Isomerism and classification based on stereochemistry

Good

1. ISOMERISM
A guide for A level students
KNOCKHARDY PUBLISHING
2008
SPECIFICATIONS

2. INTRODUCTION
This Powerpointshow is one of several produced to help students
understand selected topics at AS and A2 level Chemistry. It is based on the
requirements of the AQA and OCR specifications but is suitable for other
examination boards.
Individual students may use the material at home for revision purposes or it
may be used for classroom teaching using an interactive white board.
Accompanying notes on this, and the full range of AS and A2 topics, are
available from the KNOCKHARDY SCIENCE WEBSITE at...
www.knockhardy.org.uk/sci.htm
Navigationis achieved by...
eitherclicking on the grey arrows at the foot of each page
or using the left and right arrow keys on the keyboard
ISOMERISM
KNOCKHARDY PUBLISHING

3. CONTENTS
•Prior knowledge
•Types of isomerism
•Structural isomerism
•Stereoisomerism
•Geometrical isomerism
•Optical isomerism
•Check list
ISOMERISM

4. Before you start it would be helpful to…
•know the functional groups found in organic chemistry
•know the arrangement of bonds around carbon atoms
•know what affects the boiling point of organic molecules
ISOMERISM

5. TYPES OF ISOMERISM
Occurs due to the restricted
rotation of C=C double bonds...
two forms… Eand Z(CISand
TRANS)
STRUCTURAL ISOMERISM
STEREOISOMERISM
GEOMETRICAL ISOMERISM
OPTICAL ISOMERISM
CHAIN ISOMERISM
Same molecular formula but
different structural formulae
Occurs when molecules have a
chiral centre. Get two non-
superimposable mirror images.
Same molecular
formula but atoms
occupy different
positions in space.
POSITION ISOMERISM
FUNCTIONAL GROUP
ISOMERISM

6. STRUCTURAL ISOMERISM -INTRODUCTION
COMPOUNDS HAVE THE SAME MOLECULAR FORMULA
BUT DIFFERENT STRUCTURAL FORMULA
Chain different arrangements of the carbon skeleton
similar chemical properties
slightly different physical properties
more branching = lower boiling point

7. STRUCTURAL ISOMERISM -INTRODUCTION
COMPOUNDS HAVE THE SAME MOLECULAR FORMULA
BUT DIFFERENT STRUCTURAL FORMULA
Chain different arrangements of the carbon skeleton
similar chemical properties
slightly different physical properties
more branching = lower boiling point
Positional same carbon skeleton
same functional group
functional group is in a different position
similar chemical properties -slightly different physical properties

8. STRUCTURAL ISOMERISM -INTRODUCTION
COMPOUNDS HAVE THE SAME MOLECULAR FORMULA
BUT DIFFERENT STRUCTURAL FORMULA
Chain different arrangements of the carbon skeleton
similar chemical properties
slightly different physical properties
more branching = lower boiling point
Positional same carbon skeleton
same functional group
functional group is in a different position
similar chemical properties -slightly different physical properties
FunctionalGroupdifferent functional group
different chemical properties
different physical properties
•Sometimes more than one type of isomerism occurs in the same molecule.
•The more carbon atoms there are, the greater the number of possible isomers

9. caused by different arrangements of the carbon skeleton
similar chemical properties
slightly different physical properties
more branching = lower boiling point
There are two structural isomers of C
4H
10. One is a straight chainmolecule where all
the carbon atoms are in a single row. The other is a branched molecule where three
carbon atoms are in a row and one carbon atom sticks out of the main chain.
BUTANE
straight chain
2-METHYLPROPANE
branched
C
4H
10
STRUCTURAL ISOMERISM -CHAIN

10. STRUCTURAL ISOMERISM -CHAIN
DIFFERENCES BETWEEN CHAIN ISOMERS
Chemical Isomers show similar chemical propertiesbecause
the same functional group is present.
Physical Properties such as density and boiling point show trends according
to the of the degree of branching
Boiling Point “straight” chain isomers have higher values than branched ones
the greater the degree of branching the lower the boiling point
branching decreases the effectiveness of intermolecular forces
less energy has to be put in to separate the molecules
-0.5°C
straight chain
-11.7°C
branched
greater branching
= lower boiling point

