Alcohols

ALCOHOLS HAVE the general formula Cn H2n+1OH Using n = 1,2,3, etc When n =1, then C1H(2*1)+1OH gives CH3OH - Methanol When n = 2, then C2H(2*2)+1OH gives C2H5OH - Ethanol When n = 3, then C3(2*3)+1OH gives C3H7OH - Propanol

THE FIRST 3 MEMBERS OF THE ALCOHOLS HOMOLOGOUS SERIES

If you notice the formula of all alcohols, you will see a group of atoms that is present at all times; the OH group. This -OH group is called the functional group for alcohols. A functional group is responsible for the characteristic reactions of that homologous series. In alkenes, the functional group is the C=C double bond.

HOW ARE ALCOHOLS FORMED?

Alcohols can be made from the hydration of alkenes; that is, adding water in the presence of a catalyst (phosphoric acid, silica at 3000C, 60 atm). This is the industrial method of preparing alcohols.

Ethanol can also be prepared by another method. Foods rich in sugars such as sucrose, glucose and fructose can be fermented with yeast to produce ethanol. The yeast produces an enzyme (zymase) which breaks down the sugars, producing energy and alcohol and carbon dioxide as waste products. This process is known as fermentation.

HOW IS PURE ETHANOL OBTAINED AFTER FERMENTATION?

When the ethanol produced during fermentation reaches a concentration of about 14%, the enzyme from the yeast stops working as it becomes denatured. Fractional distillation is then used to increase the ethanol concentration to about 95%.

HOW CAN ALCOHOL/WINE BECOME SOUR?

Fermentation takes place under anaerobic conditions (without oxygen). If air is allowed in the mixture, the ethanol becomes oxidized to ethanoic acid, producing a sour or spoilt taste. C2H5OH (aq) + O2 (g) aerobic bacteria CH3COOH (aq) + H2O (l) Ethanol + oxygen Ethanoic acid + water

WHAT ARE THE CHARACTERISTIC REACTIONS OF ALCOHOLS?

Alcohols are polar due to the presence of the -OH functional group. This is similar to the -OH group in water (H-OH), hence, alcohols are also soluble in water.

Alcohols burn in air or oxygen to produce carbon dioxide, heat and energy.

2C2H5OH (l) + 7O2 (g) 4CO2 (g) + 6H2O (g)

2. Alcohols react with metals such as Na, Li, Mg and Ca to yield hydrogen gas and to form salts called alkoxides. The reaction is similar to the reaction of metals with water because of the presence of -OH group.

2Na (s) + 2C2H5OH (l) 2C2H5ONa (aq) + H2 (g) sodium ethoxide

3. Oxidation: Alcohols can be oxidized to organic acids using powerful oxidizing agents such as acidified potassium dichromate (VI) or acidified potassium permanganate (VII) solution. C2H5OH (l) + 2[O] CH3COOH(aq) + H2O (l) Ethanoic acid

The oxygen is from the oxidizing agent. If potassium permanganate is used, the reaction mixture changes from purple to colourless; and if potassium dichromate is used, the mixture changes from orange to green as the dichromate VI ion (Cr2O72-) is reduced to green chromium III ion (Cr3+). This forms the basis for the BREATHALYZER test for drunken drivers.

DEHYDRATION

Alcohols can undergo dehydration reactions to the corresponding alkene using concentrated sulphuric acid (1700C) or passing the vapour over activated alumina (Al2O3).

C2H5OH (l) C2H4 (g) + H2O. This reaction effectively removes water from the alcohol to form the alkene.

ESTERIFICATION

Alcohols react with carboxylic acids to form esters. C2H5OH + CH3COOH CH3COOC2H5 (ethylethanoate) + H2O