Physical Properties of Aldehydes and Ketones

Dhristi JEE 2022-24

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. Explain Three Physical Properties of Aldehydes and Ketone.

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.

2. What will Happen When Aldehyde Reacts with Sodium Carbonate in Presence of Iodine Solution?

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

3. What is the hybridization of carbon in aldehydes and ketones?

The hybridization of carbon in aldehydes and ketones is SP2. Aldehydes are unsaturated compounds. Ketones are saturated compounds. One can know the fact from their chemical structures. In aldehydes, the carbon atom R3 of the carbonyl group has three sp2 hybrid orbitals and one p orbital, which is not involved in the bonding. In ketones, two of the carbons (R2) of the carbonyl group have two sp2 hybrid orbitals and one p orbital each, while the other carbon (R1) has one sp2 orbital and two p orbitals. Students who need help with chemistry tuition can seek help from Vedantu. Our online tutors are available 24x7 to help the students. We provide personalized attention to each student.

4. Which test is used to identify aldehydes and ketones?

The iodoform test is used to identify aldehydes and ketones. When these compounds are treated with sodium carbonate and iodine solution, a yellow precipitate of iodoform is formed. This reaction is known as the iodoform test. Iodoform is a compound formed by the reaction of iodine and formaldehyde. The test is specific for aldehydes and ketones, which have methyl groups bonded to a carbonyl group. The test cannot be used to identify compounds that do not have a methyl group bonded to the carbonyl group. Once the presence of aldehydes and ketones has been confirmed using the iodoform test, other chemical properties can be studied.

5. What is the shape of the carbonyl molecule?

The shape of the carbonyl molecule is trigonal planar. The carbon atom in the carbonyl group is sp2 hybridized and has three bonding orbitals and one nonbonding orbital. This gives the molecule trigonal planar geometry. A trigonal planar geometry is a molecular geometry where a central atom has three ligands in an equilateral triangle around it. It can also be written as Rx Ry Rz A, where R represents the central atom, and A represents the surrounding atoms (ligands). One can also say that the carbonyl group is pyramidal. The apex of the pyramid is the carbon atom in the carbonyl group, and the base of the pyramid is the oxygen atom.

6. How do aldehydes and ketones react with hydrogen cyanide to form cyanohydrins?

Aldehydes and ketones react with hydrogen cyanide to form cyanohydrins. When aldehydes and ketones are treated with hydrogen cyanide, the carbonyl group is converted into a cyano group. This reaction is known as the cyanohydrin reaction. The mechanism for this reaction is shown below:


The carbon atom in the carbonyl group is SP2 hybridized and has three bonding orbitals and one nonbonding orbital. When it reacts with hydrogen cyanide, two of the sp2 hybrid orbitals are used to form a covalent bond with the nitrogen atom in hydrogen cyanide. This leaves one sp2 orbital as the lone pair of electrons on the carbon atom. The other two sp2 hybrid orbitals form a covalent bond with the hydrogen atom and the lone electron pair on the nitrogen atom, giving rise to an iminium ion. When this reaction is heated, one molecule (two atoms) of water is eliminated, and a carbonyl group is reformed.

7. How do aldehydes and ketones react with hydrogen cyanide to form cyanohydrins?

Aldehydes and ketones react with hydrogen cyanide to form cyanohydrins. When aldehydes and ketones are treated with hydrogen cyanide, the carbonyl group is converted into a cyano group. This reaction is known as the cyanohydrin reaction. The mechanism for this reaction is shown below:


The carbon atom in the carbonyl group is SP2 hybridized and has three bonding orbitals and one nonbonding orbital. When it reacts with hydrogen cyanide, two of the sp2 hybrid orbitals are used to form a covalent bond with the nitrogen atom in hydrogen cyanide. This leaves one sp2 orbital as the lone pair of electrons on the carbon atom. The other two sp2 hybrid orbitals form a covalent bond with the hydrogen atom and the lone electron pair on the nitrogen atom, giving rise to an iminium ion. When this reaction is heated, one molecule (two atoms) of water is eliminated, and a carbonyl group is reformed.

8. What types of chemical reactions do aldehydes and ketones undergo?

Aldehydes and ketones undergo both nucleophilic addition reactions and nucleophilic substitution reactions. In a nucleophilic addition reaction, an electrophile is attacked by a nucleophile. In the case of aldehydes and ketones, this electrophile can be positively charged on an SP2 carbon atom in the carbonyl group. The positive charge on the carbon atom attracts the negatively charged nucleophile, and a covalent bond is formed. In a nucleophilic substitution reaction, a nucleophile replaces a leaving group on a carbonyl group. In the case of aldehydes and ketones, the leaving group can be a hydrogen atom or a halogen atom. This leaving group is then replaced by the nucleophile to form a new covalent bond.

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