11. POSITION OF A DOUBLE BOND IN ALKENES
PENT-1-ENE
double bond between
carbons 1 and 2
PENT-2-ENE
double bond between
carbons 2 and 3
12 23
There are no other isomers with five C’s in the longest chain but there are three
other structural isomers with a chain of four carbons plus one in a branch.
Example 1
STRUCTURAL ISOMERISM -POSITIONAL
molecule has the same carbon skeleton
molecule has the same same functional group... BUT
the functional group is in a different position
have similar chemical properties / different physical properties

12. 1-CHLOROBUTANE
halogen on carbon 1
1 2
Moving the chlorine along the chain makes new isomers; the position is measured from
the end nearest the functional group... the third example is 2-NOT 3-chlorobutane.
There are 2 more structural isomers of C
4H
9Cl but they have a longest chain of 3
2-CHLOROBUTANE
halogen on carbon 2
BUT
is NOT
3-CHLOROBUTANE
2
POSITION OF A HALOGEN IN A HALOALKANEExample 2
STRUCTURAL ISOMERISM -POSITIONAL
molecule has the same carbon skeleton
molecule has the same same functional group... BUT
the functional group is in a different position
have similar chemical properties / different physical properties

13. STRUCTURAL ISOMERISM -POSITIONAL
1,3-DICHLOROBENZENE
metadichlorobenzene
1,2-DICHLOROBENZENE
orthodichlorobenzene
1,4-DICHLOROBENZENE
para dichlorobenzene
RELATIVE POSITIONS ON A BENZENE RINGExample 3
molecule has the same carbon skeleton
molecule has the same same functional group... BUT
the functional group is in a different position
have similar chemical properties / different physical properties

14. STRUCTURAL ISOMERISM –FUNCTIONAL GROUP
molecules have same molecular formula
molecules have different functional groups
molecules have different chemical properties
molecules have different physical properties
ALCOHOLS and ETHERS
ALDEHYDES and KETONES
ACIDS and ESTERS
MORE DETAILS FOLLOW

15. ALCOHOLS and ETHERS
Name ETHANOL METHOXYMETHANE
Classification ALCOHOL ETHER
Functional Group R-OH R-O-R
Physical properties polar O-H bond gives rise No hydrogen bonding
to hydrogen bonding. low boiling point
get higher boiling point insoluble in water
and solubility in water
Chemical properties Lewis base Inert
Wide range of reactions
STRUCTURAL ISOMERISM –FUNCTIONAL GROUP

16. ALDEHYDES and KETONES
Name PROPANAL PROPANONE
Classification ALDEHYDE KETONE
FunctionalGroup R-CHO R-CO-R
Physicalproperties polar C=O bond gives polar C=O bond gives
dipole-dipole interaction dipole-dipole interaction
Chemical properties easily oxidised to acids ofundergo oxidation under
same number of carbons extreme conditions only
reduced to 1°alcohols reduced to 2°alcohols
STRUCTURAL ISOMERISM –FUNCTIONAL GROUP

17. CARBOXYLIC ACIDS and ESTERS
Name PROPANOIC ACID METHYL ETHANOATE
Classification CARBOXYLIC ACID ESTER
Functional Group R-COOH R-COOR
Physical properties O-H bond gives rise No hydrogen bonding
to hydrogen bonding. insoluble in water
get higher boiling point
and solubility in water
Chemical properties acidic fairly unreactive
react with alcohols hydrolysed to acids
STRUCTURAL ISOMERISM –FUNCTIONAL GROUP

18. Molecules have the SAMEMOLECULAR FORMULAbut the atoms are
joined to each other in a DIFFERENT SPACIAL ARRANGEMENT -they
occupy a different position in 3-dimensional space.
There are two types...
• GEOMETRICAL ISOMERISM
• OPTICAL ISOMERISM
STEREOISOMERISM

19. GEOMETRICAL ISOMERISM IN ALKENES
Introduction
•an example of stereoisomerism
•found in some, but not all, alkenes
•occurs due to the RESTRICTED ROTATION OF C=C bonds
•get two forms...

