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Isomerism ­ Answers
1. Structural isomers are compounds with the same molecular formula but different structural isomers.
2. Structural isomers can be divided into three categories:
Carbon Chain Isomers ­ The only way in which these isomers differ is in the length of the
carbon chain. E.g. butane and methylpropane both have the molecular formula C4H10, but
butane has a carbon chain of four atoms and methylpropane has a carbon chain of three
Positional Isomers ­ functional groups can be in different positions on the carbon chain,
resulting in isomers. E.g.
Functional Group Isomers ­ Occurs when two compounds with the same molecular
formula are members of different homologous series and have different functional
groups. E.g. hex-1-ene and cyclohexane, both of which have the molecular formula of C6H12. However, a
molecule of hex-1-ene has a double bond, but cyclohexane is cyclic with no double bond.
3. Stereoisomers are compounds with the same structural formula, but which have the atoms arranged differently
in space. There are two types of stereoisomerism ­ geometric and optical.
4. Geometric isomers are isomers that differ only in the spatial arrangement of atoms in the planar part of the
molecule or above and below a ring. To get geometric isomerism, there must be:
Restricted rotation caused by a functional group, e.g. a C=C group consists of a sigma bond, which lies
along the axis between the two carbon atoms and a -bond which is above and below that axis. For
rotation around the sigma bond, the -bond would have to break and reform, but the energy required
to do this is far too great.
Two different groups on the left hand end and 2 different groups on the right
5. For the example 1,2, dichloroethene two different molecules can be formed:
In one, the two chlorine atoms are locked on opposite sides of the
double bond. This is known as the trans isomer. (trans : from latin
meaning "across")
In the other, the two chlorine atoms are locked on the same side
of the double bond. This is known as the cis isomer. (cis : from latin
meaning "on this side")
6. Geometric isomers have the same chemical properties, but their biological properties may differ. Cis-retinal
occurs in receptor cells in the retina of the human eye. When it absorbs a photon of visible light, the -bond
breaks. The molecule reforms as the trans-isomer. This change in shape causes a nerve impulse to be sent to the
7. Geometric isomerism can occur in:
Cyclic compounds in which rotations is not possible, for example when chlorine adds to cyclohexene, two
possible geometric isomers can form
Some transition metal complexes, for example, platinum (II) forms planar complexes with four ligands.
The complex exists as two geometric isomers. The cis-isomer is used in chemotherapy.
8. The cis-trans method of naming geometric isomers breaks down in more complex compounds. The E/Z system is
used instead. This system depends on the priorities of the groups attached to the carbon atoms of the double
bond. The priority is equal to the sum of the atomic numbers of the group attached, e.g. H=1, CH3=9, C2H5=17 and
The prefix Z is given to the isomer with the two higher priority groups on the same side of the double bond
The prefix E is given to the isomer with the two higher priority groups on opposite sides
9. Compounds that show optical isomerism do not have a plane of symmetry. They are said to be chiral. Such
compounds have two isomers, which are mirror images.
10. A chiral centre is caused by a molecule with an assymetric carbon atom. It results in a molecule or ion causes it to
have two optical isomers, which are called enantiomers meaning that it is non-superimposable on its mirror

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The most common cause of chirality in organic chemistry is when a carbon atom has four different groups
or atoms attached to it, e.g. the compound CHFClBr.
11. Enantiomers have identical chemical properties and the same boiling temperatures and solubilities, however they
differ in two ways:
A solution of one enantiomer rotates the plane of polarization of plane-polarised light in a clockwise
direction (+); the other rotates it in an anticlockwise direction (-).
Optical isomer soften have different biochemical reactions, e.g.…read more


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