Emily Bullas – Isomerism
Isomers are molecules which contain the same atoms, arranged differently to each other. These
molecules can have different physical and chemical properties to each other, which can have
implications on the industrial and commercial uses of the chemicals. There are different types of
isomerism, including structural isomerism and stereoisomerism.
Structural Isomerism:
Structural isomers are molecules which contain the same atoms, but they are arranged differently in
space. These isomers are often completely different shapes to the original molecules. The ways that
the isomers differ from each other are used to categorise them into different types of structural
isomers.
Chain isomers are isomers which differ in the branching of the carbon chains, for example
methylpropane is a chain isomer of butane. Both molecules have four carbon atoms at 10 hydrogen
atoms, but butane has a longest chain length of four, whereas methylpropane has a longest chain
length of 3. Figure 1 below shows the chain isomerism of C₅H₁₂. The melting point varies between
these isomers, the highest being pentane which has a melting point of –130°C, followed by 2-
methylbutane which has a melting point of –160°C and finally 2,2-dimehtylpropane which has a
melting point of –17°C (4). When a reaction produces C₅H₁₂, it often produces multiple structural
isomers of the molecule, in different proportions. The isomers produced and the proportions of
these different isomers is dependent on the reaction conditions, such as temperature, pressure and
presence of catalysts.
Figure 1: Chain Isomerism of C₅H₁₂
Positional isomers are isomers in which the carbon chain remains the same, but the groups attached
are in different positions, an example of this is 1-bromopropane and 2-bromopropane. These have
the same 3-carbon chain but 1-bromopropane has a bromine group on the end of the chain, while 2-
, bromopropane has a bromine group on the second carbon. Figure 2 below shows the five positional
isomers of C₃H₆BrCl.
Figure 2: Positional Isomerism of C₃H₆BrCl
Finally, functional group isomers are isomers which are made up of the same atoms but contain
different functional groups, so they belong to different homologous series. For example, the
molecule C₃H₆O can form either the aldehyde propanal or the ketone propanone. Furthermore, the
molecule C₂H₆O can be either an alcohol, ethanol, or an ether, methyl ether, as a result of functional
group isomerism. The structure of the two isomers is shown in figure 3 below.
Figure 3: Functional Isomerism of C₂H₆O
While they contain the same atoms, the difference in structures means that these two chemicals
have different physical and chemical properties. For example, the melting point of dimethyl ether is -
138°C, while the melting point of ethanol is -117°C. Similarly, the boiling point of dimethyl ether is -
25°C, and the boiling point of ethanol is 78. At room temperature, dimethyl ether is a gas, whereas
ethanol is a liquid. This is because of the type of bonding between the molecules – ethanol
molecules are held together primarily by hydrogen bonds, and also partly by London dispersion
forces and dipole forces, while dimethyl ether molecules are held together by regular dipole
moments. Hydrogen bonds are much stronger than these dipole moments, which makes it much
harder to pull apart ethanol molecules, compared to dimethyl ether molecules, making the melting
and boiling points of ethanol higher (1). They also undergo different chemical reactions due to their
different functional groups. Ethers are generally unreactive but, in the presence of strong acids, the
oxygen on their C-O bond can be protonated and removed using a nucleophilic substitution reaction
mechanism, this is known as acidic cleavage (2). Alcohols, however, can undergo many different
