Nomenclature, structural formulae and Isomers

Hydrocarbons

a) Alkanes, C_nH_2n+2

There are several sets of hydrocarbons, and the alkanes make a sub-set.
Alkanes all have names which end in the letters -ANE.

The Chemistry data book (page 6) gives the names of the first 8 members of the sub-set.

Methane is the simplest member of the alkane sub-set and has the formula CH₄, while the second member of the subset, is called ethane and has the formula C₂H₆.

In methane there are four hydrogen atoms join to one atom of carbon.

Formulae like this (CH₄ and C₂H₆) are called molecular formulae.

 
The arrangements of atoms in methane, ethane, propane and butane are shown below.

These formulae below show exactly how the atoms are joined and are called full structural formulae.

 
There is a pattern in the formula as shown in the diagram below:

All members of the alkane family fit the general formula C_nH_2n+2.

This can be used to work out the formula of any alkane if the number of carbon atoms is known. Try this simple program below to illustrate this :

 
Shortened structural formulae can also be drawn for alkanes (and other sub-sets).

Shortened structural formulae only show the numbers of hydrogen atoms associated with each carbon atom.

A comparison of molecular and shortened structural formulae of the alkane sub-set is shown below:

 
In this table, the name octane refers to the alkane in which 8 carbon atoms are joined in a line with 18 hydrogen atoms attached.
Heptane refers to the alkane with 7 carbon atoms joined in a line etc.

 
You can quickly test your knowledge of the above information.

 
It is possible to have chains with branches on them.
The diagram below shows two different structures for the alkane of molecular formula C₄H₁₀.

These different structures are known as isomers and are defined as compounds with the same molecular formula but with a different structural formula.

Rules for naming branched-chain alkanes

The branches are based on alkanes and have the general formula C_nH_2n+1

The branches or groups of atoms are called alkyl groups and are named by replacing the -ANE ending of the alkane with -YL e.g CH₃ = methyl group, C₂H₅ = ethyl group etc

 
Method for naming

1. Look for the longest continuous chain of carbon atoms and name this as the parent compound.
2. The parent name is placed at the end of the name of the compound.
3. Look for groups which form branches on the chain and identify them.
4. Different groups are written in alphabetical order before the parent name e.g. 'ethyl' before 'methyl'.
5. If there are 2 identical groups the prefix 'di' is placed before the name of the branch e.g. 'dimethyl', 'diethyl' etc
6. The prefixes 'tri' and 'tetra' are used if there are 3 or 4 repetitions respectively of the same group on the parent chain.
7. To specify the position of each group, the parent chain is numbered from the end which results in the lowest possible numbers in the formula.

 
The two isomers shown above are called butane and methylpropane.
No number is needed in the name methylpropane because the only possible position for the methyl branch is on the middle carbon atom of propane.

The different structures for C₅H₁₂ are shown below:

The different structures for C₆H₁₄ are shown below:

Full structure A is hexane.
Structures B and C have both have 5 carbon atoms in a chain and because of this they are said to be based on pentane. However, they are different to pentane because they each have a CH₃ group in place of a hydrogen atom.

Compounds B and C are both called methylpentane, but are different in the position of the methyl groups on the pentane chain. To distinguish them, the pentane chain is numbered from the end that gives the lowest number in the name.

Numbering from the right names B as 2-methylpentane - the correct name. (Numbering from the left would give 4-methylpentane - the incorrect name.

Compound C is called 3-methylpentane (and it doesn't matter which end the numbering starts).

Compounds D and E are based on butane with two methyl groups. The number of methyl groups is indicated using the prefix 'di-' in the name, and the positions of the methyl group indicated by separate numbers.

D is correctly named as 2,3-dimethylbutane, while E is 2,2-dimethylbutane.

 
Representing shortened branched structural formulae

Shortened structural formulae are written in the usual way and side chains are included in the formula by writing them in brackets after the carbon they are attached to.

Compound B has the formula CH₃CH₂CH₂CH(CH₃)CH₃ while compound C has the formula CH₃CH₂CH(CH₃)CH₂CH₃

Compound D has the formula CH₃CH(CH₃)CH(CH₃)CH₃ while compound E has the formula CH₃C(CH₃)₂CH₂CH₃

• Nomenclature is a special term which means 'naming'.

