Properties of aldehydes and ketones

The physical properties of aldehydes and ketones are described as follows. Methanal is a gas at room temperature. Ethanal is a volatile liquid. Other aldehydes and ketones are liquid or solid at room temperature. The boiling points of aldehydes and ketones are higher than those of hydrocarbons and ethers of comparable molecular masses. It is due to weak molecular association in aldehydes and ketones arising out of the dipole-dipole interactions. Also, their boiling points are lower than those of alcohols of similar molecular masses due to the absence of intermolecular hydrogen bonding.

The lower members of aldehydes and ketones such as methanal, ethanal and propanone are miscible with water in all proportions, because they form hydrogen bonds with water.

However, the solubility of aldehydes and ketones decreases rapidly on increasing the length of alkyl chain. All aldehydes and ketones are fairly soluble in organic solvents like benzene, ether, methanol, chloroform, etc. The lower aldehydes have sharp pungent odours. As the size of the molecule increases, the odour becomes less pungent and more fragrant. In fact, many naturally occurring aldehydes and ketones are used in the blending of perfumes and flavouring agents.

Chemical Properties. Aldehydes and ketones are created by the mild oxidation of primary and secondary alcohols.

Chemically aldehydes and ketones both contain a carbonyl carbon and thus have similar chemical reactivities. However, aldehydes are more susceptible to oxidation because of the hydrogen atom attached to the carbonyl group. This is the basis for distinguishing between these two classes of compounds.

Revision exercises

Ex.1. Answer the following questions:

1. What is the difference between aldehydes and ketones? 2. How are primary and secondary alcohols formed from aldehydes and ketones? 3. How are aldehydes and ketones foemed? 4. What are the most important aldehydes and ketones? 5. Where are aldehydes and ketones soluble? 6. What does the odour of the lower aldehydes and ketones depend on? 7. What is the basis for distinguishing between aldehydes and ketones?

Ex.2. Match the words with their definitions:

1. dione	a. a solution of silver nitrate in ammonium hydroxide;
2. acetone	b. a compound which has one hydrogen atom bonded to the carbonyl group;
3. ozonolysis	c. a compound with two aldehyde or two ketone groups;
4.primary alcohols	d. a substance which can accept hydrogen ions (protons);
5. Tollens' reagent	e. a compound in which the carbon atom is attached to one alkyl group;
6. aldehyde	f. the action of ozone on alkenes;
7. base	g. the simplest ketone 2-propanone.

Ex.3. Say whether the following statements are true or false:

1. Aldehydes and ketones are structurally similar and they show similar chemical reactions. 2. Ketones are easily oxidized under mild conditions, aldehydes are not. 3. Ketones have at least one hydrogen atom bonded to the carbonyl group. 4. Aldehydes and ketones generally react by a nucleophilic addition mechanism. 5. Aldehydes and ketones are most easily prepared by the oxidation of water. 6. Acetone is highly volatile and is also highly flammable. 7. Solubility of aldehydes and ketones increases rapidly on increasing the length of alkyl chain.

Ex.4. Insert the necessary word:

1. Aldehydes and ketones both have a carbon-oxygen double bond often called a ... group. 2. ... is the most characteristic chemical reaction of aldehydes and ketones. 3. A more convenient method for the reduction of aldehydes and ketones to ... involve the use of metal hydrides. 4. Both aldehydes and ketones can be formed by the action of ... on alkenes. 5. Acetone can be formed by fermentation of ... or starch. 6. The lower aldehydes have ... odour. 7. Aldehydes are more susceptible to ... because of the hydrogen atom attached to the carbonyl group. (alcohols, oxidation, ozone, carbonyl, pungent/sharp, addition, sugars)