The physical properties of aldehydes and ketones are very important in their uses as solvents, intermediates in synthetic pathways, and also for identification purposes. Physical properties such as the boiling point, melting point, normal boiling-point range, refractive index, density or specific gravity or solubility parameter can all be used to identify an aldehyde or ketone.
What are Aldehyde and Ketones?
Aldehydes and ketones are compounds that 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 and aryl groups.
The boiling point of an aldehyde is higher than the corresponding alcohol due to the electron-withdrawing effect of the carbonyl group. The boiling point of an aldehyde increases with increasing carbon chain length. The melting point of an aldehyde is slightly higher than the boiling point because the molecules are held together by hydrogen bonds.
The boiling point of a ketone is higher than the boiling point of alcohol due to the electron-withdrawing effect of the carbonyl group. The boiling point of a ketone increases with increasing carbon chain length. The melting point of a ketone is slightly higher than the boiling point because the molecules are held together by hydrogen bonds. The normal boiling-point range is slightly lower for ketones than aldehydes. This is because ketones are less polar than aldehydes and thus have a weaker interaction with water molecules.
Structure of Aldehydes and Ketones
Physical Properties of Aldehydes and Ketones
Methanal is a pungent-smelling gas. Ethanol is a volatile liquid. Other aldehydes and ketones continuing up to eleven carbon atoms are colorless liquids while still higher members are solids.
Except for the lower carbon aldehydes, which have unpleasant odors, all other aldehydes and ketones generally have a pleasant smell. As the size of the aldehyde and ketone molecule increases, the odor becomes less pungent and more fragrant. In fact, many naturally occurring aldehydes and ketones have been used in the blending of perfumes and flavoring agents.
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:
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.
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. The reaction is carried out in the presence of an acid catalyst such as aluminum chloride at a high temperature.
2. Reaction With Sodium Bisulphite
Both aldehydes and ketones form crystalline addition compounds called bisulfite adducts when treated with a saturated solution of sodium bisulphite. The solution is boiled to drive off the excess bisulphite and the product is then crystallized.
3. Reaction With Grignard Reagents
Aldehydes and ketones react with a Grignard reagent to form additional products. When the additional product is hydrolyzed by water, it gives alcohol. For example, ethanol reacts with excess Grignard reagent to give an additional product called diethyl ether. Hydrolysis of this additional product gives ethanol.
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.
Did You Know?
The 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 out in the presence of sodium carbonate and iodine solution. This reaction is known as the iodoform test.
The hybridization of carbon in the carbonyl group is SP2.
The shape of the carbonyl molecule is trigonal planar.
The physical properties of aldehydes and ketones are due to the presence of polar carbonyl groups. The boiling point increases with an increase in the size of the molecule. The chemical properties of aldehydes and ketones are due to the polar carbonyl group present in their molecules. Aldehydes and ketones react with hydrogen cyanide to form cyanohydrins. They also react with sodium bisulfite to form bisulfite adducts. When treated with Grignard reagents, they form additional products.
Aldehydes and ketones react with alcohols in the presence of dry HCl gas to give gem-dialkoxy compounds called acetals. The Aldehydes and ketones can also be identified using the iodoform test. The hybridization of carbon in the carbonyl group is SP2, and the shape of the molecule is trigonal planar. Students who are interested in learning more about the physical and chemical properties of aldehydes and ketones can have any standard textbook on organic chemistry. Vedantu provides online chemistry tutoring with experienced and qualified Chemistry tutors to help students understand these concepts in detail.