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  • Date Submitted: 11/19/2012 04:53 AM
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If two different substances with different chemical and/or physical properties have the same molecular formula they are said to exhibit isomerism and to be isomers of one another. This phenomenon is of interest because one can easily write the formula of one compound without realizing that it can also represent others. Isomerism is frequently encountered in organic compounds, although there are some inorganic compounds which exhibit isomerism. There are two general types of isomers: (1) constitutional isomers and (2) stereoisomers. Each general isomer class can be further subdivided into different types.

Constitutional Isomers
Constitutional isomerism arises when a formula represents compounds in which the atoms are attached to one another in different ways-i.e. bonding is different. Constitutional isomers are molecules that have the same molecular formula but different spatial arrangements of the atoms. A structural formula shows the order in which atoms are bonded together. There are three main kinds of constitutional isomers: 1) skeletal isomers 2) positional isomers 3) functional group isomers.

1. Skeletal isomerism:
The hydrocarbon chain may be arranged in different ways i.e. the carbon ‘skeletons’ differ, i.e. different structural ‘backbones’. The isomers possess the same functional group, and belong to the same homologous series.


i) Pentane
ii. C6H12 cyclohexane Methylcyclopentane 1,1-dimethylcyclobutane

Only the physical properties differ in these cases. Compounds of branched chain molecules always have lower boiling point than that of the related straight chain compound. The straight chain molecule can have greater areas in contact with one another, therefore, increase in intermolecular forces (LDF) than the branched chain molecules (in the same way as pieces of rope in contact with one another would have more area touching than would footballs placed together). As branching increases, boiling point...


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