Chemical and Physical Properties: Isomerism

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Isomerism

University of Lincoln presentation


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



1. Structural isomerism



Structural Isomerism

C2H6O

These two compounds These two compounds are structural isomersare structural isomers

methoxymethaneethanol



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



Exercise

Q What two compounds, with different functional groups, are isomers with the formula:

C3H6O2



Answer

A carboxylic acid

CH3 OH

O

CH3

O

CH3

O

An ester



2. Stereoisomerism

a. Geometric isomerism



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


CH3CH3

H H

CH3

CH3

H

H



or about a double bond,




then the compounds are called GEOMETRIC ISOMERS

– Cis– on the same side– Trans– on the opposite side




[Pt(NH3)2Cl2]

CisCis––diamminedichloroplatinum(II) diamminedichloroplatinum(II) TransTrans––diamminedichloroplatinum(II)diamminedichloroplatinum(II)




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



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



2. Stereoisomerism

b. Optical isomerism



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



For example

Nonsuperimposable mirror images

Optical isomers

are called

ENANTIOMERENANTIOMERSS



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



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



CHIRAL Carbon



For example – limonene

CH3

CH2

CH3

H

Lemons Oranges

CH3

CH2

CH3

H



For example – limonene

Lemons Oranges

CHIRAL CCHIRAL C



Acknowledgements

• JISC• HEA• Centre for Educational Research and Developmen

t• School of natural and applied sciences• School of Journalism• SirenFM• http://tango.freedesktop.org

http://www.jisc.ac.uk/

http://www.heacademy.ac.uk/

http://www.lincoln.ac.uk/cerd/

http://www.lincoln.ac.uk/cerd/

http://www.lincoln.ac.uk/fabs/

Chemical and Physical Properties: Isomerism

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