How will you distinguish between aldehyde and ketone?

Aldehydes and Ketones

Common tests of Aldehydes and Ketones

Differentiation between aldehyde and ketone and other compounds can be identified based on general tests of aldehydes and ketones. The following are common tests of aldehyde and ketone.

01. 2,4-Dinitrophenylhydrazine test: If a yellow, orange or red precipitate is obtained when heating the carbonic compound with 2,4-Dinitrophenylhydrazine solution in the presence of concentrated sulfuric acid, it exhibits the presence of aldehyde or ketone group.

02. Sodium bisulfite test: If a white precipitate is obtained when the carbonic compound is shaken with a saturated solution of sodium bisulfite, it exhibits the presence of aldehyde or ketone group.

03. Schiff‘s Reaction Test: If the carbonic compound is added to the schiff reagent, it exhibits a pink color, indicating the presence of aldehyde group.

04. Tollen reagent test: shows the presence of aldehyde group if the carbonic compound is obtained with a silver mirror when heated with tollen reagent.

05. Fehling solution test: If the carbonic compound gets red color on heating with fehling solution then it shows the presence of aldehyde group.

If a compound gives positive results in all the above tests then it is an aldehyde. If a compound gives positive results in Test no. 1 and 2 and negative in Test no. 3, 4 and 5, then it is a ketone.

Aldehyde Reactions

Polymerization of Aldehydes

The first few members of the aldehyde series exhibit polymerization reactions.

Example:

(i) Aqueous solution of formaldehyde evaporates to yield a white solid substance called para formaldehyde.

n HCHO → (HCHO)n

The value of n ranges from 6 to 100 in the above reaction. The melting point of para formaldehyde is 121°-123°C. By heating it, formaldehyde can be obtained again.

(ii) When formaldehyde gas is kept at room temperature for a long time, it polymerise to form meta formaldehyde.

Meta-formaldehyde

(iii) By pouring a few drops of concentrated H₂SO₄ at an ordinary temperature in anhydrous acetaldehyde, three molecules of acetaldehyde combine to form para aldehyde.

Para-aldehyde

Re-acetaldehyde can be obtained by distilling para aldehyde with dilute H₂SO₄.

(iv) After adding a few drops of concentrated H₂SO₄ at 0°C to anhydrous acetaldehyde, its four molecules combine to form meta aldehyde.

AC-to-meta

Schiff‘s Reaction

Rosaniline hydrochloride, also called megenta, is pink in color. The color of the solution flies away when SO₂ gas flows into its aqueous solution. This colorless solution is called schiff reagent. The schiff reagent is reaction with aldehyde and gives pink colour.

Reaction with sodium hydroxide

Aldehydes without α-hydrogen such as formaldehyde when heated with concentrated NaOH results in their disproportionation. An alcohol and salts of a fatty acid are obtained as a result of this reaction.

Example:

2HCHO + NaOH → CH₃OH + HCOONa

This reaction is called the Cannizzaros Reaction.

Aldehydes containing α-hydrogen reaction with Concentrated NaOH give complex regent-like substances. Ketone does not show these reactions.

Reaction with ammonia

The reaction of formaldehyde and ammonia results in the formation of Hexamethylenetetramine [(CH₂)₆N₄] which is also called urotropin.

6HCHO + 4NH₃ → (CH₂)₆N₄ + 6H₂O

Other aldehyde forms additive products with ammonia, which are called aldehyde ammonia.

Example:

diacetoneamine

Reducing Properties

aldehyde is a strong reducing agent, hence reducing tollens reagent and fehling solution. Ketone does not reduce these reagents.

Tollen’s Reagent | Fehling & Benedict’s Solution

1. Oxidation of aldehydes a) Oxidation by Tollen’s reagent b) Oxidation by Fehling solution c) Oxidation by Benedict’s solution 2. Oxidation of ketones a) By strong oxidizing agents b) Popoff’s rule

CH₃CHO + Ag₂O → CH₃COOH + 2Ag

CH₃CHO + 2CuO → CH₃COOH + Cu₂O

Ammonia silver nitrate solution is called tollens reagent. By reducing aldehyde tollens reagent, it forms metallic silver which is obtained in the form of silver mirror. On heating with fehling solution, they give red precipitates due to formation of Cu₂O.

Reaction with Alcohol

In the presence of dry hydrogen chloride gas, the reaction of alcohols and aldehydes forms a temporary compound called hemiacetal which then reacts with alcohol to form a permanent compound called acetal.

hemiacetal

Ketones reactions

Reaction with chloroform

In the presence of caustic potash (KOH), chloroform reacts with kitones to form chloro hydroxy compounds.

Example:

chloretone

Reaction with nitrous acid

Ketone reacts with nitrous acid to make oximino ketones.

example:

CH₃ – CO – CH₃ + O = N – OH → CH₃COCH = NOH + H₂O

Reaction with Sodamide

Ketone reacts with ether solution of sodium or sodamide to form sodium products. Example:

CH₃COCH₃ + NaNH₂ → CH₃ – CO – CH₂Na + NH₃

Stay Connected

Editor's Choice

Introduction of Inductive-Effect || How does Inductive Effect Work?

2023-09-19