ALCOHOLS

The alcohols form a homologous series with the general formula CnH2n+1OH. They contain the functional group –OH, which is called a hydroxyl group. It is the presence of this functional group which gives alcohols their characteristic properties. The first members of the series are:

methanol ethanol propan-1-ol

ETHANOL C2H5OHEthanol can be produced in two ways:

by the fermentation of sugars by the hydration of ethene.

1. Fermentation of SugarsFermentation is used to produce ethanol as an industrial chemical. The raw material, usually glucose, is dissolved in water to make a solution containing about 15% by mass of glucose. Yeast is then added, and the temperature is maintained at about 25oC for 3 to 5 days. Yeast is a living organism which contains the enzyme, zymase. Enzymes are biological catalysts. Under these conditions, the sugar reacts to form ethanol and carbon dioxide.

C6H12O6 2C2H5OH + 2CO2

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H H H

H H H

H C C C OH

H H

H H

H C C OH

H

H

H C OH

fermentationmixture

The carbon dioxide is allowed to escape and air is prevented from entering the fermentation vessel, often by using a water trap.

When the fermentation is complete, the yeast is filtered off, and the ethanol is separated from the reaction mixture by fractional distillation.

Fermentation is also used: to produce ethanol in beer and wine to produce the bubbles of CO2 which make bread dough

rise

N.B. The simple laboratory test for carbon dioxide is that it turns limewater milky.

2. Hydration of EtheneEthene undergoes an addition reaction with steam at high temperature (~300oC) and pressure (60-70 atm) in the presence of a strong acid catalyst (phosphoric acid), to form ethanol.

C2H4 + H2O C2H5OH ethene ethanol

COMPARISON OF THE TWO PROCESSES

Fermentation Disadvantages:

a slow reaction (3 to 5 days) the product is a mixture containing only about 10% ethanol, from

which the ethanol is separated by distillation the process is a batch process

Advantages the raw material (glucose) is renewable the process is low-tech: it needs only inexpensive equipment and

unskilled labour

Hydration Advantages:

a fast reaction (seconds) the product is pure the process is a continuous process

Disadvantages the raw material (ethene) is obtained from crude oil, which is

non-renewable the process is high-tech: it requires expensive equipment and

semi-skilled labour

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ETHANOL AS A FUELCombustionEthanol is a fuel and burns to produce carbon dioxide and water.

C2H5OH + 3O2 2CO2 + 3H2O

Ethanol is mixed with petrol in biofuels. The impact on the environment of burning a biofuel is less than burning a fossil fuel as the carbon dioxide produced during combustion is equal to that which was taken in by the plant during photosynthesis.

Carboxylic acids

The carboxylic acids form a homologous series with the general formula CnH2nO2. They contain the functional group –COOH, which is called an acid group. It is the presence of this functional group which gives carboxylic acids their characteristic properties. The first members of the series are:

Methanoic acid Ethanoic acid Propanoic acid

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OH

O H

H C C

H OH

O H H

H C C C

H HOH

O

H C

Carboxylic acids react in the same way as the mineral acids you are familiar with i.e. they react with metals to produce salts and hydrogen gas.

2CH3COOH + Mg Mg(CH3COO) 2 + H2

They are weak acids and so have fewer H+ ions in solutions and hence reactions are not as vigorous as strong acids of equal concentration. Weak acids typically have a pH of around 3-4.

Carboxylic acids are made by oxidising alcohols. This happens in the air and explains why wine which has been left without a cork in it for some time, tastes sour. The ethanol has been oxidised to ethanoic acid which is the acid present in vinegar.On an industrial scale, alcohols are oxidised to carboxylic acids much more quickly by using oxidising agents such as acidified potassium manganate (VII).

C2H5OH + [O] CH3COOH

Here [O] is used to show oxygen gained from an oxidising agent.

Esters

Carboxylic acids are also used to make useful compounds called esters. Esters are made by reacting carboxylic acids with alcohols using a conc. sulphuric acid catalyst. These esters are used as food flavourings (many fruity sweets contain esters) and as solvents. The characteristic smell you get when you remove the lid on nail varnish is an ester.

Naming esters is as simple as a,b,c… (alcohol before carboxylic acid).

Propanoic acid ethanol ethyl propanoate water

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H H

H C C C H H OH

O+ + H2O

O

O H H HO C C H H H

H H

H C C C

H H

H H

HH

C C H

This is a condensation reaction since a small molecule, in this case water, is also produced.

Topic 17b Organic chemistrySummery questions

1

2

3

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4

5

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6

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7

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8

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(d)

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9 Which of the following will react together to form propyl ethanoate?

1 2

3 4

A) 1 and 3 B) 2 and 3 C) 2 and 4 D) 1 and 4

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10

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