Properties of aldehydes and ketones

ВВВВВВВВ 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.

ВВВВВВВВ 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.

ВВВВВВВВ 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.

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

ВВВВВВВВ 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)В

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UNIT XIII РЎARBOXYLIC ACID

РЎarboxylic acid is any of a class of organic compounds in which a carbon (C) atom is bonded to an oxygen (O) atom by a double bond and to a hydroxyl group (в?’OH) by a single bond. A fourth bond links the carbon atom to a hydrogen (H) atom or to some other univalent combining group. The carboxyl (COOH) group is so-named because of the carbonyl group (C=O) and hydroxyl group.

The chief chemical characteristic of the carboxylic acids is their acidity. They are generally more acidic than other organic compounds containing hydroxyl groups but are generally weaker than the familiar mineral acids (e.g., hydrochloric acid, HCl, sulfuric acid, H₂SO₄, etc.).

Carboxylic acids occur widely in nature. The fatty acids are components of glycerides, which in turn are components of fat. Hydroxyl acids, such as lactic acid (found in sour-milk products) and citric acid (found in citrus fruits), and many keto acids are important metabolic products that exist in most living cells. Proteins are made up of amino acids, which also contain carboxyl groups.

Compounds in which the в?’OH of the carboxyl group is replaced by certain other groups are called carboxylic acid derivatives, the most important of which are acyl halides, acid anhydrides, esters, and amides.

Carboxylic acid derivatives have varied applications. For example, in addition to its use as a disinfectant, formic acid, the simplest carboxylic acid, is employed in textile treatment and as an acid reducing agent. Acetic acid is extensively used in the production of cellulose plastics and esters. Aspirin, the ester of salicylic acid, is prepared from acetic acid. Palmitic acid and stearic acid are important in the manufacture of soaps, cosmetics, pharmaceuticals, candles, and protective coatings. Stearic acid also is used in rubber manufacture. Acrylic acid is employed as an ester in the production of polymers (long-chain molecules) known as acrylates. Methacrylic acid serves as an ester and is polymerized to form Lucite. Oleic acid is used in the manufacture of soaps and detergents and of textiles.