Alcohols

(Separate Chemistry only) Alcohols (4.7.2.3) are assessed only in GCSE Chemistry (Triple), not in Combined Science.

Alcohols are a family of organic compounds that all contain the same reactive group: the –OH group (an oxygen atom joined to a hydrogen atom). This is the alcohol functional group, and because every alcohol has it, they all react in similar ways.

The names of alcohols always end in -ol. You need to know the first four members.

Name	Carbons	Molecular formula	Structural formula
Methanol	1	CH₄O	CH₃OH
Ethanol	2	C₂H₆O	C₂H₅OH
Propanol	3	C₃H₈O	C₃H₇OH
Butanol	4	C₄H₁₀O	C₄H₉OH

The displayed formula of ethanol shows the –OH group joined to the carbon chain. Every carbon has four bonds; the oxygen sits between a carbon and the final hydrogen.

Ethanol (C₂H₅OH)

    H   H
    |   |
H — C — C — O — H
    |   |
    H   H

Alcohols react with sodium metal. A piece of sodium added to an alcohol fizzes as hydrogen gas is given off, and the sodium gradually disappears as it reacts.

When an alcohol reacts with sodium, hydrogen gas is produced. You would see the sodium fizzing and moving as bubbles of hydrogen are released.

This is similar to how sodium reacts with water, but the reaction with an alcohol is gentler. The hydrogen given off can be tested with a lit splint, which produces a squeaky pop. You are not expected to write a balanced equation for this reaction, only to describe what happens: fizzing, sodium dissolving, and hydrogen gas released.

Alcohols are flammable and burn in air, releasing energy. This is why they are used as fuels. Complete combustion of an alcohol produces carbon dioxide and water, just like a hydrocarbon.

Worked example — write the balanced equation for the complete combustion of ethanol, C₂H₅OH.

Ethanol has 2 carbons (giving 2 CO₂) and 6 hydrogens (giving 3 H₂O):

${\text{C}}_2{\text{H}}_5\text{OH}+{\text{O}}_2\to 2{\text{CO}}_2+3{\text{H}}_2\text{O}$

Now balance oxygen. The right side has $(2\times 2)+(3\times 1)=4+3=7$ oxygen atoms. The left side already has 1 oxygen inside the ethanol, so the O₂ must supply the other 6, which is 3 O₂ molecules:

${\text{C}}_2{\text{H}}_5\text{OH}+3{\text{O}}_2\to 2{\text{CO}}_2+3{\text{H}}_2\text{O}$

Check: left has 2 C, 6 H and $1+(3\times 2)=7$ O; right has 2 C, 6 H and 7 O. Balanced.

When balancing an alcohol's combustion, remember the –OH oxygen counts. The alcohol already contains one oxygen atom, so the O₂ on the left supplies the rest.

Adding to water. The first four alcohols mix completely with water, dissolving to form a solution that is neutral (about pH 7). Unlike acids, alcohols do not turn the solution acidic; the solution has no effect on universal indicator beyond staying green.

Reaction with an oxidising agent. When an alcohol is warmed with an oxidising agent (for example acidified potassium dichromate), it is oxidised to a carboxylic acid. Ethanol is oxidised to ethanoic acid, the acid found in vinegar.

$\text{ethanol}\stackrel{\text{oxidising agent}}{\to }\text{ethanoic acid}$

This link matters: it is why an open bottle of wine turns sour over time. Oxygen from the air slowly oxidises the ethanol in the wine into ethanoic acid, giving a vinegary taste. The same oxidation done deliberately with an oxidising agent produces the carboxylic acid in the lab.

Reaction	What you observe / product
With sodium	fizzing, hydrogen gas released
Combustion in air	burns, releasing energy; makes CO₂ and water
Added to water	dissolves to a neutral (pH 7) solution
With an oxidising agent	oxidised to a carboxylic acid

The main uses of the first four alcohols are as fuels and as solvents. Ethanol dissolves many substances that water cannot, so it is used in products such as perfumes and inks, and it is the alcohol present in alcoholic drinks.

Ethanol for drinks is made by fermentation. Yeast contains enzymes that break down sugar (glucose) into ethanol and carbon dioxide.

$\text{sugar}\stackrel{\text{yeast}}{\to }\text{ethanol}+\text{carbon dioxide}$

You must know the conditions for fermentation:

• aqueous solution (the sugar is dissolved in water),
• anaerobic conditions, with air (oxygen) kept out, otherwise the ethanol is oxidised to ethanoic acid,
• a warm temperature of about 30–40 °C, warm enough for the yeast enzymes to work quickly but not so hot that they are denatured.

Fermentation conditions: sugar dissolved in water, with yeast, no oxygen (anaerobic), kept warm at roughly 30–40 °C.

1. Forgetting the alcohol's own oxygen when balancing combustion

An alcohol such as C₂H₅OH already contains one oxygen atom. Count it: the O₂ on the left only has to supply the remaining oxygen needed by the CO₂ and water.

2. Saying alcohols are acidic in water

The first four alcohols dissolve to give neutral solutions (about pH 7). They are not acids. It is their oxidation product, the carboxylic acid, that is acidic.

3. Giving the wrong gas from the sodium reaction

Sodium plus an alcohol releases hydrogen gas (which gives a squeaky pop), not oxygen or carbon dioxide.

4. Getting the fermentation conditions wrong

Fermentation needs yeast, an aqueous sugar solution, no air (anaerobic), and a warm temperature of about 30–40 °C. Too hot denatures the enzymes; letting air in oxidises the ethanol to ethanoic acid.

5. Confusing the oxidation product with an aldehyde or hydrocarbon

Oxidising an alcohol with an oxidising agent produces a carboxylic acid (ethanol → ethanoic acid). At GCSE the product you name is the carboxylic acid.