Tests

Tests for alkenes and alkynes

Addition of Bromine

Alkenes are unsaturated hydrocarbons and therefore these undergo electrophilic addition reactions such as the reaction with Bromine.

If Bromine is added to an alkene in the dark there will be reaction and the solution will be decolourised whilst if Bromine is added to an alkane no visible reaction will be observed.

Addition of KMnO₄

In this test, pink colour of KMnO₄ disappears, when alkaline KMnO₄ is added to an unsaturated hydrocarbon. The disappearance of pink colour may take place with or without the formation of brown precipitate of MnO₂.

Terminal Alkynes

Terminal alkynes react with Tollen’s reagents or Fehling’s solution where a white precipitate or a red-brown precipitate is observed respectively.

Test for alcohols

A test for alcohols is the Lucas’ Test, in which one can analyse whether an alcohol is a primary alcohols, secondary alcohols or tertiary alcohol.

The Lucas test is a test analysing the type of alcohol, by producing a substitution reaction with HCl in the presence of dry ZnCl₂. The reaction is as follows:

Due to the fact that the reaction favours the SN1 mechanism, tertiary alcohols would react faster than secondary alcohols while these would have reacted faster than primary alcohols.

• no visible reaction: primary
• the solution turns cloudy in 3-5 minutes: secondary
• the solution turns cloudy immediately, and/or phases separate: tertiary alcohols

Iodoform Test

Apart from the Lucas’ test, which can be performed on all alcohols, the Iodoform test can be produced on any alcohol which has got –CH(OH)CH₃ group, or better still the C=OCH₃ group.

The iodoform test is as follows:

It must be noted that for ketones and aldehydes the same reaction can take place, but the first step, ie. the oxidation of the alcohol group to a carbonyl group would not take place.

PCl₅

When PCl₅ is added to a compound containing the -OH group white misty fumes can be observed. These misty fumes will be fumes of HCl.

Tests for Aldehydes and Ketones

2,4-DNPH

2,4-dinitrophenyl hydrazine can be used to qualitatively detect the carbonyl group of an aldehyde or a ketone. A positive result is indicated by the formation of anyellow or orange-red precipitate of 2,4-dinitrophenyl hydrazone.

Tollen’s Test

Tollen’s reagent is ammoniacal silver nitrate. Aldehydes react with Tollen’s reagent to form elemental silver, accumulated onto the inner surface of the reaction vessel, producing silver mirror on the inner surface of the vessel.

Fehling’s Test

This is an important test to distinguish aldehydes from ketones. The reagents used in this test are Fehling’s solution A and Fehling’s solution B. Fehling’s solution A is an aqueous solution of copper sulfate and Fehling’s solution B is a clear solution of sodium potassium tartrate (Rochelle salt) and strong alkali (usually NaOH).

The final Fehling’s solution is obtained by mixing equal volumes of both Fehling’s solution A and Fehling’s solution B that has a deep blue colour. In Fehling’s solution, copper (II) ions form a complex with tartrate ions in alkali. Aldehydes reduces the Cu(II) ions in the fehling’s solution to red precipitate of cuprous oxide(copper (I) oxide).