenzene, however, does not undergo this reaction. So, this reaction can be used to distinguish between bromobenzene and bromocyclohexane.

(c) Distinguishing between bromobenzene and 3-bromo-1-hexene

Bromobenzene and 3-bromo-1-hexene can be distinguished using the Baeyer’s test. The reagent used in this test is a dilute alkaline potassium permanganate solution. Bromobenzene does not react because it does not have a carbon-carbon double bond, while 3-bromo-1-hexene decolorizes the purple colour of the solution due to the presence of a carbon-carbon double bond. So, Baeyer’s test can be used to distinguish between bromobenzene and 3-bromo-1-hexene.

(d) Distinguishing between ethylbenzene and benzyl alcohol

We can distinguish between ethylbenzene and benzyl alcohol using Lucas test. The reagent used in this test is Lucas' reagent, which is a mixture of zinc chloride (\(ZnCl_2\)) and concentrated hydrochloric acid (HCl). Benzyl alcohol quickly turns white turbid due to the formation of benzyl chloride, ethylbenzene, however, does not react. So, the Lucas test can be used to distinguish between ethylbenzene and benzyl alcohol.

Key concepts

Oxidation Test

The oxidation test is a chemical procedure used to identify compounds that can be oxidized. This test particularly distinguishes between hydrocarbons like benzene and toluene. The key reagent for the oxidation test is potassium permanganate \(KMnO_4\). Potassium permanganate is a strong oxidizing agent and, in alkaline conditions, reacts differently with various compounds.

Benzene consists of a stable aromatic ring without an alkyl side chain; therefore, it remains unreactive to the oxidation test. On the other hand, toluene, which has a methyl group attached to the benzene ring, can be oxidized to produce benzoic acid. This happens because the \(KMnO_4\) oxidizes the methyl group completely into a carboxyl group, forming benzoic acid. Hence:

• No reaction with benzene.
• Benzoic acid formation with toluene.

This test is simple yet effective in distinguishing hydrocarbons with different functional groups.

Substitution Reaction

Substitution reactions are fundamental to organic chemistry. They involve the replacement of an atom or group of atoms in a molecule with another atom or group. An example of such reaction is used to differentiate bromobenzene from bromocyclohexane.

When bromocyclohexane is heated with sodium metal, it undergoes a substitution reaction resulting in the formation of cyclohexene. This occurs due to the liberation of hydrogen and the formation of bonds to sodium from the carbon atoms of cyclohexane, which eventually leads to the release of cyclohexene, characterized by its distinctive smell.

Bromobenzene does not undergo this reaction with sodium, primarily due to its stability provided by the aromatic benzene ring, which resists such substitutions. Therefore,

• Bromocyclohexane reacts, producing cyclohexene.
• Bromobenzene remains unchanged.

This difference helps in the identification of the two types of compounds.

Baeyer’s Test

Baeyer’s test is a simple chemical test used to detect unsaturation, particularly the presence of carbon-carbon double bonds (), in organic compounds. The test utilizes dilute alkaline potassium permanganate (), which ends up being decolorized when reacting with alkenes due to the formation of a diol.

In this case, bromobenzene and 3-bromo-1-hexene, the test acts as a distinguishing factor. Bromobenzene, having a stable aromatic ring, does not decolorize the potassium permanganate solution since it lacks a double bond. However, 3-bromo-1-hexene contains a \(C=C\) bond which makes it unsaturated. It reacts with the potassium permanganate solution, leading to its decolorization.

• Bromobenzene - no reaction, no color change.
• 3-bromo-1-hexene - reaction occurs, color is decolorized.

Thus, Baeyer’s test efficiently distinguishes between compounds with and without unsaturation.

Lucas Test

Lucas test is used to differentiate alcohols based on their reactivity with Lucas' reagent, which consists of zinc chloride (\(ZnCl_2\)) and concentrated hydrochloric acid (HCl). This test is particularly useful to distinguish primary, secondary, and tertiary alcohols, but in this context, it helps distinguish ethylbenzene from benzyl alcohol.

When benzyl alcohol is introduced to the Lucas reagent, it reacts within minutes, forming an oily layer of benzyl chloride that turns the solution turbid or cloudy. This turbidity occurs due to the substitution reaction where hydrochloric acid replaces the hydroxyl group in benzyl alcohol.

• Benzyl alcohol reacts quickly, solution becomes turbid.
• Ethylbenzene does not react, solution remains clear.

Conversely, ethylbenzene does not contain the hydroxyl group, and thus does not react with Lucas' reagent, helping to identify its structural differences from benzyl alcohol.