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Isomers

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Isomers

It is possible for two organic compounds to have the same molecular formula but a different structural formula. Look at the two organic compounds that are shown in the figure below for example.

Isomers

Isomers of a 4-carbon organic compound with the molecular formula \(\text{C}_{4}\text{H}_{10}\). (a) butane and (b) 2-methylpropane.

Fact:

Both butane and 2-methylpropane (isobutane) are used in camping stoves and lighters.

Isomers

If you were to count the number of carbon and hydrogen atoms in each compound, you would find that they are the same. They both have the same molecular formula \(\text{C}_{4}\text{H}_{10}\), but their structure is different and so are their properties. Such compounds are called isomers.

Definition: Isomer

In chemistry, isomers are molecules with the same molecular formula but different structural formula.

Isomers are molecules with the same molecular formula and often (though not always) with the same kinds of chemical bonds between atoms, but with the atoms arranged differently.

The isomers shown above differ only in the location of the carbon atoms. The functional groups are the same but butane has all four carbons in one chain, while 2-methylpropane has three carbons in the longest chain and a methyl group attached to the second carbon in the chain. It is also possible to have positional isomers (see figure below). In this case the \(-\text{OH}\) functional group can be on different carbon atoms, for example carbon 1 for pentan-1-ol, on carbon 2 for pentan-2-ol or on carbon 3 for pentan-3-ol.

Isomers

The structural and condensed structural representations of the isomers (a) pentan-1-ol, (b) pentan-2-ol and (c) pentan-3-ol.

Positional isomers are also found in esters. If one ester was made from ethanol and hexanoic acid and another was made from hexanol and ethanoic acid the two esters produced are isomers (see figure below).

Isomers

Isomers

The structural representations of the isomers (a) ethyl hexanoate and (b) hexyl ethanoate. 

It is important to note that molecules need not have the same functional groups to be isomers. For example propanone (commonly known as acetone) and propanal (see figure above) have the same molecular formula (\(\text{C}_{3}\text{H}_{6}\text{O}\)) but different functional groups and properties (see table below). These types of isomers are functional isomers.

Isomers

Two \(\text{C}_{3}\text{H}_{6}\text{O}\) isomers (a) propanone and (b) propanal. 

Name

Functional Group

Melting point (℃)

Boiling point (℃)

Reactivity

propanone

ketone

\(-\text{95}\)

\(\text{56}\)

less reactive

propanal

aldehyde

\(-\text{81}\)

\(\text{48}\)

more reactive

Table: Some properties of the isomers propanone and propanal.

Heptanoic acid and butyl propanoate (see figure below) are another example of functional isomers (containing different functional groups). They both have the same molecular formula of \(\text{C}_{7}\text{H}_{14}\text{O}_{2}\) but have different functional groups and different properties (see table below).

Isomers

The structural representations of the isomers (a) heptanoic acid and (b) butyl propanoate. 

Name

Functional group

Melting point (℃)

Boiling point (℃)

heptanoic acid

carboxylic acid

\(-\text{7.5}\)

\(\text{223}\)

butyl propanoate

ester

\(-\text{89.5}\)

\(\text{145}\)

Table: The different physical properties of the isomers heptanoic acid and butyl propanoate.

 

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