A Level Chemistry Revision: Organic Chemistry – Alkanes

by Lucy Bell-Young

Alkanes are saturated hydrocarbons. This means that each carbon atom is single-bonded to another carbon atom. Alkanes exist in straight chain, branched (isomeric), and cyclic forms. You can use a general formula to determine the specific formulas based on the number of carbon atoms. You can also easily name alkanes using the IUPAC standards.

The study of specific substances in organic chemistry, like alkanes, is always focused on four main aspects: 

  • Formula and structure
  • Nomenclature
  • Physical and chemical properties
  • Reactions

In this article, we’ll try to be as comprehensive as possible in discussing alkanes, but you must also ensure that you have a good foundational understanding of the basic concepts in inorganic and organic chemistry if you want to get the most out of your A level chemistry revision.

What Are Alkanes?

Alkanes are hydrocarbons, i.e. they only contain carbon and hydrogen. Their general formula can be written as CnH2n+2.

As you can see in the formula, you can easily determine the number of hydrogen atoms in an alkane if you have the number of carbon atoms. 

Alkanes are saturated, having only single covalent, sigma bonds, which are very strong and difficult to break. Alkanes are relatively unreactive because they have high enthalpy:

H = E + PV

H = enthalpy

E = energy

P = pressure

V = volume

You would need a higher energy to break the bonds of alkanes. For instance, to start a combustion reaction, you need a spark or a flame to initiate combustion. Alkanes can also react with the highly reactive halogens, but they are virtually inert with other elements.

How Do Alkanes Bond?

Alkanes have covalent, sigma bonds, which allow them to have highly stable molecules. The four valence electrons of carbon allow the atom to form carbon chains with hydrogens attached to them.

The similar electronegativities of hydrogen and carbon create strong bonds. They’re also non-polar, making them insoluble in water. These bonds form three types of molecular structures: straight chain, branched chain, and cyclic.

  1. The straight-chain alkanes

Carbons form the backbone of straight-chain (or linear) alkanes, while the hydrogens are attached to each carbon. From the simplest methane molecule (CH4), more carbons can be added to form a chain, following the general formula CnH2n+2.

For alkanes with three or more carbons, the extreme ends have CH3 groups while those in between are CH2 groups. The two valence electrons are bonded with two carbons in the chain while the remaining two are bonded with hydrogens. They’re called a homologous series because more carbons can be indefinitely added to the chain.

The simplest example of an alkane is methane (CH4), but it’s technically not a straight chain alkane because it only has one carbon. Therefore, the simplest example of an alkane chain is ethane (C2H6), which has two carbons that form a chain, as shown in the illustration below:

Structure of straight-chain alkanes
  1. The branched alkanes

Branched alkanes are isomeric forms of alkanes. While the chemical formula is the same, the position of one atom or group of atoms is different. Instead of just a hydrogen attached to the peripherals of the main chain of carbons, one or more hydrocarbon substituents are attached to one or more main chain carbons.

For example, the chemical formula for both butane and isobutane is C4H10. They both have four carbons and ten hydrogens. However, their respective molecular structures are different. The illustration below shows the structural difference between butane and its isomer:

Illustration showing the structural difference between butane and its isomer
  1. The cycloalkanes

Cycloalkanes, otherwise known as naphthenes, are also saturated hydrocarbons but have cyclic or ring-like molecular structures. More precisely, cycloalkanes can be represented by skeletal formulas. These formulas are simplified illustrations on a flat surface, while the real three dimensional molecular structures are more like puckered rings.

For example, the simplest cycloalkane is cyclopropane (C3H6), which has three carbon atoms. Its skeletal formula is represented by a triangle.

Cycloalkanes have the general formula CnH2n. Their skeletal formulas have regular polygon representations up to nine carbon atoms. Beyond that, they’re still monocyclic, but the shape changes from regular polygons to connected incomplete regular hexagons. For example, the skeletal formula for cyclodecane (C10H20) can be represented by the drawing below. This pattern continues for larger cycloalkanes:

Skeletal formula of cycloalkanes

Each corner or vertex represents a carbon atom. If you count the number of corners in this illustration, the total number is ten.

How Are Alkanes Named?

The naming convention for alkanes is based on the International Union of Pure and Applied Chemistry (IUPAC) standards. The composition, particularly the number of carbons, the positions, and shape of the molecules, are all factored into consideration when naming an alkane.

  1. Naming straight-chain alkanes

The straight-chain alkanes are relatively easy to name based on IUPAC standards. The names describe the number of carbon atoms in the chain. For the first four straight-chain alkanes, you can use the preferred IUPAC names, which are also the common names, instead of the systematic names.

For example, the systematic IUPAC name of butane is tetracarbane, which is almost never used. In naming the straight-chain alkanes, you simply use the Greek prefixes referring to the number of carbons and always add -ane suffixes at the end. Below is a list of the first ten alkanes and their molecular formulas:

  • Methane: CH4
  • Ethane: C2H6
  • Propane: C3H8
  • Butane: C4H10
  • Pentane: C5H12
  • Hexane: C6H14
  • Heptane: C7H16
  • Octane: C8H18
  • Nonane: C9H20
  • Decane: C10H22
  1. Naming branched-chain alkanes

Branched-chain alkanes nomenclature can be a bit tricky. You need to follow a few rules, which include:

  • Rule 1: Determine the parent chain by counting the number of carbons of each straight chain. The longest unbranched chain, or the one with the most carbon atoms, is the parent chain. This is sometimes illustrated with bends rather than a straight line. You can find it by referring to the CH3– group in its extreme ends.
  • Rule 2: Specify the location of the branch based on the ordinal number of the carbon of the parent chain to which it is attached. The numbering must be from the end nearest to the branch or functional group.
  • Rule 3: Name the branch or branches attached to the parent chain. Then, specify how many branches are attached. If there are more than one branch, you can use prefixes such as di-, tri-, tetra-, penta-, and hexa-.
  • Rule 4: When naming alkyl groups, they should be written in alphabetical order.
  1. Naming cycloalkanes

Cycloalkanes without branches or functional group attachments are very easy to name. There are only two rules: first, always use the prefix “cyclo-”, and second, use the same name of straight-chain alkanes based on the number of carbons. The simplest cycloalkane is cyclopropane, which has three carbon atoms. Here are some examples:

  • Cyclopropane: C3H6
  • Cyclobutane: C4H8
  • Cyclopentane: C5H10
  • Cyclohexane: C6H12
  • Cyclooctane: C8H16
Diagram explaining the nomenclature of alkanes

How Do Alkanes React With Other Substances?

Alkanes are very stable and relatively unreactive compounds. It would take a high amount of energy to start a reaction, such as an electrical spark or a flame. 

Alkanes have three main types of direct chemical reactions: combustion, halogen substitution, and pyrolysis (cracking). Meanwhile, oxygen and halogens, such as chlorine, are highly reactive with other elements and substances.

  1. Combustion of alkanes

The combustion of an alkane is basically an oxidation reaction in which oxygen strips some electrons from an alkane. It’s a rapid exothermic reaction. The final products of a complete combustion of an alkane are water and carbon dioxide.

  1. Halogen substitution

Halogen substitution, otherwise known as halogenation of alkanes, is simply the replacement of one or more hydrogens with a halogen. One of the simplest examples is the chlorination of methane.

  1. Pyrolysis of alkanes

The pyrolysis of alkanes, which is also known as ‘cracking’ in the petroleum industry, involves the application of high temperatures (400-450oC) and catalysts to split an alkane into various products. 

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