Test for Aldehydes: Chemical Reactions, Procedure & Observation

Test For Aldehydes: The aldehydes and ketones contain a carbonyl group. In a carbonyl group, a carbon atom is double-bonded to an oxygen atom \({\rm{C = O}}{\rm{.}}\) At the same time, the carbon is also bonded to two other atoms or groups.

The carbonyl group in aldehydes is connected to a hydrogen atom as well as aliphatic or aromatic radicals. Formaldehyde is an unusual example in which two hydrogen atoms are bonded to the carbonyl in the compound. Two aliphatic or aromatic groups are attached to the carbonyl group in ketones.

In this article, we have discussed in detail about Aldehydes, their tests, chemical reactions, procedure, observations, etc. Continue reading to know more.

Aldehydes and Ketones

In an aldehyde, the carbonyl group is always on an end carbon. 

To name aldehydes, use the longest parent chain name of the alkane and replace \({\rm{‘e’}}\) of \({\rm{‘ – ane’}}\) with the suffix \({\rm{‘ – al’}}{\rm{.}}\) Thus, the name of \({\rm{C}}{{\rm{H}}_3}{\rm{CHO}}\) is ethanal, and \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{CHO}}\) is propanal. A homologous series for aldehydes is methanal, ethanal, propanal, and butanal with chemical formulas of\({\rm{HCHO}},{\rm{C}}{{\rm{H}}_3}{\rm{CHO}},{\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{CHO}},\) and \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}{\rm{CHO,}}\) respectively, where each successive compound differs from the previous one by a \({\rm{ – C}}{{\rm{H}}_{\rm{2}}}\) group.

The general formula of the homologous aldehyde series is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{{\rm{2n}}}}{\rm{O}}\), where \({\rm{n}} = 1,2,3 \ldots \)

In a ketone, the carbonyl group is never on an end carbon. Acetone, for example, is a ketone and is the main component in nail polish remover, while cumin aldehyde is an aldehyde present in cumin seeds. To name ketones, use the longest parent chain name of the alkane and replace ‘e’ of ‘–ane’ with the suffix ‘–one’. For parent chains with more than four carbon atoms, the position of the carbonyl group must be indicated. The general formula of the homologous ketone series is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{{\rm{2n}}}}{\rm{O}}\), where \({\rm{n}} = 1,2,3 \ldots \)

Learn Oxidation of Aldehydes

Tests to Distinguish Aldehyde and Ketones from Other Functional Groups

The following tests are performed to distinguish aldehyde and ketones from other functional groups.

1. 2,4-dinitrophenylhydrazine Test

2,4-Dinitrophenylhydrazine (DNPH), also known as Brady’s Reagent or Borche’s Reagent, is an organic compound with the formula \({{\rm{C}}_6}{{\rm{H}}_3}{\left( {{\rm{N}}{{\rm{O}}_2}} \right)_2}{\rm{NHN}}{{\rm{H}}_2}{\rm{ }}{\rm{. }}\) A positive test is signalled by the formation of a yellow, orange or red precipitate of the dinitrophenylhydrazone. Aromatic carbonyls give red precipitates, whereas aliphatic carbonyls give a more yellow colour.

The chemical reaction is given below.

This reaction is a condensation reaction in which two molecules join together with a loss of water. Mechanistically, it is an example of an addition-elimination reaction: nucleophilic addition of the \({\rm{ – N}}{{\rm{H}}_{\rm{2}}}\) group to the \({\rm{C = O}}\) carbonyl group, followed by eliminating an \({{\rm{H}}_{\rm{2}}}{\rm{O}}\) molecule.

2. Sodium Bisulfite Test

Sodium hydrogen sulphite, also known as sodium bisulphite, is used to detect carbonyl groups from other functional groups. This reaction only works well for aldehydes because, in aldehydes, bulky groups are not attached to the carbonyl group. Whereas, in the case of ketones, the presence of bulky hinders the progress of the reaction. One of the hydrocarbon groups attached to the carbonyl group needs to be a methyl group to have an effective result. 

Aldehydes and ketones combine with sodium bisulfite to form well crystallised water-soluble products known as “aldehyde bisulfite” and “ketone bisulfite”.

The chemical reaction is given below.

With propanone, the equation is:

The general formula of the aldehyde and ketone depicts that the carbonyl group is attached to the end carbon atom in aldehydes. In contrast, in ketones, the carbonyl group is never on the end carbon atom; this enables aldehydes to get easily oxidised to carboxylic acids, which is not the case with ketones.

Test for Distinguishing Aldehydes from Ketones

Aldehydes can be distinguished from ketones by the following tests.

1. Schiff’s Test

When sulphur dioxide is passed through a solution of dye fuchsin, a colourless addition product is formed called the Schiff’s Reagent. Aldehydes abstract sulfurous acid from the Schiff’s Reagent and restores the pink colour. The colouration is due to the formation of a complex compound. Ketones, in general, do not respond to this reaction.

The reaction should not be subjected to heat. Some ketones give a light pink colour with Schiff’s Reagent. Therefore, light pink colour formation is not a positive test.

Observation: Appearance of pink, red or magenta colour indicates the presence of the aldehyde group.

2. Test of Alhedydes using Fehling’s Solution Test

Fehling’s solution is a mixture of two solutions. Fehling’s solution \({\rm{A + }}\) Fehling’s solution \({\rm{B}}\).
Fehling’s solution \({\rm{A}}\) contains \({\rm{7}}\,{\rm{g}}\) of \({\rm{CuS}}{{\rm{O}}_{\rm{4}}}{\rm{.7}}{{\rm{H}}_{\rm{2}}}{\rm{O}}\) in \({\rm{100}}\,{\rm{ml}}\) of water.
Fehling’s solution \({\rm{B}}\) contains \({\rm{24}}\,{\rm{g}}\) of \({\rm{KOH}}\) and \({\rm{34.6}}\,{\rm{g}}\) of potassium sodium tartrate in \({\rm{100}}\,{\rm{ml}}\) water.

