Chemical Properties of Alcohols

The functional group of alcohols, the hydroxyl, determines the main chemical properties of these compounds. Alcohols are characterized by great chemical activity.

Acid and basic properties. Formation of alcoxides. Alcohols are practically neutral substances: they do not change the colour of indicators and do not react with either aqueous solutions of alkalies or dilute acids. However, in certain reactions, alcohols exhibit properties of a very weak acids and bases, i.e. they are amphoteric, like water. When alkali metals act on alcohols in unhydrous medium, hydroxyl hydrogen is forced out and alkoxides are formed. Alkoxides have the nature of salts of a very weak acid with a base. The basic properties of alcohols are exhibited in their interaction with strong acids. In this case, alcohols like water yield oxonium salts.

Formation of ethers. Ethers are obtained when alkoxides react with alkyl halides.

Dehydration of alcohols. Alcohols are capable of losing water under definite conditions. Concentrated sulphuric or phosphoric acid can be used as dehydrating agent. Alcohols dehydrate when their vapours are passed over a heated solid catalyst (silica gel, aluminium oxide, etc.).

Oxidation of alcohols. Different products can be formed, depending on the nature of alcohols and reaction conditions. Primary alcohols yield at first aldehydes, and then acids having the same number of carbon atoms. Ketones are formed from secondary alcohols. Tertiary alcohols are more stable towards oxidation.

Catalytic dehydrogenation of alcohols. Alcohols can be converted into aldehydes and ketones also by dehydrogenation, i.e. by passing alcohol vapours over a heated metallic catalyst such as copper or silver at 300C0. A typical dehydrogenation reaction is the conversion of ethyl alcohol into acetaldehyde. This reaction is endothermic.

Preparation of Alcohols

Functional Group Transformation. Alcohols can be prepared by nucleophilic substitution of alkyl halides, hydrolysis of esters, reduction of carboxylic acids or esters, reduction of aldehydes or ketones, electrophilic addition of alkenes, hydroboration of alkenes, or substitution of ethers.

C–C Bond Formation. Alcohols can also be obtained from epoxides, aldehydes, ketones, esters, and acid chloride as a consequence of C–C bond formation. These reactions involve the addition of carbanion equivalents through the use of Grignard or organolithium reagents.

Revision exercises

Ex.1. Answer the following questions:

1.What are alcohols? 2. What is the general formula of alcohols? 3. What groups are alcohols classified into? 4. What does isomerism of alcohols determine? 5. What are the methods of alcohol preparation? 6. What are the physical properties of alcohols?

Ex.2. Match the words with their definitions:

1. volatility	a. the quantity per unit volume, unit area, or unit length as the mass of a substance per unit volume;
2. derivative	b. the state of looking or being like someone or something else, correspondence in appearance or superficial qualities, a point of likeness, similarity;
3. density	c. something that comes from something else, substance that is made from another substance;
4. resemblance	d. the property of certain substances in very small concentrations to stimulate chemical sense receptors that sample the air or water surrounding the animal;
5. odour	e. a substance that has a bitter taste and that forms a salt when mixed with an acid; a soluble salt obtained from the ashes of plants and consisting largely of potassium or sodium carbonate; a substance (as a hydroxide or carbonate of alkali metal) having marked basic properties;
6. alkali	f. state of being readily vaporizable at a relatively low temperature;

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

1. Alcohols dehydrate when their vapours are passed over a heated solid catalyst. 2. The prefix cyclo- is used for alcohols with straight-chain alkyl groups. 3. Alcohols can be converted into aldehydes and ketones also by hydration. 4. The first three members of the saturated alcohols mix with water in all proportions. 5. The isomerism of alcohols is caused by the structure of the chain. 6. The lower alcohol representatives have a characteristic alcoholic odour and a burning taste.

Ex.4. Insert the necessary word:

1. Alcohols are among the most common organic compounds. 2. They are used as … and in making perfumes, are valuable intermediates in the synthesis of other compounds, and are among the most abundantly produced organic … in industry. 3. Perhaps the two best-known alcohols are … and methanol (or methyl alcohol). 4. Ethanol is used in toiletries, pharmaceuticals, and fuels, and it is used to … hospital instruments. 5. It is, moreover, the alcohol … . 6. The anesthetic ether is also made from … . 7. Methanol is used as solvent, as raw material for the manufacture of formaldehyde and special resins, in special fuels, in antifreeze, and for cleaning metals. 8. Alcohols may be classified as primary, secondary, or tertiary, according to which carbon of the alkyl group is bonded to the … group. 9. Most alcohols are colourless liquids … at room temperature. 10. Alcohols of low molecular weight are highly … in water; with increasing molecular weight, they become less soluble in water, and their boiling points, vapour pressures, densities, and viscosities increase. (solids, beverages, chemicals, ethanol(×2), sweeteners, soluble, hydroxyl, sterilize).

Ex.5. Translate the following text: