Classifying Organic Reactions

Section 1 of 7Three types of reaction

Substitution

• Substitution One atom or group in a molecule is replaced by a different atom or group.
  • Two products are formed.

Addition

• Addition Two or more molecules join to make a single larger molecule, with no other product.
  • Unsaturated compounds with a C=C bond do this.

Combustion

• Combustion Burning: an organic compound reacts with oxygen to release energy and form oxides, usually carbon dioxide or carbon monoxide, and water.

Section 2 of 7Substitution example

1Reactants

• Reactants Methane, CH₄, and bromine, Br₂, are mixed.

2Ultraviolet light

• Ultraviolet light UV radiation supplies the energy needed to start the reaction.

3Atom swapped

• Atom swapped A hydrogen atom in methane is replaced by a bromine atom.

4Two products

• Two products CH₄ + Br₂ → CH₃Br + HBr, giving bromomethane and hydrogen bromide.

Section 3 of 7Addition example

1

Ethene, C₂H₄, is an unsaturated molecule containing a C=C double bond.

2

A bromine molecule approaches and the double bond opens up.

3

One bromine atom attaches to each carbon of the former double bond.

4

The only product is dibromoethane: C₂H₄ + Br₂ → C₂H₄Br₂, a saturated compound.

Section 4 of 7Complete and incomplete combustion

Type	Oxygen supply	Products	Example equation
Complete combustion	Plenty of oxygen or air	Carbon dioxide and water	CH4 + 2O2 → CO2 + 2H2O
Complete combustion	Plenty of oxygen or air	Carbon dioxide and water	C3H8 + 5O2 → 3CO2 + 4H2O
Incomplete combustion	Limited oxygen or air	Carbon monoxide and water	CH4 + 1½O2 → CO + 2H2O

Section 5 of 7How to decide which type

Spot the reaction type

Count the products

• Count the products Addition gives only one product. Substitution gives two. Combustion gives oxides.

Look for oxygen

• Look for oxygen If oxygen is a reactant and carbon dioxide or carbon monoxide and water form, it is combustion.

Check for a double bond

• Check for a double bond A C=C bond that disappears after the reaction signals addition.

Consequence of poor air supply

• Consequence of poor air supply Incomplete combustion makes toxic carbon monoxide and can produce soot (carbon), so appliances need good ventilation.

Section 6 of 7Key terms

Term	Meaning
Substitution reaction	A reaction in which an atom or group is replaced by another atom or group.
Addition reaction	A reaction in which two molecules join to form one larger molecule only.
Combustion	A reaction with oxygen that releases energy, commonly called burning.
Unsaturated	Containing at least one C=C double bond.
Incomplete combustion	Burning in a limited oxygen supply, producing carbon monoxide or carbon instead of carbon dioxide.

Section 7 of 7Exam technique

1

Calling the methane and bromine reaction an addition reaction. Two products form, so it is substitution.

2

Forgetting that substitution with an alkane needs ultraviolet light.

3

Writing carbon dioxide as a product of incomplete combustion. The products there are carbon monoxide and water.

4

Not balancing combustion equationscount carbon first, then hydrogen, then oxygen last.

5

Saying alkanes undergo addition. Only unsaturated compounds such as alkenes add atoms across a double bond.