chemical and physical properties: isomerism
DESCRIPTION
Lecture materials for the Introductory Chemistry course for Forensic Scientists, University of Lincoln, UK. See http://forensicchemistry.lincoln.ac.uk/ for more details.TRANSCRIPT
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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 isomerism2. Stereoisomerism
• Geometric (cis/trans)• Optical
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1. Structural isomerism
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Structural Isomerism
C2H6O
These two compounds These two compounds are structural isomersare structural isomers
methoxymethaneethanol
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Structural Isomerism
CH3
N+
O-
O CH3
N+
O-
O
Structural isomers often have Structural isomers often have different chemical & physical different chemical & physical propertiesproperties
CH3
N+
O-
O
o– m– p–
–nitromethylbenzene
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Exercise
Q What two compounds, with different functional groups, are isomers with the formula:
C3H6O2
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Answer
A carboxylic acid
CH3 OH
O
CH3
O
CH3
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
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CH3CH3
H H
CH3
CH3
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(NH3)2Cl2]
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
CH3CH3
H H
CH3
CH3
H
H
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Geometric isomerism helps you see!
This reaction is the central feature of human This reaction is the central feature of human visionvision
11-cis-retinal
11-trans-retinal
11 12
R R’
R’ 11 12
RCis Trans
O
CH3
CH3CH3CH3
CH3
H
O
CH3
CH3CH3CH3 CH3 H
L ig 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
ENANTIOMERENANTIOMERSS
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Enantiomers
Labels L– and D– are given to two enantiomers to distinguish them from
each other
OH
OH O
OHNH2
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
CH3
CH2
CH3
H
Lemons Oranges
CH3
CH2
CH3
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 Developmen
t• School of natural and applied sciences• School of Journalism• SirenFM• http://tango.freedesktop.org