Physical Properties of Aldehydes and Ketones

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What are Aldehyde and Ketones?

Aldehydes and ketones are compounds which contain a carbonyl group and therefore, these compounds are collectively called carbonyl compounds. There is a double bond (one sigma and one pi bond) between carbon and oxygen. Due to the difference in electronegativity between carbon and oxygen, the carbonyl bond is polar in nature. In aldehydes, the carbonyl group is attached to one hydrogen atom and one alkyl or aryl group, whereas in ketones, it is attached to both alkyl or aryl groups.


Structure of Aldehydes and Ketones

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Physical Properties of Aldehydes and Ketones

  • Physical State

Methanal is a pungent-smelling gas. Ethanal is a volatile liquid. Other aldehydes and ketones continuing up to eleven carbon atoms are colourless liquids while still higher members are solids.

  • Smell

Except the lower carbon aldehydes which have unpleasant odours, other all aldehydes and ketones have generally pleasant smell. As the size of the aldehyde and ketone molecule increases, the odour becomes less pungent and more fragrant. In fact, many naturally occurring aldehydes and ketones have been used in the blending of perfumes and flavouring agents. 

  • Solubility 

Aldehydes and ketones up to four carbon atoms are miscible with water. This is due to the presence of hydrogen bond association between the polar carbonyl group and water molecules as shown below:

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However, the solubility of aldehydes and ketones in water decreases rapidly on increasing the length of the alkyl chain (carbon chain). As a result, the higher members with more than four carbon atoms are practically insoluble in water. All aldehydes and ketones are soluble in organic solvents (like dissolves like) such as benzene, ether, chloroform, and alcohol. 

  • Boiling Point

The boiling points of aldehydes and ketones are higher than those of non-polar compounds (hydrocarbons) or weakly polar compounds of comparable molecular masses. However, their boiling point is lower than those of corresponding alcohols or carboxylic acids. This is because aldehydes and ketones are polar compounds having sufficient intermolecular (between the molecules) dipole-dipole interactions between the opposite ends of carbonyl dipoles. 


Chemical Properties of Aldehydes and Ketones

The chemical properties of aldehydes and ketones are due to the polar carbonyl group present in their molecules. 


1. Reaction With Hydrogen Cyanide

Both aldehydes and ketones react with hydrogen cyanide to form an additional product known as cyanohydrins.

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2. Reaction With Sodium Bisulphite

Both aldehydes and ketones form crystalline addition compounds called bisulphite adducts when treated with a saturated solution of sodium bisulphite.

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3. Reaction With Grignard Reagents

Aldehydes and ketones react with Grignard reagent to form addition products. When the addition product is hydrolysed by water, it gives alcohol.

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4. Reaction With Alcohols

Aldehydes react with alcohols in the presence of dry HCl gas to give gem- dialkoxy compounds. These compounds are called acetals.

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Did You Know?

  • Formation of a yellow precipitate of iodoform is used as a test for certain aldehydes and ketones which have methyl groups bonded to a carbonyl group. This test is carried in presence of sodium carbonate and iodine solution. This reaction is known as the iodoform test.

  • The hybridisation of carbon in the carbonyl group is SP2.

  • The shape of the carbonyl molecule is trigonal planar.

FAQ (Frequently Asked Questions)

Question: Explain Three Physical Properties of Aldehydes and Ketone.

Answer: Three physical properties of aldehydes and ketones are given below-


Physical State- Methanal is a pungent-smelling gas. Ethanal is a volatile liquid. Other aldehydes and ketones continuing up to eleven carbon atoms are colourless liquids while still higher members are solids.


Smell- With the exception of lower carbon number aldehydes which have unpleasant odours, aldehydes and ketones have generally pleasant smell. As the size of the aldehyde and ketone molecule increases, the odour becomes less pungent and more fragrant. 


Solubility- Aldehydes and ketones up to four carbon atoms are miscible with water. This is due to the presence of a hydrogen bond association between the polar carbonyl group and water.

Question: What will Happen When Aldehyde Reacts with Sodium Carbonate in Presence of Iodine Solution?

Answer: On reacting an aldehyde with sodium carbonate in presence of iodine, it forms a yellow precipitate of iodoform indicating positive iodoform test.