Introduction

What you'll learn

What hydrocarbons are, and why carbon is so important in organic chemistry.
How to represent organic molecules using empirical, molecular, general, structural and displayed formulae.
What homologous series, functional groups and isomerism mean.
How to name simple organic compounds and classify substitution, addition and combustion reactions.

Organic chemistry: the carbon chemistry topic

Organic chemistry is the study of carbon-containing compounds. In IGCSE Chemistry, you mainly meet molecules made from carbon, hydrogen and sometimes oxygen or halogens.

Carbon is useful because each carbon atom forms four covalent bonds. This lets carbon atoms join together in chains, branches and rings, making many different compounds.

Definition

Hydrocarbon

A hydrocarbon is a compound made of hydrogen and carbon only. For example, methane, $CH_4$ , and ethene, $C_2H_4$ , are hydrocarbons. Ethanol, $C_2H_5OH$ , is not a hydrocarbon because it contains oxygen.

Common Mistake

Not every carbon compound is a hydrocarbon

Carbon dioxide, $CO_2$ , contains carbon but no hydrogen, so it is not a hydrocarbon. Ethanol contains carbon and hydrogen, but also oxygen, so it is not a hydrocarbon either.

Ways to write organic formulae

Chemists use different types of formula depending on what they want to show. The same molecule can be represented in several ways.

The diagram below compares the main formula types using ethane, which is a simple alkane.

Formula representations for ethane

Empirical formula

The empirical formula shows the simplest whole-number ratio of atoms in a compound.

For ethane, the molecular formula is $C_2H_6$ . The ratio 2 carbon : 6 hydrogen simplifies to 1 carbon : 3 hydrogen, so the empirical formula is $CH_3$ .

Molecular formula

The molecular formula shows the actual number of atoms of each element in one molecule.

For example, propane has molecular formula $C_3H_8$ , meaning one molecule contains 3 carbon atoms and 8 hydrogen atoms.

General formula

A general formula describes every member of a homologous series using $n$ for the number of carbon atoms.

For example, alkanes have the general formula $C_nH_{2n+2}$ . If $n = 3$ , the formula is $C_3H_8$ , which is propane.

Structural formula

A structural formula shows how atoms are arranged, but groups atoms together to save space.

For example, propane can be written as $CH_3CH_2CH_3$ .

Displayed formula

A displayed formula shows every atom and every covalent bond.

Displayed formulae are especially useful when you need to show a double bond, a branch, or the exact position of a functional group.

Example

Finding formulae for propene

Propene has molecular formula $C_3H_6$ .

Compare the numbers of atoms to find the empirical formula: the ratio 3 carbon : 6 hydrogen divides by 3, giving 1 carbon : 2 hydrogen, so the empirical formula is $CH_2$ .
Check the general formula for alkenes: alkenes have formula $C_nH_{2n}$ , and when $n = 3$ , this gives $C_3H_6$ .
Write a structural formula by arranging 3 carbons with one C=C double bond and enough hydrogens for each carbon to have four bonds: $CH_2=CHCH_3$ .

Common Mistake

General formulae have limits

A general formula only applies to the correct homologous series. Do not use the alkane formula $C_nH_{2n+2}$ for alkenes, alcohols or carboxylic acids.

Homologous series and functional groups

A homologous series is a family of organic compounds with the same functional group and similar chemical reactions. Consecutive members usually differ by $CH_2$ .

Definition

Functional group

A functional group is the atom, group of atoms, or bond that gives an organic compound its characteristic chemical reactions. For example, alkenes contain a C=C double bond, and alcohols contain the -OH group.

Important homologous series in this specification include:

Alkanes: saturated hydrocarbons with only single C-C bonds, such as methane and ethane.
Alkenes: unsaturated hydrocarbons with a C=C double bond, such as ethene and propene.
Alcohols: compounds containing the -OH functional group, such as ethanol.
Carboxylic acids: compounds containing the -COOH functional group, such as ethanoic acid.
Esters: compounds containing the -COO- group, often with sweet or fruity smells.

Key Idea

The big pattern

Members of the same homologous series have similar chemical reactions because they have the same functional group. Their physical properties, such as boiling point, change gradually as the carbon chain gets longer.

Naming organic compounds: IUPAC basics

The International Union of Pure and Applied Chemistry, usually shortened to IUPAC, provides systematic rules for naming compounds. In this specification, you should be able to name compounds containing up to six carbon atoms.

Carbon chain prefixes

Learn these six prefixes very securely:

1 carbon: meth-
2 carbons: eth-
3 carbons: prop-
4 carbons: but-
5 carbons: pent-
6 carbons: hex-

Suffixes show the homologous series

The ending of the name shows the type of compound:

-ane means alkane, for example propane.
-ene means alkene, for example propene.
-anol means alcohol, for example ethanol or propan-1-ol.
-anoic acid means carboxylic acid, for example ethanoic acid.
alkyl alkanoate means ester, for example ethyl ethanoate.

If a functional group or double bond could be in more than one position, use a number to show where it is. Number the chain from the end that gives the important group the lowest possible number.

Example

Naming an alkene

Name the compound $CH_3CH_2CH=CH_2$ .

Find the longest chain: there are 4 carbon atoms, so the prefix is but-.
Identify the functional group: there is a C=C double bond, so the suffix is -ene.
Number from the end nearest the double bond: the double bond starts at carbon 1, so the name is but-1-ene.

