4.1 Introduction Notes

Edexcel IGCSE Chemistry: Revision notes

Key facts

  • A hydrocarbon contains hydrogen and carbon only.
  • Five formula types: empirical, molecular, general, structural, displayed.
  • A homologous series shares a general formula and functional group, and differs by CHX2\ce{CH2}.
  • Isomers have the same molecular formula but different structures.
  • Stems: meth-, eth-, prop-, but-, pent-, hex- for 1 to 6 carbons.
  • Organic reactions: substitution, addition, combustion.

Hydrocarbons

Carbon forms four bonds and bonds to itself, so it builds chains and rings.

A hydrocarbon contains hydrogen and carbon atoms only. Carbon forms four strong covalent bonds, including to other carbon atoms, so it builds long chains, branched chains and rings. This is why there are millions of organic compounds, built mainly from carbon, hydrogen, oxygen and sometimes halogens or nitrogen. Most are a hydrocarbon backbone with a small functional group that gives the characteristic behaviour.

Hydrocarbon

  • CX2HX6\ce{C2H6}
  • Carbon and hydrogen only

Not a hydrocarbon

  • CX2HX5OH\ce{C2H5OH}, CHX3Cl\ce{CH3Cl}
  • Contains oxygen or chlorine

Which of these is a hydrocarbon?

Types of formula

Each type of formula shows a different amount of detail; butane is the example.

Displayed and structural formulae are the most useful for showing isomerism, because two molecules can share a molecular formula but have different structures.

Empirical

What it shows:
Simplest whole-number ratio
Example (butane):
CX2HX5\ce{C2H5}

Molecular

What it shows:
Actual atoms in one molecule
Example (butane):
CX4HX10\ce{C4H10}

General

What it shows:
Pattern for a whole series
Example (butane):
CnH2n+2\mathrm{C}_n\mathrm{H}_{2n+2}

Structural

What it shows:
Atoms grouped along the chain
Example (butane):
CHX3CHX2CHX2CHX3\ce{CH3CH2CH2CH3}

A displayed formula draws out every atom and every bond.

What does a displayed formula show?

Homologous series and isomers

A homologous series is a family with one general formula and functional group; isomers share a formula but not a structure.

A homologous series has the same general formula, differs by a CHX2\ce{CH2} unit from one member to the next, has the same functional group (so similar chemistry), and shows a gradual change in physical properties such as boiling point. Alkanes fit CnH2n+2\mathrm{C}_n\mathrm{H}_{2n+2}.

Isomerism: same molecular formula, different structures. CX4HX10\ce{C4H10} can be butane or methylpropane, with slightly different physical properties.

CCCCCCCC
Carbon skeletons of butane (left) and methylpropane (right)

Which describes a pair of isomers?

Naming compounds

A name has a stem for the carbon chain and a suffix or prefix for the functional group.

IUPAC names have two parts: a stem showing the number of carbon atoms in the longest chain, and a suffix or prefix showing the functional group. A four-carbon alkane is butane; a two-carbon alcohol is ethanol. Branches are named as substituents such as "methyl", with a number showing which carbon.

1 to 3 carbons

  • meth-
  • eth-
  • prop-

4 to 6 carbons

  • but-
  • pent-
  • hex-

What is the stem for a chain of three carbon atoms?

Types of reaction

Organic reactions fall into three broad types: substitution, addition and combustion.

Substitution: an atom or group is replaced by another, for example an alkane reacting with a halogen. Addition: two molecules join to form one product with no atoms lost, for example an alkene with bromine. Combustion: a compound reacts with oxygen, releasing heat.

Substitution

What happens:
Atom replaced by another
Example:
Alkane + halogen

Addition

What happens:
Two molecules join, nothing lost
Example:
Alkene + bromine

Combustion

What happens:
Reaction with oxygen, heat released
Example:
Burning fuels

Ethene reacts with bromine to form a single product. What type of reaction is this?

Try an exam question

Explain what is meant by a homologous series and by isomers.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.