Alkanes

What are alkanes?

Alkanes are a homologous series of saturated hydrocarbons with the general formula C_nH_2n+2. This means that they are made of hydrogen and carbon only and contain only carbon-carbon single bonds.

Since they are in the same homologous series, they have:

• The same functional group
• The same general formula
• Similar chemical properties
• Differ in their chemical formula by one -CH₂- group
• Have gradually changing physical properties

Where do alkanes come from?

Although they are relatively available, alkanes are produced by two main processes:

1. Fractional distillation of crude oil
• This process involves separating a mixture of hydrocarbons into different fractions according to their boiling points. 
• For more specific information on this, go to this revision note "Fractional Distillation"
2. Hydrogenation of alkenes
• This process is generally used to produce specific alkanes and convert them into more reactive halogenoalkanes

What are the names of the first straight-chain alkanes?

The systematic nomenclature / names of the alkanes follow the International Union of Pure and Applied Chemistry (IUPAC) rules that are used to name all organic compounds.

• More information: Nomenclature of organic compounds

For straight-chain alkanes, there is a prefix that indicates the number of carbon atoms in the chain and the ‘-ane’ suffix that indicates the molecule contains single carbon-carbon bonds. 

Number of carbon atoms	Prefix of the alkane	Name of the alkane	Molecular formula of the straight-chain alkane
1	meth-	methane	CH4
2	eth-	ethane	C2H6
3	prop-	propane	C3H8
4	but-	butane	C4H10
5	pent-	pentane	C5H12
6	hex-	hexane	C6H14
7	hept-	heptane	C7H16
8	oct-	octane	C8H18

What are branched chain and cyclic alkanes?

Straight-chain alkanes have carbon atoms that are connected to each other in one continuous chain, e.g. butane

Branched chain alkanes

These alkanes have side groups attached to the main carbon chain, e.g. 2-methylbutane

To name branched chain alkanes:

1. Identify the longest carbon chain and assign the corresponding alkane prefix
• Longest carbon chain = 4 carbons so the prefix is but-. 
2. Check all of the carbon-carbon bonds are single and assign the appropriate suffix
• All carbon-carbon bonds are single so the suffix is -ane.
3. Identify any side chains / branches
• There is a methyl / CH₃ group on carbon 2, which gives 2-methyl-.
4. Combining all of these fragments gives the overall 2-methylbutane name.

This process can be reversed to draw the displayed formula of an alkane from its name.

Cyclic alkanes 

These alkanes have their carbon atoms connected to form a ring shape, e.g. cyclobutane

Careful: Cyclic alkanes do not have the C_nH_2n+2 general formula

What are isomers in alkanes?

Structural isomers have the same molecular formula but a different structural formula.

• More information: "Structural Isomerism"

Straight-chain alkanes can have branched chain isomers. For example, methylpropane is a branched chain isomer of butane as they both have the molecular formula C₄H₁₀ but they have different structural formulae:

Cyclic alkanes are not isomers of straight-chain alkanes because they have different formulae, they are isomers of alkenes. The general formula of a cyclic alkane is C_nH_2n compared to the C_nH_2n+2 general formula of an alkane.

Chemical and physical properties of alkanes

All alkanes have similar chemical properties which means that they undergo similar chemical reactions.

Alkanes are not soluble in water because they are not polar and cannot overcome / disrupt the hydrogen bonding between water molecules.

The alkanes show patterns in physical properties such as boiling point:

As the alkane gets longer, the boiling point increases. This is because a longer alkane chain has increased intermolecular forces which require more energy to overcome and cause the state change from liquid to gas.

What reactions do alkanes do?

Alkanes are relatively unreactive due to the presence of strong carbon-carbon single bonds and strong carbon-hydrogen bonds. The reactions that alkanes undergo are combustion, cracking and halogenation.

Combustion of alkanes 

Combustion is the reaction of a chemical with oxygen, commonly called burning. Alkanes can undergo complete or incomplete combustion giving slightly different products:

• Complete combustion
• Alkane + oxygen → carbon dioxide + water
• Incomplete combustion (with limited oxygen)
• Alkane + oxygen → carbon monoxide + water
• Alkane + oxygen → carbon + water

• More information: “Combustion of alkanes”

Cracking of alkanes

The smaller chain alkanes are in high demand as they are commonly used for heating, cooking, transportation and electricity generation. But the supply of the shorter chain alkanes is much lower than the demand, while the supply of longer chain alkanes is much greater than their demand.

To provide enough short-chain alkanes to meet the demand, the excess long-chain alkanes are cracked to produce more useful short-chain alkanes as well as alkenes. Alkenes are used to make polymers.

• More information: “Cracking of alkanes”

Halogenation of alkanes

Halogenation reactions of alkanes are actually free radical substitution reactions. During this reaction, a hydrogen atom on the alkane is replaced / substituted by a halogen atom. For example, the halogenation of methane:

CH₄ + Cl₂ → CH₃Cl + HCl

 This reaction actually occurs in three steps:

1. Initiation - where the halogen bond is broken, typically using UV light, to give two radicals
• Cl₂ → 2Cl^⬤
2. Propagation - where the radicals interact with other chemical species to create new radicals
• CH₄ + Cl^⬤ → CH₃^⬤ + HCl
• CH₃^⬤ + Cl₂ → CH₃Cl + Cl^⬤
3. Termination - where two radicals combine to form an uncharged species
• CH₃^⬤ + Cl^⬤ → CH₃Cl

The halogens that are commonly used are chlorine (resulting in chlorination) and bromine (resulting in bromination)

For more information about the cracking of alkanes, see “Free Radical Substitution”.

What keyword definitions do I need to know for alkanes? 

Some keyword definitions you need to know are:

• Hydrocarbon - a compound that contains hydrogen and carbon only

• Saturated - a compound that contains carbon-carbon single bonds

• Halogenation - the reaction of an alkane with a halogen to form a halogenoalkane

• Combustion - the reaction of a chemical with oxygen

• Cracking - the breakdown of a long-chain hydrocarbon into shorter, more useful hydrocarbons

This is a quick summary of some key concepts on alkanes - remember to go through the full set of revision notes, which are tailored to your specification, to make sure you know everything you need for your exams!