Question

: Describe simple chemical tests (if any) that would distinguish between: (a) styrene and ethylbenzene; (b) styrene and phenylacetylene; (c) allylbenzene and 1 -nonene; (d) allylbenzene and allyl alcohol $({\mathrm{CH}}_2=\mathrm{CH}-{\mathrm{CH}}_2\mathrm{OH})$. Tell exactly what you would do and see.

Short answer

(a) To distinguish between Styrene and Ethylbenzene, perform a Bromine water test. Styrene decolorizes the brown bromine water, while Ethylbenzene does not. (b) To distinguish between Styrene and Phenylacetylene, perform Baeyer's test using dilute potassium permanganate. The solution turns brown with Styrene but remains purple with Phenylacetylene. (c) To distinguish between Allylbenzene and 1-Nonene, perform a Ferric chloride test. Allylbenzene will give a negative result (no color change), while 1-Nonene remains unaffected. (d) To distinguish between Allylbenzene and Allyl Alcohol, perform the Lucas test using Lucas reagent. Allyl alcohol reacts to form a turbid solution or a separate layer, while Allylbenzene remains clear.

Step-by-step solution

(a) Distinguishing between Styrene and Ethylbenzene

Styrene has a vinyl group ($C{H}_2=CH-$) and Ethylbenzene doesn't. This difference allows for a Bromine water test, where the Bromine adds across the double bond. Procedure: Add bromine water to separate samples of Styrene and Ethylbenzene. Result: Styrene decolorizes the brown bromine water because of the addition reaction that occurs at the double bond. Ethylbenzene, which has no such double bond, does not react; thus, the bromine water retains its brown color.

(b) Distinguishing between Styrene and Phenylacetylene

Styrene has an alkene group, while Phenylacetylene has an alkyne group. This difference provides the basis for a Baeyer's test. Procedure: To separate samples of Styrene and Phenylacetylene, add dilute potassium permanganate solution. Result: The solution turns from purple to brown with Styrene (indicating a positive test for an alkene), but it remains purple with Phenylacetylene since alkyne groups are resistant to oxidation under these mild conditions.

(c) Distinguishing between Allylbenzene and 1-Nonene

Allylbenzene is an aromatic compound, while 1-nonene is an alkene. You can distinguish between them using the Ferric chloride test, which tests for phenols. Procedure: Add a few drops of Ferric chloride solution to separate samples of Allylbenzene and 1-Nonene. Result: Allylbenzene will give a negative result (no color change), as there is no phenol group, while 1-Nonene will remain unaffected.

(d) Distinguishing between Allylbenzene and Allyl Alcohol

Allylbenzene is an aromatic compound, whereas allyl alcohol is an alcohol. They can be distinguished using the Lucas test which is for alcohols. Procedure: Add Lucas reagent (a solution of Zinc chloride in concentrated Hydrochloric acid) to separate samples of Allyl benzene and Allyl alcohol. Result: Allyl alcohol reacts with Lucas reagent to form a turbid solution or a separate layer (as indication of the presence of a secondary or primary alcohol, respectively), while Allylbenzene remains clear (no reaction) because it does not have a hydroxyl group.

Key concepts

Bromine Water Test

The Bromine Water Test is a simple and effective chemical test to detect unsaturated compounds like alkenes and alkynes. Alkenes have a double bond, which makes them reactive towards bromine.
Here's what happens during the test:

• Add bromine water, which has a visible brown-red color, to the sample you are testing.
• If the sample contains a double bond, like in the case of styrene, it will react with bromine, causing the solution to lose its color, turning from brown to colorless.
• If no double bond is present, such as in ethylbenzene, the solution remains brown as there is no reaction.

This color change indicates a positive test for an unsaturated compound. It's quick to perform and doesn’t require complicated equipment, making it an accessible method in both academic settings and industrial applications.

Baeyer's Test

Baeyer's Test is designed to identify alkenes through the use of dilute potassium permanganate, a strong oxidizing agent. This test takes advantage of the oxidizing ability of potassium permanganate:

• When dilute potassium permanganate is added to a sample containing an alkene like styrene, the solution changes from purple to brown. This signifies that the alkene has been oxidized to a diol, a compound containing two hydroxyl groups.
• If the compound is an alkyne, such as phenylacetylene, it will not react under these mild conditions, and the purple color persists. This indicates a negative result for an alkene.

Baeyer's test is both simple and effective, particularly suitable for distinguishing alkenes from alkynes, but it's important to note that it can also detect other compounds that might react with oxidizing agents.

Ferric Chloride Test

The Ferric Chloride Test is specifically used to detect phenols by producing a characteristic color change. Here’s how the test works:

• When a few drops of ferric chloride solution are added to the test sample, if a phenol group is present, a distinct color (such as violet, blue, or green) appears.
• If no phenol group is present, like in allylbenzene or 1-nonene, the solution remains unchanged, indicating a negative result.

This test helps distinguish phenolic compounds from those without phenol groups and is often used in organic chemistry research and education due to its straightforward nature. However, be aware that not all phenols will show the same color, and some compounds without phenol groups might occasionally show false positives.

Lucas Test

The Lucas Test is a classic method used to differentiate between different types of alcohols (primary, secondary, and tertiary). It relies on the reactivity of alcohols with the Lucas reagent, which is a mixture of concentrated hydrochloric acid and zinc chloride:

• To perform the test, add Lucas reagent to your alcohol sample. If the alcohol is secondary or tertiary, you'll see a reaction producing a cloudy emulsion or a separate layer fairly quickly for tertiary alcohols and slower for secondary alcohols.
• Primary alcohols react very slowly or not at all, and as such do not cause cloudiness during the testing period.
• When testing allyl alcohol and allylbenzene, only allyl alcohol will react, demonstrating the presence of a hydroxyl group by forming turbidity, while allylbenzene remains clear as there’s no hydroxyl group to react with the Lucas reagent.

This test is useful in laboratory settings where rapid identification of alcohol types is necessary, but care should be taken as some non-alcohols might react slowly.