Isomerism Summary

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NOCKHARDY
STEREOISOMERISM
K NOTES ISOMERISM GEOMETRICAL ISOMERISM CH 3 CH3
AT A GL ANC E
A LEVEL CHEMISTRY · a form of stereoisomerism C C
· found in alkenes H H
CIS
· occurs due to the restricted rotation of C=C double bonds
ISOMERISM
· doesn't occur with single bonds because they can rotate CH 3 H
· the two forms are known as CIS and TRANS
STRUCTURAL ISOMERS STEREOISOMERS C C
· doesn't occur when two similar groups / atoms are on
Same molecular formula - Same structural formula, different H CH3
different structural formula
the same end of the double bond
arrangement of groups in space. TRANS
GEOMETRICAL ISOMERS OPTICAL ISOMERS
Groups fixed in different space as Groups fixed in different space as a OPTICAL ISOMERISM
a result of the restricted rotation result of the asymmetry of the structure.
of double bonds etc. Non-superimposable mirror images of each other. · a form of stereoisomerism
· the different forms are known as optical isomers or enantiomers
· occurs when compounds have an asymmetric carbon atom
STRUCTURAL ISOMERISM can occur due to variations in . . . · occurs when compounds have 4 different groups attached to the same carbon
· the carbon skeleton butane and 2-methylpropane CH3
CH 3 C2H5 C OH
CH3 CH 2 CH 2 CH 3 CH3 CH CH 3 Br
· positions of a functional 1-chloropropane and 2-chloropropane · TWO isomers which are non-superimposable mirror images of each other
group on a chain CH3CH2CH2Cl CH3CHClCH3
· D D
but-1-ene and but-2-ene
C C
H CH2 CH3 CH3 CH 3 A A
B B
C C C C
H H H H
· isomers differ in their reaction to plane-polarised light
· functional group acids and esters · one isomer rotates light to the right - DEXTROROTATORY
alcohols and ethers · the other rotates light to the left - LAEVOROTATORY
H H H H · rotation of light is measured using a polarimeter
H H C C
· rotation is measured by observing the polarised light coming towards the observer
C C OH O H
· an 50-50 mixture of the two enantiomers is a racemic mixture
H H H H
© J. L. HOPTON 1999

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