 
The alkanes make a collection called a homologous series because all alkanes fit the same general formula, have similar chemical properties and show a gradual but regular change in physical properties such as melting and boiling points.

 
You can quickly test your knowledge of the above information.

 
b) Alkenes, C_nH_2n

Alkenes form another homologous series of hydrocarbons with names ending in the letters
-ENE.

Alkenes have a double bond between 2 carbon atoms, and this is the reason for two fewer hydrogen atoms than alkanes.

The table gives the names, molecular formulae and shortened structure for the first 7 members of the alkene series.

Isomers exist for alkenes by using branches, but the presence of the double bond also increases the possibility of isomers because the position of the double bond can vary.

When naming alkenes, the position of the double bond is indicated by numbering the carbon chain from the end that gives the lowest number for the double bond.

Although the double bond is between 2 carbon atoms, only the number of the lowest carbon atom is included in the name.

In the carbon skeleton C-C-C=C, the double bond is between carbon atoms 1 and 2, and so the chain is called but-1-ene.

In the carbon skeleton C-C=C-C, the double bond is between carbon atoms 2 and 3, and so the chain is called but-2-ene.

Some examples of names, full structural formulae and shortened structural formulae are given below.

 
c) Cycloalkanes, C_nH_2n

Cycloalkanes form another sub-set of hydrocarbons.

Their names are related to the alkane sub-set by ending in the letters -ANE, and they can be identified as cycloalkanes because they begin 'CYCLO-'

They have only single bonds between carbon atoms (like alkanes), but do no exist as chains.
Instead, one end of an alkane chain has joined to the other end of the alkane chain to make a 'cyclic' (or ring) structure.
To allow this to happen, one hydrogen atom from each end of the alkane chain has to be removed to allow the chain to form a 'ring' structure.

The general formula for cycloalkanes is C_nH_2n (the same as for the alkene sub-set).

 
The smallest cycloalkane is cyclopropane (C₃H₆), and continues with cyclobutane (C₄H₈) etc.

The shortened structural formulae for several cycloalkanes are shown below:

• Remember that cycloalkanes have the same general formula as alkenes and so some isomers can belong to a different homologous series e.g propene and cyclopropane are isomers.

 
Alkanols and Alkanoic Acids

Alkanols, C_nH_2n+1OH

Alkanols make a homologous series of compounds based on the Alkanes. The ending -OL indicates membership of the family, and the presence of the hydroxyl group, -OH instead of one of the hydrogen atoms.

 
Isomers exist for alkanols, and the presence of the -OH or hydroxyl group increases the possibility of isomers because the position of the hydroxyl group can vary.

When naming alkanols, the position of the hydroxyl group on the carbon chain longest chain is numbered so that the hydroxyl group in attached to the lowest numbered carbon atom.

The shortened formula for propan-1-ol is CH₃-CH₂-CH₂-OH

 
Alkanoic Acids

Alkanoic acids have names which end in -OIC and contain the carboxyl group, -COOH.

Esters

Esters contain the ester linkage, -COO- and can be recognised because their names end in the letters -OATE.
Esters are made when an alkanol and an an alkanoic acid join together e.g. methyl ethanoate is made when methanol joins with ethanoic acid.

To name an ester replace the 'ANOL' ending of the alkanol with 'YL'; replace the 'OIC ACID' ending of the alkanoic acid with 'OATE' and put the two names together.

The shortened structural formula for methyl ethanoate is CH₃-COO-CH₃

 
You can quickly test your knowledge of the above information.

New words and their meanings

Hydrocarbon - compounds which contain the elements hydrogen and carbon only.

Molecular formulae - formulae which show the elements present, and the number of atoms of each element.

Full structural formulae - formulae which show how the different elements present are joined together.

General formula - a formula which shows the ratio of different atoms applicable to all members of the homologous series.

Shortened structural formulae - structures in between molecular formulae and full structural formula. They show the number of hydrogen atoms joined to each carbon atom without showing how the atoms are joined together.

Isomers - compounds with the same molecular formula but with a different structural formula.

Homologous series - family of compounds with similar chemical properties, a gradation in physical properties and where all members of the family fit the same general formula.