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Revise Functional Groups in Organic Molecules (A-Level Organic Chemistry)

Naming Alkanes

Definition of alkanes:

Alkanes are chemical compounds that consist only of the elements carbon (C) and hydrogen (H) linked exclusively by single bonds.

Each carbon atom forms 4 bonds (either C-H or C-C bonds).
Each hydrogen atom is connected to a single carbon atom, by a H-C bond.

Alkanes are one of the simplest types of organic compounds and are therefore generally taught early in basic courses in organic chemistry, e.g. at GCSE level in UK schools.

Alkanes are hydrocarbons because they consist only of carbon and hydrogen atoms.
These atoms combine in proportions according to the general formula to form alkanes:

CnH2n+2

where the letter n represents the number of carbon atoms in each molecule of the compound.

In general there are three basic types of alkanes.
They are linear alkanes, branched alkanes and cyclic alkanes (which may also be referred to as cycloalkanes). Of these, linear alkanes are the simplest to draw and explain and are therefore usually the first to be introduced in chemistry lessons.

 

Linear Alkanes:

The homologous series of linear alkanes is illustrated in the following table:

Name of Alkane
Number of Carbon atoms
Chemical Formula
Simple Structure
(Molecular Diagram)
Also known as
(other names):
Number of isomers

Methane

1

C H4

Methane (Alkane)

  • natural gas
  • marsh gas
  • methyl hydride

1

Ethane

2

C2H6

Ethane (Alkane)

  • dimethyl
  • methyl methane
  • ethyl hydride

1

Propane

3

C3H8

Propane (Alkane)

  • dimethyl methane
  • propyl hydride

1

n-Butane

4

C4H10

n-Butane (Alkane)

  • methylethyl methane
  • butyl hydride

2

n-Pentane

5

C5H12

n-Pentane (Alkane)

  • amyl hydride
  • Skellysolve A

3

n-Hexane

6

C6H14

n-Hexane (Alkane)

  • dipropyl
  • Gettysolve-B
  • hexyl hydride
  • Skellysolve B

5

n-Heptane

7

C7H16

n-Heptane (Alkane)

  • dipropyl methane
  • Gettysolve-C
  • heptyl hydride
  • Skellysolve C

9

n-Octane

8

C8H18

n-Octane (Alkane)

  • dibutyl
  • octyl hydride

18

n-Nonane

9

C9H20

n-Nonane (Alkane)

  • nonyl hydride
  • Shellsol 140

35

n-Decane

10

C10H22

n-Decane (Alkane)

  • decyl hydride

75


In the case of the names of alkanes beginning with n- , the n- part is included to specify the linear (as opposed to a branched or cyclic) form of that particular alkane. In some cases the atoms may be arranged in different ways, hence the alkane may exist in the forms of several different structural isomers. Some examples of structural isomers are shown below.


Branched Alkanes

Alkane molecules that include more than 3 carbon atoms can be arranged in more than one way.
The simplest example is butane, which can take either of two different forms called structural isomers, as shown below.

The 2 Structural Isomers of Butane

Butane
Methylpropane
Butane Methylpropane (Branched Alkane)

n-butane
normal butane
unbranched butane

isobutane
i-butane


Another way to draw methylpropane that is less accurate, but perhaps easier to see initially, is :

The diagram on the right may be thought of as the result of exchanging the carbon-group on the right-hand-side of the butane structure shown below, with the hydrogen atom indicated. However, the diagram on the (top) right is a less accurate representation of the actual molecule because the angles between the atoms in real 3-dimensional space are more similar to each other, as indicated in the molecular structure shown in the table above.

It is useful to be aware of structural isomers of organic molecules as the result of exchanging the positions of certain atoms and/or groups because this concept applies to many different types of organic molecules, not just to alkanes - which include the simplest examples, such as butane and methylpropane.

Another note about these diagrams is that, because they are very simple representations of the arrangements of atoms within molecules, the lengths and thicknesses of the lines representing chemical bonds do not generally signify anything about the bonds they represent. Although it is a good idea to draw these lines as consistently as possible, it is not unusual to need to vary the lengths slightly in order to fit everything in to the space available, e.g. as on the right.

Structural Isomers of Butane: Methylpropane and n-Butane

 

In molecular diagrams of organic molecules single lines represent single bonds, double lines represent double bonds, and triple lines represent triple bonds. The molecular diagrams on this page all include only single bonds, which shows that alkanes are saturated molecules.

The 3 Structural Isomers of Pentane

Pentane is an alkane that has 5 carbon atoms in each molecule.
In organic chemistry generally, the larger the molecule, the more different ways in which the atoms can join together.
There are 3 different structural isomers of pentane, as shown below:

Pentane
Methylbutane
Dimethylpropane
pentane methylbutane dimethylpropane

n-pentane
normal pentane
unbranched pentane

isopentane

neopentane

As indicated in the right-hand column of the table of linear alkanes further up this page, molecules of alkanes that have more carbon atoms per molecule can exist in many more different isomeric forms. Introductory courses, such as GCSE Chemistry, do not usually require detailed knowledge of all the isomers of the larger alkanes but it can be useful to work out and draw the molecular diagrams of some them in order to aid understanding and memory of the concept of structural isomerism.

 

More about alkanes generally (Revision Notes):

  • Alkanes form a homologous series of organic compounds (meaning that all alkanes can be described by a single general chemical formula, specifically CnH2n+2 in the case of alkanes) in which the members of the series differ by a constant relative molecular mass of 14, which is the molecular mass of one carbon atom and two hydrogen atoms.
  • Alkanes are saturated compounds, meaning that the atoms that form the molecules of alkane compounds are linked exclusively by single bonds.
  • There is no limit to the number of carbon atoms that can be linked together in such a way as to form a linear alkane.
  • The conditions that alkane molecules must satisfy are that they are:
    1. hydrocarbons (only contain atoms of carbon and hydrogen),
    2. saturated (only contain single bonds; so include no double- or triple- carbon bonds),
    3. linear alkanes (specifically) are acyclic, meaning that the carbon atoms are generally connected in such a way as to form a line, if so arranged, rather than a ring ... which would be a cycloalkane.
  • Saturated oils and waxes are examples of larger alkanes in which the number of carbon atoms in the carbon chain, which is sometimes called a "carbon backbone", is greater than 10.
  • Alkanes are not very reactive and have little biological activity.
    However, alkanes are sometimes described in terms of being a "molecular tree" (i.e. a basic structure) onto which more complex and biologically active atoms or "functional groups" of atoms may be attached, the simplest example being halogenoalkanes (also known as haloalkanes).

See also: Boiling Points of Alkanes


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