Isomers of Alkanes

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Isomers of Alkanes

• Catherine Mwenya
• 10th April 2008
• Grade 12

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Isomers of pentaneChemistryMunali Boys High
Schools
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Definitions

• Isomerism
• The occurrence of two or more compounds with the
  same molecular formula but different structural
  formula. Isomers are therefore compounds with the
  same molecular formula but with different
  structural formula. Isomerism exists in all
  homologous series including the alkane family.
• Alkanes
• Organic compounds having single carbon-carbon
  bonds in their structures and also conform to the
  general formula CnH2n2

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Characteristics

• Isomers have different physical properties but
  similar chemical properties because the belong to
  the same family

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Rules for Naming Isomers

• Identify the longest continuous carbon chain and
  name it. This will be the isomers last name or
  suffix. Bear in mind the homologous series of the
  isomer you are trying to draw.
• Identify the attached alkyl groups and their
  positions. This will give the isomers first name
  or prefix.
• Number the longest continuous carbon chain in
  such a way that these attached groups are at the
  least numbered carbon atoms in the chain.

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Rules for Naming Isomers, Cont.

• Name the isomer, starting with the position of
  the attached alkyl group e.g methyl, ethyl, etc.
• Use di, tri etc. when you have more than one of
  the same alkyl attached.
• Use commas between figures and hyphens between a
  figure and a letter.

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Example Draw and name the isomers of Pentane

• Isomer 1 is n-pentane, the straight chain normal
  structure for pentane

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• Isomer 2 is 2-methylbutane, a branched chain with
  a carbon atomjoined onto three other carbon
  atoms

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• Isomer 3 is  2,2-dimethylpropane, a branched
  chain with the central carbon atomjoined onto
  four other carbon atoms.
• Note that for all the isomers, each carbon atom
  has four bonds (valency 4),and each hydrogen
  atom has one bond (valency 1).
• Valency is the combining power of an atom.

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Exercise

1. What is an isomer?
2. Why do isomers have different physical
   properties?
3. Draw and name the isomers of hexane

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Answers

1. Compounds with the same molecular formula but
   different structural formula
2. Because they have different structures
3. Normal hexane, 2-methylpentane,
   2,3-dimethylbutane, 2,2-dimethylbutane