Saturated Hydrocarbons – SS2 Chemistry Lesson Note

Alkanes are aliphatic hydrocarbons with the general molecular formula C_nH_2n+2. They have no functional group. The first five members are methane, ethane, propane, butane and pentane. This note covers IUPAC naming of aliphatic compounds, the laboratory preparation, properties and uses of methane, and isomerism.

Behavioural objectives

By the end of the lesson, students should be able to:

• state the general formula of the alkanes and name the first five members;
• name the root, suffix and prefix in an IUPAC name and apply the rules for naming;
• describe the laboratory preparation of methane;
• state the physical and chemical properties and the uses of methane;
• define isomerism and list its types.

What are alkanes?

The alkanes are aliphatic hydrocarbons. Their general molecular formula is C_nH_2n+2. There is no functional group in the alkane series. The first five members are:

The series continues in the same way for higher values of n.

How are aliphatic compounds named using IUPAC rules?

Parts of an IUPAC name

In IUPAC nomenclature, every name of an organic compound consists of root, suffix and prefix names.

1. Root name: the name of the parent aliphatic hydrocarbon of the longest carbon chain in a molecule.
2. Suffix name: the name of the principal functional group on the longest carbon chain in a molecule.
3. Prefix name: the name of the other substituents on the longest carbon chain which are not functional groups.

For example, 1-chloroethan-1-ol has 1-chloro as the prefix, ethane as the root and -1-ol as the suffix.

Rules for IUPAC nomenclature

1. Select the longest continuous carbon chain as the parent hydrocarbon.
2. Number the longest carbon chain starting from the end that gives the lowest possible number to the suffix (functional group).
3. Indicate the other substituents by prefixes preceded by numbers to show their position on the carbon chain.
4. If the same alkyl or other substituent group occurs more than once as a side chain, show how many are present by using the prefix di, tri, tetra and so on. Indicate by numbers the position of each group on the longest carbon chain.
5. If there are several different alkyl groups attached to the parent chain, name them in alphabetical order.
6. If there are halogens together with alkyl groups attached to the parent chain, name the halogens first in alphabetical order, then the alkyl groups as explained earlier.

How is methane prepared in the laboratory?

Methane is prepared in the laboratory by heating an ethanoate salt with the corresponding alkali, for example sodium ethanoate with soda-lime. Soda-lime is quicklime slaked with a concentrated solution of sodium hydroxide. It is used in preference to caustic soda because it is not deliquescent and does not attack glass so readily.

What are the properties of methane?

Physical properties

1. Methane is a colourless and odourless gas.
2. It is slightly soluble in water.
3. It is less dense than air.
4. It has no action on litmus paper.

Chemical properties

1. Combustion: Methane burns in air or oxygen to produce steam, carbon(IV) oxide and a lot of heat.

CH₄(g) + 2O₂(g) → 2H₂O(g) + CO₂(g)

The general equation for the combustion of alkanes is:

C_xH_y(g) + (x + y/4)O₂(g) → (y/2)H₂O(g) + xCO₂(g)

2. Substitution reaction: A substitution reaction involves the direct displacement of an atom or group by another atom or group. Methane reacts with chlorine in the presence of ultraviolet light to yield a mixture of products.

CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g)

What are the uses of methane?

1. Methane is used as fuel by itself or mixed with other gases.
2. It is used for making hydrogen gas.
3. It is used in making carbon black.
4. Trichloromethane is used in surgical operations as an anaesthetic.

What is isomerism?

Isomerism is the existence of two or more organic compounds with the same molecular formula but different molecular structures. There are two types: structural isomerism and stereoisomerism.

Structural isomerism

Structural isomerism occurs in organic compounds with the same molecular formula but different structural arrangements of the carbon atoms. It has three types:

1. Chain isomerism: this occurs because of differences in the way the carbon atoms are arranged in the chain.
2. Functional isomerism: this is due to differences in functional groups.
3. Positional isomerism: this occurs as a result of the difference in the way the functional group is positioned.

Stereoisomerism

Stereoisomerism arises as a result of differences in the spatial orientation of atoms or groups of atoms about a carbon-carbon double bond, a ring structure, or a carbon atom surrounded by four different groups. It has two types:

1. Geometric isomerism: this is the existence of compounds with the same molecular formula but different spatial structural formulae.
2. Optical isomerism: the isomers have different configurations and they rotate plane polarised light.

Evaluation

1. State the general formula of the alkanes and give the names and formulae of the first four members.
2. Name the three parts of an IUPAC name and say what each one represents.
3. Write the equation for the combustion of methane.
4. Why is soda-lime used in preference to caustic soda in the preparation of methane?
5. Define isomerism and name the types of structural isomerism.

Assignment

1. List four uses of methane.
2. Write the general equation for the combustion of alkanes.
3. Distinguish between chain isomerism and positional isomerism.