Chemical Tests for the Presence of Aldehydes, Ketones and Carboxylic Acids

This is part of the HSC Chemistry course under the section: Analysis of Organic Substances.

HSC Chemistry Syllabus

Conduct qualitative investigations to test for the presence in organic molecules of the following functional groups:

– carbon–carbon double bonds

– hydroxyl groups

– carboxylic acids (ACSCH130)

Chemical Tests For Aldehydes, Ketones and Carboxylic Acids

This video explores how qualitative chemical tests such as oxidation can distinguish between an aldehyde and ketone. The video also explores qualitative tests for the presence of carboxylic acids.

Chemical Tests For Aldehydes and Ketones

Acidified dichromate (Cr₂O₇^2–/H⁺) Oxidation Test

This test is used to differentiate between:

tertiary alcohols from primary and secondary alcohols
ketones and aldehydes.

When Cr⁶⁺ ions (in dichromate) are reduced to form Cr³⁺, the solution turns from orange to green.

$$\frac{1}{2}Cr_2O_7^{2-} + 7H^+ + 3e^- \rightleftharpoons Cr^{3+} + \frac{7}{2}H_2O$$

Dichromate will oxidise primary and secondary alcohols but not tertiary alcohols

Solution turns green when added to primary or secondary alcohols
Solution remains orange when added to tertiary alcohols.

Dichromate will oxidise an aldehyde into a carboxylic acid but does not react with ketone.

Solution turns green in the presence of an aldehyde.
Solution remains orange in the presence of a ketone.

Cr⁶⁺(orange) reduces to form Cr³⁺(green)

Observation for different types of alcohol

Acidified Permanganate (MnO₄^–/H⁺) Oxidation Test

This test is used to differentiate between:

tertiary alcohols from primary and secondary alcohols
ketones and aldehydes.

When Mn⁷⁺ ions (in permanganate) are reduced to form Mn²⁺, the solution turns from purple to colourless

$$MnO_4^- + 8H^+ + 5e^- \rightleftharpoons Mn^{2+} + 4H_2O$$

Permanganate will oxidise primary and secondary alcohols but not tertiary alcohols

Solution decolourises when added to primary or secondary alcohols
Solution remains purple when added to tertiary alcohols.

Permanganate will oxidise an aldehyde into a carboxylic acid but does not react with ketone.

Solution decolourises in the presence of an aldehyde
Solution remains purple in the presence of a ketone

Mn⁷⁺ (purple) reduces to form Mn²⁺ (colourless)

Chemical Tests For Carboxylic Acids

Using pH Indicators

A solution containing carboxylic acid will have a pH < 7 at 25ºC. This means a blue litmus paper will turn red, bromothymol blue will be yellow.

Ketones and aldehydes do not contain acidic hydrogens so their solutions will be neutral.

Sodium Carbonate Test

Carboxylic acids undergo acid-base reaction with carbonate and hydrogen carbonates to produce salt, water and carbon dioxide. No acid and base reactions occur between an aldehyde/ketone and carbonates.

When Na₂CO₃ or NaHCO₃ is added to a test tube, formation of bubbles indicates the production of carbon dioxide, which in turn indicates the presence of a carboxylic acids.

Production of CO₂ can be further confirmed by using limewater test by adding calcium hydroxide (lime water). The presence of carbon dioxide would react with lime water to produce calcium carbonate, causing the solution to turn cloudy and milky.

CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)

Limewater test of carbon dioxide: solution of calcium hydroxide (left) and calcium carbonate formation after carbon dioxide is bubbled into the solution (right).