20. GEOMETRICAL ISOMERISM IN ALKENES
Introduction
•an example of stereoisomerism
•found in some, but not all, alkenes
•occurs due to the RESTRICTED ROTATION OF C=C bonds
•get two forms...
CIS (Z)
Groups/atoms are on the
SAME SIDEof the double bond
TRANS (E)
Groups/atoms are on OPPOSITE
SIDES across the double bond

21. GEOMETRICAL ISOMERISM
RESTRICTED ROTATION OF C=C BONDS
Single covalent bonds can easily rotate. What appears to be a different structure is
not. It looks like it but, due to the way structures are written out, they are the same.
ALL THESE STRUCTURES ARE THE SAME BECAUSE C -C BONDS HAVE ‘FREE’ ROTATION
Animation doesn’t
work in old
versions of
Powerpoint

22. GEOMETRICAL ISOMERISM
RESTRICTED ROTATION OF C=C BONDS
C=C bonds have restricted rotationso the groups on either end of the bond are
‘frozen’ in one position; it isn’t easy to flip between the two.
This produces two possibilities. The two structures cannot interchange easily
so the atoms in the two molecules occupy different positions in space.
Animation doesn’t
work in old
versions of
Powerpoint

23. GEOMETRICAL ISOMERISM IN ALKENES
E/Z or CIS-TRANS
E / Z Z (zusammen) higher priority groups / atoms on
the SAMEside of C=C bond
E (entgegen) higher priority groups / atoms on
OPPOSITEsides of C=C bond

24. GEOMETRICAL ISOMERISM IN ALKENES
E/Z or CIS-TRANS
E / Z Z (zusammen) higher priority groups / atoms on
the SAMEside of C=C bond
E (entgegen) higher priority groups / atoms on
OPPOSITEsides of C=C bond
To determine priority, the Cahn, Ingold and Prelog convention is used.
eg C
2H
5> CH
3> H and I> Br > Cl> F > C > H

25. GEOMETRICAL ISOMERISM IN ALKENES
E/Z or CIS-TRANS
E / Z Z (zusammen) higher priority groups / atoms on
the SAMEside of C=C bond
E (entgegen) higher priority groups / atoms on
OPPOSITEsides of C=C bond
To determine priority, the Cahn, Ingold and Prelog convention is used.
eg C
2H
5> CH
3> H and I> Br > Cl> F > C > H

26. GEOMETRICAL ISOMERISM IN ALKENES
E/Z or CIS-TRANS
E / Z Z (zusammen) higher priority groups / atoms on
the SAMEside of C=C bond
E (entgegen) higher priority groups / atoms on
OPPOSITEsides of C=C bond
To determine priority, the Cahn, Ingold and Prelog convention is used.
eg C
2H
5> CH
3> H and I> Br > Cl> F > C > H
E Z Z E

27. GEOMETRICAL ISOMERISM IN ALKENES
E/Z or CIS-TRANS
CIS /
TRANS
Should only be used when there are two H’s and two non-
hydrogen groups attached to each carbon.
cis non-hydrogen groups / atoms on the
SAMEside of C=C bond
trans non-hydrogen groups / atoms on
OPPOSITEsides of C=C bond

28. GEOMETRICAL ISOMERISM IN ALKENES
E/Z or CIS-TRANS
CIS /
TRANS
Should only be used when there are two H’s and two non-
hydrogen groups attached to each carbon.
cis non-hydrogen groups / atoms on the
SAMEside of C=C bond
trans non-hydrogen groups / atoms on
OPPOSITEsides of C=C bond

29. GEOMETRICAL ISOMERISM IN ALKENES
E/Z or CIS-TRANS
CIS /
TRANS
Should only be used when there are two H’s and two non-
hydrogen groups attached to each carbon.
cis non-hydrogen groups / atoms on the
SAMEside of C=C bond
trans non-hydrogen groups / atoms on
OPPOSITEsides of C=C bond
cis trans cis trans

