Introduction
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.
Aldehydes
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.
Ketones
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
Physical State
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.
Smell
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.
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:
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. 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.
Conclusion
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.
FAQs on Physical Properties of Aldehydes and Ketones
1. What are the main physical properties of aldehydes and ketones?
The key physical properties of aldehydes and ketones, as per the CBSE Class 12 syllabus for 2025-26, are determined by the polar carbonyl group. These include:
- Physical State: Methanal is a gas at room temperature, ethanal is a volatile liquid, while other aldehydes and ketones up to 11 carbon atoms are liquids. Higher members are solids.
- Boiling Point: They have higher boiling points than non-polar hydrocarbons of similar mass due to dipole-dipole interactions, but lower boiling points than corresponding alcohols.
- Solubility: Lower members (up to 4 carbons) are soluble in water because they can form hydrogen bonds with water molecules. Solubility decreases as the carbon chain length increases.
- Odour: Lower aldehydes have a strong, pungent smell. As the molecules get larger, they tend to have a more pleasant, fragrant odour and are often used in perfumes.
2. Why do aldehydes and ketones have higher boiling points than hydrocarbons of comparable mass?
Aldehydes and ketones have higher boiling points than hydrocarbons because of the presence of the polar carbonyl group (C=O). This polarity creates significant intermolecular dipole-dipole interactions between molecules, which are much stronger than the weak van der Waals forces found in non-polar hydrocarbons. More energy is required to overcome these stronger forces, resulting in higher boiling points.
3. Why are the boiling points of aldehydes and ketones lower than those of corresponding alcohols?
The boiling points of aldehydes and ketones are lower than those of corresponding alcohols because alcohols can form strong intermolecular hydrogen bonds with each other. Aldehydes and ketones lack a hydrogen atom attached directly to the highly electronegative oxygen atom, so they cannot form these strong hydrogen bonds among themselves. They only exhibit weaker dipole-dipole interactions, which require less energy to break.
4. Are lower aldehydes and ketones soluble in water? Explain the reason.
Yes, lower aldehydes and ketones (like methanal, ethanal, and propanone) are soluble in water. The reason is that the oxygen atom of the polar carbonyl group can form hydrogen bonds with the hydrogen atoms of water molecules. However, as the length of the hydrophobic alkyl chain increases, the non-polar part of the molecule dominates, and the solubility in water rapidly decreases.
5. How do the physical properties of aldehydes and ketones generally compare?
The physical properties of aldehydes and ketones are very similar because both contain the carbonyl functional group. They follow similar trends in boiling points, solubility, and physical state. However, ketones are generally slightly more polar than their isomeric aldehydes. This slight increase in polarity results in ketones having marginally higher boiling points than aldehydes of the same molecular mass.
6. What is the structure and geometry of the carbonyl group in these compounds?
The carbonyl carbon atom in both aldehydes and ketones is sp2 hybridised. This results in a trigonal planar geometry around the carbon atom, with bond angles of approximately 120°. The C=O bond consists of one strong sigma (σ) bond and one weaker pi (π) bond. Due to the high electronegativity of oxygen, the C=O bond is highly polar, which is the primary cause of their distinct physical properties.
7. What is the typical trend observed in the smell of aldehydes and ketones?
There is a distinct trend in the odour of aldehydes and ketones. The lower aldehydes, such as methanal and ethanal, have a very sharp, pungent, and often unpleasant smell. As the molecular size increases, the odour becomes less pungent and progressively more fragrant. Many naturally occurring higher aldehydes and ketones are valued for their pleasant smells and are used in perfumes and flavouring agents, like vanillin (from vanilla beans) and cinnamaldehyde (from cinnamon).
8. How does the physical state of aldehydes and ketones change with increasing molecular mass?
The physical state of aldehydes and ketones is directly related to their molecular mass. At room temperature, the simplest aldehyde, Methanal, is a gas. The next member, ethanal, is a volatile liquid. Most other common aldehydes and ketones are colourless liquids. As the carbon chain gets very long (beyond 11 carbons), the strength of the intermolecular van der Waals forces increases significantly, causing the higher members to exist as solids.
