Chemical and Physical Properties: Isomerism

Lecture materials for the Introductory Chemistry course for Forensic Scientists, University of Lincoln, UK. See http://forensicchemistry.lincoln.ac.uk/ for more details.

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Isomerism
University of Lincoln
presentation

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Isomerism
ISOMERS are compounds with the same
molecular formula
(but different arrangements of atoms)

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Types of Isomerism
1.Structural isomerism
2.Stereoisomerism
•Geometric (cis/trans)
•Optical

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1. Structural isomerism

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Structural Isomerism
C
2
H
6
O
These two compounds are These two compounds are
structural isomersstructural isomers
methoxymethane
ethanol

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Structural Isomerism
CH
3
N
+
O
-
O CH
3
N
+
O
-
O
Structural isomers often have different Structural isomers often have different
chemical & physical propertieschemical & physical properties
CH
3
N
+
O
-
O
o–
m– p–
–nitromethylbenzene

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Exercise
QWhat two compounds, with different
functional groups, are isomers with the
formula:
C
3
H
6
O
2

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Answer
A carboxylic acid
CH
3
OH
O
CH
3
O
CH
3
O
An ester

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2. Stereoisomerism
a. Geometric isomerism

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Geometric Isomerism (cis/trans)
If two compounds have the same formulae
and the same structural framework,
but differ in the spatial arrangement of
different atoms or groups about a central
atom

11. CH
3
CH
3
H H
CH
3
CH
3
H
H
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Geometric Isomerism (cis/trans)
or about a double bond,

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Geometric Isomerism (cis/trans)
then the compounds are called GEOMETRIC
ISOMERS
–Cis– on the same side
–Trans– on the opposite side

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Geometric Isomerism (cis/trans)
[Pt(NH
3
)
2
Cl
2
]
CisCis––diamminedichloroplatinum(II) diamminedichloroplatinum(II) TransTrans––diamminedichloroplatinum(II)diamminedichloroplatinum(II)

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Geometric Isomerism (cis/trans)
There is restricted rotation
about a C=C double bond
and the two isomers are not
inter-convertible without
first breaking the C=C bond
Cis-but-2-ene
Trans-but-2-ene
CH
3
CH
3
H H
CH
3
CH
3
H
H

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Geometric isomerism helps you see!
This reaction is the central feature of human visionThis reaction is the central feature of human vision
11-cis-retinal
11-trans-retinal
11 12
R R’
R’
11 12
R
Cis Trans
O
CH
3
CH
3
CH
3
CH
3
CH
3
H
O
CH
3
CH
3
CH
3
CH
3
CH
3
H
L i g h t

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2. Stereoisomerism
b. Optical isomerism

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Optical Isomerism
Optical isomerism is caused by a lack of
symmetry in a molecule’s structure so that
the molecule and its mirror image are
actually different
–like a left hand and a right hand:
They are non-superimposable

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For example
Nonsuperimposable
mirror images
Optical isomers
are called
ENANTIOMERENANTIOMER
SS

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Enantiomers
Labels L– and D– are given to
two enantiomers to
distinguish them from each
other
OH
OH
O
OH
NH
2
H
Only one optical
isomer of DOPA
called L-DOPA, is
effective in the
treatment of
Parkinson’s disease

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Asymmetric Carbon
A compound that is NOT SUPERIMPOSABLE
on its mirror image is described as being
CHIRAL
Many chiral compounds contain an
asymmetric carbon atom – a carbon atom
with FOUR different groups attached

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CHIRAL Carbon

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For example – limonene
CH
3
CH
2
CH
3
H
Lemons Oranges
CH
3
CH
2
CH
3
H

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For example – limonene
Lemons Oranges
CHIRAL CCHIRAL C

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Acknowledgements
•JISC
•HEA
•Centre for Educational Research and Development
•School of natural and applied sciences
•School of Journalism
•SirenFM
•http://tango.freedesktop.org