30. GEOMETRICAL ISOMERISM
Isomerism in butene
There are 3 structural isomers of C
4H
8that are alkenes*. Of these ONLY
ONE exhibits geometrical isomerism.
but-1-ene 2-methylpropenetransbut-2-ene
(E) but-2-ene
cisbut-2-ene
(Z)but-2-ene
*YOU CAN GET ALKANES WITH FORMULA C
4H
8 IF THE CARBON ATOMS ARE IN A RING

31. GEOMETRICAL ISOMERISM
How to tell if it exists




Two different
atoms/groups
attached
Two different
atoms/groups
attached
Two similar
atoms/groups
attached
Two similar
atoms/groups
attached
Two similar
atoms/groups
attached
Two different
atoms/groups
attached
Two different
atoms/groups
attached
Two different
atoms/groups
attached
GEOMETRICAL ISOMERISM
GEOMETRICAL ISOMERISM
Once you get two similar
atoms/groups attached to
one end of a C=C, you
cannot have geometrical
isomerism

32. OPTICAL ISOMERISM
Occurrence another form of stereoisomerism
occurs when compounds have non-superimposable mirror images
Isomers the two different forms are known as optical isomers or enantiomers
they occur when molecules have a chiral centre
a chiral centre contains an asymmetric carbon atom
an asymmetric carbon has four different atoms (or groups)
arranged tetrahedrally around it.

33. OPTICAL ISOMERISM
Occurrence another form of stereoisomerism
occurs when compounds have non-superimposable mirror images
Isomers the two different forms are known as optical isomers or enantiomers
they occur when molecules have a chiral centre
a chiral centre contains an asymmetric carbon atom
an asymmetric carbon has four different atoms (or groups)
arranged tetrahedrally around it.
There are four different colours
arranged tetrahedrally about
the carbon atom
2-chlorobutane exhibits optical isomerism
because the second carbon atom has four
different atoms/groups attached
CHIRAL CENTRES

34. OPTICAL ISOMERISM
SPOTTING CHIRAL CENTRES
Look at each carbon atom in the chain and see what is attached to it. For a chiral centre
you need an asymmetric carbon with four different atoms/groups)arranged tetrahedrally around it.
IF A CARBON HAS MORE THAN ONE OF ANY ATOM/GROUP ATTACHED, IT CAN’T BE CHIRAL
CH
3CH
2CH
2CH
2Cl
C 3 H’s around it NOT chiral
C 2 H’s around it NOT chiral
C 2 H’s around it NOT chiral
C 2 H’s around it NOT chiral
1-chlorobutane 

35. OPTICAL ISOMERISM
SPOTTING CHIRAL CENTRES
Look at each carbon atom in the chain and see what is attached to it. For a chiral centre
you need an asymmetric carbon with four different atoms/groups)arranged tetrahedrally around it.
IF A CARBON HAS MORE THAN ONE OF ANY ATOM/GROUP ATTACHED, IT CAN’T BE CHIRAL
CH
3CH
2CHClCH
3
CH
3CH
2CH
2CH
2Cl
C 3 H’s around it NOT chiral
C 2 H’s around it NOT chiral
C 2 H’s around it NOT chiral
C 2 H’s around it NOT chiral
C 3 H’s around it NOT chiral
C 2 H’s around it NOT chiral
C H, CH
3, Cl,C
2H
5around itCHIRAL
C 3 H’s around it NOT chiral
1-chlorobutane
2-chlorobutane 


36. OPTICAL ISOMERISM
SPOTTING CHIRAL CENTRES
Look at each carbon atom in the chain and see what is attached to it. For a chiral centre
you need an asymmetric carbon with four different atoms/groups)arranged tetrahedrally around it.
IF A CARBON HAS MORE THAN ONE OF ANY ATOM/GROUP ATTACHED, IT CAN’T BE CHIRAL
CH
3CH
2CHClCH
3
CH
3CH
2CH
2CH
2Cl
C 3 H’s around it NOT chiral
C 2 H’s around it NOT chiral
C 2 H’s around it NOT chiral
C 2 H’s around it NOT chiral
C 3 H’s around it NOT chiral
C 2 H’s around it NOT chiral
C H, CH
3, Cl,C
2H
5around itCHIRAL
C 3 H’s around it NOT chiral
(CH
3)
3CCl C 3 H’s around it NOT chiral
C 3 CH
3’s around it NOT chiral
1-chlorobutane
2-chlorobutane
2-chloro-2-methylpropanane
(CH
3)
2CHCH
2Cl
C 3 H’s around it NOT chiral
C 2 CH
3’s around it NOT chiral
C 2 H’s around it NOT chiral
1-chloro-2-methylpropanane





