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Organic
Chemistry
Lesson 2
Isomers
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List two characteristics of a
homologous series.
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Lesson 2: Isomers
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Objectives:
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Describe the term structural isomer
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Draw and name the non-cyclic alkanes
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Draw and name the straight-chain alkenes
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Draw and name the straight-chain alkynes
Human isomers
8 members of the class stand up.
Each person is a carbon atom.
You are only allowed to link hands or one leg with one, two
or three people. Assume any open hands or legs are
bonded to an imaginary hydrogen atom.
How many different arrangements can you make?
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What changed?
What stayed the same?
Main
Isomers
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Compounds with the same
molecular formula but
different structural formula
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The 20 different C4H8O
compounds from last
lesson are isomers of each
other
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These are all structural
isomers
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Same number of each atom,
but bonded in a different
order
You would have even more
if you included geometric
and optical isomers
Structural Isomers of the Alkanes
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The (non-cyclic) alkanes have the general formula
CnH2n+2
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Draw full and condensed structural formulas for
every isomer of every one of the alkanes up to
n=6
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If you finish early, draw each as a skeletal formula
Did you get them all?
And skeletally
Naming straight-chain alkanes
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Suffix:
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Tells us the functional group
of the molecule
For alkanes it is ‘-ane’
Prefix:
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Tells us the length of the
longest carbon chain:
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1 carbon: meth2 carbons: eth3 carbons: prop4 carbons: but5 carbons: pent6 carbons: hex7 carbons: hept8 carbons: oct9 carbons: non10 carbons: dec-
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Example : ethane, CH3CH3
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Example : butane
CH3CH2CH2CH3
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Task: write in the names of
the 4 straight chain
alkanes next to your
diagrams from last slide
Naming branched-chain alkanes
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Start with the number and name of the longest chain of carbon atoms
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Look for the branches
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Branch Size:
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1 carbon: methyl-
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2 carbons: ethyl-
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3 carbons: propyl-
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4 carbons: butyl-
Find the number of the carbon atom to which the branch is attached:
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Number from whichever end gives the lowest number
Example: 2-methylpropane
Example:
2,3-dimethylbutane
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Complete by adding the number and name of the branch(es) to the longest
carbon chain
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When there is more than one branch of the same kind use:
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One branch – nothing
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Two branches – di-
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Three branches – tri-
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Four branches – tetra-
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When there are different kinds of branches, list them in alphabetical order
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Use commas between numbers, e.g. 2,2 or 2,3
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Use hyphens to separate numbers from letters, e.g. 2,2-dimethylpropane
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No gaps between names, e.g. 2-methylpropane
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Task: name the remaining
alkanes
The straight-chain alkenes and alkynes
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Alkenes are the same as alkanes but have one C=C double bond.
Alkynes have one C≡C triple bond.
The suffix for the alkene homologous series is ‘-ene’.
The suffix for the alkyne homologous series is ‘-yne’.
Task: draw full structural and skeletal formulas for each of the straight-chain alkenes up to C6 and
name them.
Do the branched ones as well
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Naming: First identify the longest chain of carbon atoms that contains the carbon-carbon double
bond.
You need to state the position of the double bond, but only if there is the possibility of multiple
isomers. Number the carbon chain starting from the end closest to the double bond. The position
of the double bond is indicated by placing the lower of the pair of numbers assigned to the
double-bonded carbon atoms in front of the name of the alkene.
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i.e. ‘but-2-ene’ or ‘hex-1-ene’
There is only one ‘ethene’ so no need to write ‘eth-1-ene’
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For branched chains, add the location and name of any substituent atoms or groups
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Alkynes are named in the same manner.
Did you get them?
Key Points
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Structural isomers have the same number of each
atom but they are connected differently
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When naming compounds
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The number of the longest carbon chain forms the Stem
The functional group tells you the Suffix
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Sometimes numbers need to be used to tell you where this
functional group is
The Prefix includes the side chains and other groups
(substituents) which are named according to what they
are, how many there are and their position
PREFIX
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STEM
SUFFIX