3 isomerism lect 3
TRANSCRIPT
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ISOMERISM
Isomers are molecules which have the samemolecular formula but different structures.
There are a number of different types of isomerism inorganic compounds, we will consider structural
isomerism and stereoisomerism
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Structural Isomerism Structural isomers are molecules which have the
same molecular formula but a differentarrangement of covalent bonds.
The different arrangement of covalent bonds can resultfrom:
1. The functional group being in different positions(positional isomerism)
2. A different arrangement of the carbon skeleton(chain isomerism)
3. A different functional group (functionalisomerism)
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Positional isomerism Positional isomers are molecules with the same
molecular formula but which have the functionalgroup on different positions in the molecule.
Alkanes do not show functional isomerism as they have no
functional group.
Alkenes with four or more carbon atoms show positionalisomerism:
Eg but-1-ene and but-2-ene
C
H
H
H
HC C
HH
C
H
H
C C C
H
H
HHH
H
C
H
H
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Positional isomerism Haloalkanes with three or more carbon atoms show
positional isomerism
Eg 1-chloromethylpropane and2-chloromethylpropane
True for most other functional groups
CH3
CHCH3
ClCH2
CH3
CH3
Cl
C CH3
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Chain isomerism The arrangement of carbon atoms in an organic
molecule is known as the carbon skeleton.
Carbon skeletons containing up to three carbon atomscan only be arranged in one way i.e. a straight chain
with no branching:
Carbon skeletons containing four carbon atoms can bearranged in two ways:
C C C
C C CC C C C
C
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Chain isomerism Carbon skeletons containing five carbon atoms can be
arranged in three ways:
Carbon skeletons containing six carbon atoms can bearranged in five ways:
All molecules containing four or more carbon atoms
can thus show chain isomerism:
C C C CC
C C C
C
C
C C C
C
C
C C C CCC
C C C
C
CC
C CC
C
CC
C C
C
C
CC C C
C
C
CC
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Chain isomerism Eg butane and methylpropane
Eg pent-1-ene and 2-methylbut-1-ene
H
C
H
H
H C
H
H
C
H
H
HC
H
C
H
H
HC
H
C
H
HH
C
H
H
H
CC
H
H
H
H
H
HC C
H
H
C
H
H
C
HH
C
H
H
H
H
HC C
H
C
H
H
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Functional isomerism Functional isomers are molecules with the same
molecular formula but different functionalgroups.
Alkanes which have a ring rather than a straight chainarrangement are known as cycloalkanes. They have thegeneral formula CnH2n, which is the same as alkenes.Cycloalkanes and alkenes can thus show functionalisomerism.
Eg cyclohexane and hex-1-ene
C C
CC
CC
H HH
H
H
H
HH
H
H
H
H
CC
H
H
H
H
H
C C
H
H
C
H
H
C
H
H
H
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Functional group isomerism Eg 1-propanol, 2-ppropanol, and methyl-ethyl ether
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Stereoisomerism Molecules with the same atoms, and functional
groups, with the same connectivity, but differentorientation in space.
Includes geometrical (cis/trans) isomerism, andoptical isomerism.
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Geometrical isomerism
In a double bound molecules are not free to rotate inspace
In a double bond, the second bond is a -bond. This iscaused by the side-on overlap of two p-orbitals:
The result is an overlap in two places. This means that
the bond cannot be twisted and as a result there isrestricted rotation about the -bond. This is whythe cis and trans isomers cannot be interconverted andare therefore different
CH2 CH2 + or
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Geometrical isomerism In a double bound molecules are not free to rotate in
space
Because of this substituents s are held in place on afixed side the double bond. When similar substituents
are on the same side of the double bond, the moleculesis in the cis- conformation, while if the are across fromeach other they are in the trans configuration
CH2 CH2+ or
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Geometrical isomerism
C C
OH
Cl
CH3
Cl
C C
Cl
Cl
CH3
H
Cis Trans
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QuestionsName the following compounds:
a) b)
c) d)
CH3
CH3
H C
H
H
C
H
H
HCC
H
H
C C
H
H
H
H
C
H
H
CBr
H
C C C
H
H
HHH
H
C
H
H
C
H
H
CC
H
H
H
Cl HC
H
H
C
HH
H
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QuestionsName the following compounds:
e) CH3CH(CH3)CH3
f) CH3CHBrCH2Br g) CH2=CHCH2CH3
h) CH3CH=C(CH3)CH3
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QuestionsDraw the structures of the following compounds:
a) methylbutane b) cyclohexane
c) but-1-ene
d) 3-ethyl,2-methylhex-1-ene e) 3-chlorobut-1-
ene
f) 1,1-dichloropropane g) 2,2,4-trimethylheptaneh) pent-2-ene
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Questions Draw the four structural isomers of C4H9Cl and name
them.
Draw the three isomers of C4H8 containing a C=C bond andname them.
Draw the five structural isomers with the formula C6H14
and name them. Then arrange them in order of increasingboiling point, starting with the lowest.
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Questions Given two molecular formulae C4H10 and C4H8;
Select a compound which could be:
an alkane a cycloalkane
an alkene
In each case, draw one possible structure to show howyour choice is correct.