37. OPTICAL ISOMERISM
Spatial differences between isomers
•two forms exist which are NON-SUPERIMPOSABLE MIRROR IMAGES of each other
•non-superimposable means you you can’t stack one form exactly on top of the other

38. OPTICAL ISOMERISM
Spatial differences between isomers
•two forms exist which are NON-SUPERIMPOSABLE MIRROR IMAGES of each other
•non-superimposable means you you can’t stack one form exactly on top of the other
Some common objects are mirror images and superimposable spoons
superimposable but not mirror images books
non-superimposable mirror images hands

39. OPTICAL ISOMERISM
Spatial differences between isomers
•two forms exist which are NON-SUPERIMPOSABLE MIRROR IMAGES of each other
•non-superimposable means you you can’t stack one form exactly on top of the other
Some common objects are mirror images and superimposable spoons
superimposable but not mirror images books
non-superimposable mirror images hands
NB For optical isomerism in molecules, both conditions must apply...
they must be mirror images AND be non-superimposable

40. OPTICAL ISOMERISM
What is a non-superimposable mirror image?
Animation doesn’t
work in old
versions of
Powerpoint

41. OPTICAL ISOMERS -DIFFERENCE
•isomers differ in their reaction to plane-polarised light
•plane polarised light vibrates in one directiononly
•one isomer rotates light to the right, the other to the left
•rotation of light is measured using a polarimeter
•rotation is measured by observing the polarised light coming out towards the observer

42. OPTICAL ISOMERS -DIFFERENCE
•isomers differ in their reaction to plane-polarised light
•plane polarised light vibrates in one directiononly
•one isomer rotates light to the right, the other to the left
•rotation of light is measured using a polarimeter
•rotation is measured by observing the polarised light coming out towards the observer
•If the light appears to have turned to the right turned to the left
DEXTROROTATORY LAEVOROTATORY
dor +form lor -form

43. OPTICAL ISOMERS -DIFFERENCE
•isomers differ in their reaction to plane-polarised light
•plane polarised light vibrates in one direction only
•one isomer rotates light to the right, the other to the left
•rotation of light is measured using a polarimeter
•rotation is measured by observing the polarised light coming out towards the observer
•If the light appears to have turned to the right turned to the left
DEXTROROTATORY LAEVOROTATORY
d or + form l or -form
Racemate a 50-50 mixtureof the two enantiomers (dl) or (±) is a racemic mixture.
The opposite optical effects of each isomer cancel each otherout
Examples Optical activity is common in biochemistry and pharmaceuticals
• Most amino acids exhibit optical activity
• many drugs must be made of one optical isomer to be effective
-need smaller doses (safer and cost effective)
-get reduced side effects
-improved pharmacological activity

44. OPTICAL ISOMERISM
The polarimeter
If the light appears to have turned to the right turned to the left
DEXTROROTATORY LAEVOROTATORY
ALight source produces light vibrating in all directions
BPolarising filter only allows through light vibrating in one direction
CPlane polarised light passes through sample
DIf substance is optically active it rotates the plane polarised light
EAnalysing filter is turned so that light reaches a maximum
FDirection of rotation is measured coming towards the observer
A B
C D
E
F

45. OPTICAL ISOMERISM
How optical isomers can be formed
Carbonyl compounds undergo nucleophilic addition. If there are two different
groups attached to the C=O bond, the possibility of forming optical isomers arises.
THE NUCLEOPHILIC ADDITION OF HCN TO ETHANAL
If the nucleophilic cyanide ion
attacks from above one
optical isomer is formed

46. OPTICAL ISOMERISM
How optical isomers can be formed
Carbonyl compounds undergo nucleophilic addition. If there are two different
groups attached to the C=O bond, the possibility of forming optical isomers arises.
THE NUCLEOPHILIC ADDITION OF HCN TO ETHANAL
If the nucleophilic cyanide ion
attacks from above one
optical isomer is formed
However, attack from below,
gives the non-superimposable
mirror image of the first

