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Alkanes
1 Introduction





An Alkane: is a hydrocarbon in which there are only single covalent bonds
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In any alkane, all the carbon-carbon bonds are single covalent bonds, and all the other
bonds are carbon-hydrogen bonds
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1
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


Chemists often use the condensed formula in which the bonds around each carbon atom
in the compound are not written
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 A Homologous Series: A group of compounds that forms if there is a constant increment of
change in molecular structure from one compound in the series to the next
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This change is
summarized below for the initial part of the homologous series
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1
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Every alkane has a name that ends with the suffix -ane
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They are
methane, ethane, propane, and butane, respectively
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The prefixes are Pent- for 5,hex- for 6,hept- for Z and so on
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2to memorize the names of the first ten straight-chain
alkanes
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2 Substituents


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Alkanes and other hydrocarbons do not always have carbon atoms bonded in
straight chains
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In organic chemistry, branches on a hydrocarbon chain are discussed as if they were
substituted for a hydrogen atom on the chain
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 Look at the diagrams below
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 All other carbon atoms or groups of carbon atoms are regarded as substituents
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2
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You can think of an alkyl group as just an alkane with one of the hydrogens removed
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Alkyl groups are named by removing the -ane ending from the parent hydrocarbon
name and adding (-yl)
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1
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Branched Chain Alkane: An alkane with one or more alkyl groups
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Constitutional Isomerism: Compounds that have the same molecular formula but
different structural formulas
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For example, isobutane boils at -12C whereas butane boils at 0 C
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p
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p
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p
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5 C
b
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9
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For example, there are three isomers of pentane, five of hexane and 75 of
decane (C10H22)
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4 Cycloalkanes


They have the general formula CnH2n
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Five- and six-membered rings are most stable
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5 Reactions of Alkanes


Alkanes are unreactive molecules at normal temperature due to presence of only C—C and C—
H single bonds
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1
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1 Combustion


Alkanes, and other hydrocarbons can be oxidized (by burning) in the presence of excess
molecular oxygen
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CnH2n+2 + O2  CO2 + H2O + heat energy

For example, a propane gas burns according to the reaction:
C3H8 (g) + 5O2 (g)  3 CO2 + 4 H2O (l); H = -2220kJ/mol

1
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2 Substitution reactions:


 Ex: halogenation reactions
Alkanes can also react with a halogen (usually chlorine or bromine) in a reaction called
halogenation
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

The products of this reaction are an alkyl halide or haloalkane and a hydrogen halide
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

Alkyl halides are very useful reactants for the synthesis of other organic compounds
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This is
important in the pharmaceutical industry for the synthesis of some drugs
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

Halogenation can occur only in the presence of heat and/or light, the general equation
for the halogenations of an alkane as follows, the R in the general structure for the
alkane may be either a hydrogen atom or an alkyl group

Examples:



If the halogenation reaction is allowed to continue, the alkyl halide formed may react with
other halogen atoms
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For
example:

C H 4 + C l2

light

C H 3C l + H C l

C H 3C l

+

C l2

light

C H 2C l 2

+

C l2

light

C H C l3

+

C l2

light

C H 2C l 2 +
C H C l3
C C l4

+
+

HCl
HCl
HCl



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