Chemistry, Third edition

332 18 · COMMON CLASSES OF ORGANIC COMPOUNDS

Ethanol

Although there are many alcohols, when the non-chemist refers to ‘alcohol’ they gen-

erally mean ethanol. Ethanol can be made by the decomposition of carbohydrates, a

process known as fermentation.

Fruit juices, such as grape juice, contain the sugar glucose (molecular formula

C 6 H 12 O 6 ). Glucose can be fermented into alcohol by the addition of yeast. Yeast is a

fungus that contains the enzyme zymase, which catalyses the decomposition of

glucose into ethanol. Enzymesare biological catalysts. The fermentation reaction is

carried out at about 20 °C – high temperatures kill the yeast and at lower tempera-

tures the process is too slow. The equation for the reaction may be written as

C 6 H 12 O 6 (aq)2C 2 H 5 OH(aq)2CO 2 (g)

glucose ethanol

Fermentation ceases when the ethanol content of the mixture reaches about 14%

(v/v); higher concentrations of ethanol kill the yeast. If a higher concentration of

alcohol is required, the mixture is fractionally distilled. ‘Spirits’ such as whisky or

brandy may contain about 40% ethanol. Although fermentation is used to produce

alcoholic beverages, it is rarely used to synthesize the chemical ethanol on a large

scale. Alcohols, such as ethanol, can be produced commercially by the addition of

water across an alkene double bond (a hydration reaction). The reaction is catalysed

by phosphoric acid:

H 3 PO 4

CH 2 ==CH 2 H 2 O —CH 3 CH 2 OH

ethene steam 300 °C, ethanol

70 atm

Because alcohols form hydrogen bonds with other alcohol molecules, the boil-

ing points of the alcohols tend to be higher than other organic compounds of simi-

lar molecular mass. Their ability to hydrogen bond with water allows the lower

molecular mass alcohols to dissolve in, or be miscible with, water. Ethane-1,2-diol,

or ethylene glycol, is used as a component in antifreeze because it has a high solubil-

ity in water, it contains two hydroxy groups which can hydrogen bond with the

water, and it has a high boiling point and low freezing point.

2. Alcohols are good solvents

The presence of an –OH group means that they have some of the solvent properties

of water and can dissolve ionic compounds. The R–group allows them to behave as

solvents to organic, covalent substances.

Properties of alcohols

Chloroethane, butane and propan-1-ol have boiling points of 286 K, 272 K and 371 K,

respectively.

(i) Calculate the molecular mass of each compound.

(ii)Assuming all their melting points are lower than room temperature, what is the state of

each substance under conditions of 20 °C and 1 atm pressure?

(iii)Explain why the boiling point of propan-1-ol is much higher than those of the other two

compounds.

Exercise 18C