47. OPTICAL ISOMERISM
How optical isomers can be formed
Carbonyl compounds undergo nucleophilic addition. If there are two different
groups attached to the C=O bond, the possibility of forming optical isomers arises.
THE NUCLEOPHILIC ADDITION OF HCN TO ETHANAL
If the nucleophilic cyanide ion
attacks from above one
optical isomer is formed
However, attack from below,
gives the non-superimposable
mirror image of the first
The reactionproduces a mixtureof the two optical
isomers because both modes of attack are possible

48. OPTICAL ISOMERISM
How optical isomers can be formed
Carbonyl compounds undergo nucleophilic addition. If there are two different
groups attached to the C=O bond, the possibility of forming optical isomers arises.
THE NUCLEOPHILIC ADDITION OF HCN TO ETHANAL
ANIMATION
The reactionproduces a mixtureof the two optical
isomers because both modes of attack are possible

49. OPTICAL ISOMERISM
Synthesis of 2-hydroxypropanoic acid (lactic acid)
LACTIC ACID can be formed from ethanal in a two stage process.
1. Nucleophilic additionof hydrogen cyanide to ethanal
2 Hydrolysisof the nitrile group
HCN H
+
/ H
2O

50. OPTICAL ISOMERISM
Synthesis of 2-hydroxypropanoic acid (lactic acid)
LACTIC ACID can be formed from ethanal in a two stage process.
1. Nucleophilic additionof hydrogen cyanide to ethanal
2 Hydrolysisof the nitrile group
HCN H
+
/ H
2O
During the first stage, the nucleophilic CN
-
ion
can attack from below, or above, the aldehyde.
A mixtureof the two enantiomers is formed.

51. OPTICAL ISOMERISM
Synthesis of 2-hydroxypropanoic acid (lactic acid)
LACTIC ACID can be formed from ethanal in a two stage process.
1. Nucleophilic additionof hydrogen cyanide to ethanal
2 Hydrolysisof the nitrile group
HCN H
+
/ H
2O
During the first stage, the nucleophilic CN
-
ion
can attack from below, or above, the aldehyde.
A mixture of the two enantiomers is formed.
Acid hydrolysis of the mixture provides a
mixtureof the two lactic acid forms.

52. OPTICAL ISOMERISM -THALIDOMIDE
The one obvious difference between optical isomers is their response to plane
polarised light. However, some naturally occurring molecules or specifically
synthesised pharmaceuticals show different chemical reactivity.
The drug, THALIDOMIDEis a chiral molecule and can exist as two enantiomers.In the
1960’s it was used to treat anxiety and morning sickness in pregnant women.
Tragically, many gave birth to children with deformities and missing limbs.
It turned out that only one of the enantiomers (the structure on the right) was effective
and safe; its optically active counterpart was not. The major problem was that during
manufacture a mixture of the isomers was produced. The drug was banned world-
wide, but not after tens of thousands of babies had been affected.

53. OPTICAL ISOMERISM –Other points
The following points are useful when discussing reactions producing optical isomers.
The formation of racemic mixtures is more likely in a laboratory reaction
than in a chemical process occurring naturally in the body.
If a compound can exist in more than one form, only one of the optical
isomers is usually effective.
The separation of isomers will make manufacture more expensive.
A drug made up of both isomers will require a larger dose and may cause
problems if the other isomer is ‘poisonous’ like thalidomide.

54. REVISION CHECK
What should you be able to do?
Recall the definitions of structural isomerism and stereoisomerism
Explain and understand how structural, geometrical and optical isomerism arise
Work out all the possible isomers for a given formula
Recalland understandthe importance of optical activity in natural product chemistry
CAN YOU DO ALL OF THESE? YES NO

55. You need to go over the
relevant topic(s) again
Click on the button to
return to the menu

56. WELL DONE!
Try some past paper questions

57. © 2004 JONATHAN HOPTON & KNOCKHARDY PUBLISHING
ISOMERISM
The End