Tip

Name = chain length + family

Most simple organic names are built from two clues: the prefix tells you the number of carbon atoms, and the suffix tells you the homologous series.

Isomerism

Sometimes two molecules have the same molecular formula but different arrangements of atoms. This is why organic chemistry has so many different compounds.

Definition

Isomerism

Isomerism occurs when compounds have the same molecular formula but different structural formulae. These compounds are called isomers.

The diagram below shows structural isomerism for $C_4H_{10}$ .

Structural isomerism for C4H10

Writing possible formulae from a molecular formula

When you are asked to write possible structural or displayed formulae, work systematically:

Use all atoms in the molecular formula.
Check carbon forms four bonds.
Check hydrogen forms one bond.
Try a straight chain first, then try branched chains.
Do not count the same structure twice just because it has been drawn backwards.

Example

Writing isomers of C4H10

Write the possible structural formulae for $C_4H_{10}$ .

Start with a straight chain of 4 carbon atoms. Add enough hydrogens so every carbon has four bonds: $CH_3CH_2CH_2CH_3$ , called butane.
Try a shorter main chain of 3 carbon atoms with the fourth carbon as a branch on the middle carbon: $CH_3CH(CH_3)CH_3$ , called methylpropane.
Check for duplicates: putting the branch on either end of the 3-carbon chain just gives the straight-chain structure again, so there are only two structural isomers of $C_4H_{10}$ .

Common Mistake

Carbon cannot have five bonds

When drawing displayed formulae, count carefully around each carbon. A carbon atom should have four covalent bonds in these organic molecules.

Classifying organic reactions

You do not need reaction mechanisms for this topic. That means you do not need to show exactly how electrons move during the reaction. You do need to recognise the type of reaction.

Substitution reactions

A substitution reaction happens when one atom or group of atoms is replaced by another.

Alkanes can react with halogens in ultraviolet light. For example, methane reacts with chlorine to form chloromethane and hydrogen chloride:

CH4(g)+Cl2(g)→CH3Cl(g)+HCl(g)CH_4\text{(g)} + Cl_2\text{(g)} \to CH_3Cl\text{(g)} + HCl\text{(g)}

One hydrogen atom in methane has been replaced by a chlorine atom.

Addition reactions

An addition reaction happens when atoms are added across a C=C double bond. Alkenes do addition reactions because they are unsaturated.

For example, ethene reacts with hydrogen to form ethane:

C2H4(g)+H2(g)→C2H6(g)C_2H_4\text{(g)} + H_2\text{(g)} \to C_2H_6\text{(g)}

The double bond becomes a single bond, and the extra atoms are added to the molecule.

Combustion reactions

A combustion reaction is a reaction with oxygen that releases energy. Complete combustion of hydrocarbons produces carbon dioxide and water.

For example, methane burns completely in oxygen:

CH4(g)+2O2(g)→CO2(g)+2H2O(l)CH_4\text{(g)} + 2O_2\text{(g)} \to CO_2\text{(g)} + 2H_2O\text{(l)}

Example

Classifying reaction types

Classify each reaction.

A: $C3H8(g)+5O2(g)→3CO2(g)+4H2O(l)C_3H_8\text{(g)} + 5O_2\text{(g)} \to 3CO_2\text{(g)} + 4H_2O\text{(l)}$
B: $C2H4(g)+H2(g)→C2H6(g)C_2H_4\text{(g)} + H_2\text{(g)} \to C_2H_6\text{(g)}$
C: $C2H6(g)+Cl2(g)→C2H5Cl(g)+HCl(g)C_2H_6\text{(g)} + Cl_2\text{(g)} \to C_2H_5Cl\text{(g)} + HCl\text{(g)}$

In A, the hydrocarbon reacts with oxygen and forms carbon dioxide and water, so A is combustion.
In B, hydrogen is added to an alkene and the formula changes from $C_2H_4$ to $C_2H_6$ , so B is addition.
In C, one hydrogen atom in ethane is replaced by chlorine and hydrogen chloride is also formed, so C is substitution.

Exam technique

In the exam

For formula questions, decide what each formula type is being tested: simplest ratio, actual atoms, family pattern, atom arrangement, or all bonds.
For naming, count the longest carbon chain first, then identify the functional group, then number the chain if needed.
For reaction classification, look for the chemical change: replacement means substitution, adding across C=C means addition, and reaction with oxygen forming $CO_2$ and $H_2O$ means combustion.

Self review

Check yourself

Why is $C_2H_5OH$ not a hydrocarbon?
What is the difference between a molecular formula and a displayed formula?
How would you recognise an addition reaction from a balanced equation?

What you'll learn

What hydrocarbons are, and why carbon is so important in organic chemistry.
How to represent organic molecules using empirical, molecular, general, structural and displayed formulae.
What homologous series, functional groups and isomerism mean.
How to name simple organic compounds and classify substitution, addition and combustion reactions.

Organic chemistry: the carbon chemistry topic

Organic chemistry is the study of carbon-containing compounds. In IGCSE Chemistry, you mainly meet molecules made from carbon, hydrogen and sometimes oxygen or halogens.

Carbon is useful because each carbon atom forms four covalent bonds. This lets carbon atoms join together in chains, branches and rings, making many different compounds.

Definition

Hydrocarbon

Common Mistake

Not every carbon compound is a hydrocarbon

Carbon dioxide, $CO_2$ , contains carbon but no hydrogen, so it is not a hydrocarbon. Ethanol contains carbon and hydrogen, but also oxygen, so it is not a hydrocarbon either.