1. On heating, the sample with the Fehling’s solution, bistartarocuprate \(\left( {{\rm{II}}} \right)\) complex, oxidises the aldehydes to corresponding carboxylic acids.
2. The copper \(\left( {{\rm{II}}} \right)\) ions of the complex are reduced to insoluble yellow or red-coloured precipitate or cuprous \(\left( {{\rm{I}}} \right)\) oxide \(\left( {{\rm{C}}{{\rm{u}}_2}{\rm{O}}} \right)\) ions.
3. The ketones, on the other hand, are oxidised to yield shorter chains of acids.
4. If Fehling’s solution is heated in the absence of reducing sugars, it forms a black precipitate of cupric oxide.

Aromatic aldehydes do not respond to Fehling’s test. Therefore, an aqueous solution of the compound may be used instead of an alcoholic solution. Formic acid also gives this test.

Observation: Fehling’s solution gives an appearance of red precipitate, confirming the presence of an aldehydic group.

3. Test for Aldehydes using Tollens’ reagent

Tollen’s test is also known as the silver mirror test. Tollens reagent consists of a silver ammonia complex in an ammonia solution. Tollens’ Reagent is prepared as follows-

Aldehydes react with Tollens’ Reagent to give a grey-black precipitate which appears like a silver mirror on the walls of the test tube. The tollens’ Reagent oxidise aldehydes to their corresponding carboxylic acid. The oxidation state of silver in Tollens reagent is reduced from \(+1\) to its elemental form. Generally, ketones do not respond to this test.

Observation: Tollens’ reagent gives the appearance of a shiny silver mirror confirming the presence of aldehydes.

4. Test with Chromic Acid

A positive test is marked by the formation of a green to blue colour opaque suspension within \(5\) seconds upon addition of the orange-yellow chromic acid reagent to aldehydes. Ketones do not react with chromic acid. Primary and secondary alcohols also respond to this test but fail in the dinitrophenylhydrazine test.

The chemical reaction is given below.

Observation: The appearance of green or blue colour precipitate confirms the presence of aldehydes.

5. Sodium Nitroprusside Test

Ketones respond to this test positively. Ketones react with alkalis to form an anion. This anion reacts further with sodium nitroprusside solution forming a red-coloured complex ion. Aldehydes do not respond to this test.

The chemical reaction is given below:

\({\rm{C}}{{\rm{H}}_3}{\rm{COC}}{{\rm{H}}_3} + {\rm{O}}{{\rm{H}}^ – } \to {\rm{C}}{{\rm{H}}_3}{\rm{COCH}}_2^ – + {{\rm{H}}_2}{\rm{O}}\)

\({\left[ {{\rm{Fe}}{{({\rm{CN}})}_5}{\rm{NO}}} \right]^{2 – }} + {\rm{C}}{{\rm{H}}_3}{\rm{COCH}}_2^ – \to {\left[ {{\rm{Fe}}{{({\rm{CN}})}_5}{\rm{NO}}.{\rm{C}}{{\rm{H}}_3}{\rm{COC}}{{\rm{H}}_2}} \right]^{3 – }}\)

Observation: The appearance of red colouration shows the presence of ketone.

Summary

Aldehydes belong to a particular class of organic compounds. As both aldehyde and ketones contain a carbonyl group, it is pretty essential to distinguish them. This article summarises the various tests through which we can identify carbonyl groups from other functional groups. It also gives a brief idea about the various tests that could distinguish between an aldehyde and ketone.

Frequently Asked Questions

We have provided some frequently asked questions here:

Q.1. What are aldehydes found in?
Ans: Aldehydes possess a delightful odour. They are used in various cosmetics, perfumes, cleaning products, odorant dispensers and grooming aids. In addition, aldehydes are the starting materials for many medicinal products and polymers.

Q.2. What is the general test for the presence of aldehydes?
Ans: Tollen’s Reagent is used to test the presence of aldehydes. The addition of Tollen’s Reagent to the aldehyde sample gives a silver mirror-like appearance observed on the test tube’s sidewalls. The test was used as one of the confirmatory tests for aldehydes.

Q.3. Does aldehyde give Fehling’s test?
Ans: Yes, aldehydes respond positively to the Fehlings’ solution test. Fehling’s solution is a mixture of two solutions- Fehling’s A (Copper sulphate) and Fehling’s B (alkaline solution of sodium-potassium tartrate). The appearance of a brick-red suspension on heating the aldehyde sample with Fehling’s solution confirms the presence of the aldehyde group. The brick-red colouration is due to the formation of cuprous oxide \(\left( {{\rm{C}}{{\rm{u}}_{\rm{2}}}{\rm{O}}} \right).\)

Q.4. How do you know if something is an aldehyde?
Ans: The most common confirmatory test to detect the presence of aldehyde is Fehlings’ solution test and Silver mirror test.
Fehlings’ solution test: The appearance of a red coloured opaque suspension on adding the aldehyde sample to the Fehlings’ solution confirms the presence of aldehydes.
Silver Mirror test: The addition of Tollen’s Reagent to the aldehyde sample gives a silver mirror-like appearance observed on the test tube’s sidewalls. The test was used as one of the confirmatory tests for aldehydes.

Q.5. How would you distinguish an aldehyde from a ketone by chemical tests?
Ans: An aldehyde can be distinguished from a ketone by Fehling’s solution test, Tollen’s reagent test, Schiff’s test and sodium nitroprusside test.

Learn About Aldehyde Nucleophilic Addition

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