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ISOMERISM A guide for A level students KNOCKHARDY PUBLISHING.
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Transcript of ISOMERISM A guide for A level students KNOCKHARDY PUBLISHING.
ISOMERISMISOMERISMA guide for A level studentsA guide for A level students
KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING
INTRODUCTION
This Powerpoint show 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 if an interactive white board is available.
Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...
www.argonet.co.uk/users/hoptonj/sci.htm
Navigation is achieved by...
either clicking on the grey arrows at the foot of each page
or using the left and right arrow keys on the keyboard
ISOMERISMISOMERISMKNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING
CONTENTS
• Prior knowledge
• Types of isomerism
• Structural isomerism
• Stereoisomerism
• Geometrical isomerism
• Optical isomerism
• Check list
ISOMERISMISOMERISM
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
ISOMERISMISOMERISM
TYPES OF ISOMERISMTYPES OF ISOMERISM
Occurs due to the restricted rotation of C=C double bonds... two forms - CIS and 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
STRUCTURAL ISOMERISM - STRUCTURAL ISOMERISM - INTRODUCTIONINTRODUCTION
COMPOUNDS HAVE THE SAME MOLECULAR FORMULABUT DIFFERENT STRUCTURAL FORMULA
Chain different arrangements of the carbon skeletonsimilar chemical propertiesslightly different physical propertiesmore branching = lower boiling point
STRUCTURAL ISOMERISM - STRUCTURAL ISOMERISM - INTRODUCTIONINTRODUCTION
COMPOUNDS HAVE THE SAME MOLECULAR FORMULABUT DIFFERENT STRUCTURAL FORMULA
Chain different arrangements of the carbon skeletonsimilar chemical propertiesslightly different physical propertiesmore branching = lower boiling point
Positional same carbon skeletonsame functional groupfunctional group is in a different positionsimilar chemical properties - slightly different physical properties
STRUCTURAL ISOMERISM - STRUCTURAL ISOMERISM - INTRODUCTIONINTRODUCTION
COMPOUNDS HAVE THE SAME MOLECULAR FORMULABUT DIFFERENT STRUCTURAL FORMULA
Chain different arrangements of the carbon skeletonsimilar chemical propertiesslightly different physical propertiesmore branching = lower boiling point
Positional same carbon skeletonsame functional groupfunctional group is in a different positionsimilar chemical properties - slightly different physical properties
Functional Group different functional groupdifferent chemical propertiesdifferent 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
caused by different arrangements of the carbon skeletonsimilar chemical propertiesslightly different physical propertiesmore branching = lower boiling point
There are two structural isomers of C4H10. One is a straight chain molecule 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.
BUTANEstraight chain
2-METHYLPROPANEbranched
C4H10
STRUCTURAL ISOMERISM - STRUCTURAL ISOMERISM - CHAINCHAIN
STRUCTURAL ISOMERISM - STRUCTURAL ISOMERISM - CHAINCHAIN
DIFFERENCES BETWEEN CHAIN ISOMERS
Chemical Isomers show similar chemical properties becausethe 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 onesthe greater the degree of branching the lower the boiling pointbranching decreases the effectiveness of intermolecular forcesless energy has to be put in to separate the molecules
- 0.5°Cstraight chain
- 11.7°Cbranched
greater branching = lower boiling point
POSITION OF A DOUBLE BOND IN ALKENES
PENT-1-ENEdouble bond between
carbons 1 and 2
PENT-2-ENEdouble bond between
carbons 2 and 3
1 2 2 3
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 - STRUCTURAL ISOMERISM - POSITIONALPOSITIONAL
molecule has the same carbon skeletonmolecule has the same same functional group... BUTthe functional group is in a different positionhave similar chemical properties / different physical properties
1-CHLOROBUTANEhalogen 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 C4H9Cl but they have a longest chain of 3
2-CHLOROBUTANEhalogen on carbon 2
BUT
is NOT3-CHLOROBUTANE
2
POSITION OF A HALOGEN IN A HALOALKANEExample 2
STRUCTURAL ISOMERISM - STRUCTURAL ISOMERISM - POSITIONALPOSITIONAL
molecule has the same carbon skeletonmolecule has the same same functional group... BUTthe functional group is in a different positionhave similar chemical properties / different physical properties
STRUCTURAL ISOMERISM - STRUCTURAL ISOMERISM - POSITIONALPOSITIONAL
1,3-DICHLOROBENZENEmeta dichlorobenzene
1,2-DICHLOROBENZENEortho dichlorobenzene
1,4-DICHLOROBENZENEpara dichlorobenzene
RELATIVE POSITIONS ON A BENZENE RINGExample 3
molecule has the same carbon skeletonmolecule has the same same functional group... BUTthe functional group is in a different positionhave similar chemical properties / different physical properties
STRUCTURAL ISOMERISM – STRUCTURAL ISOMERISM – FUNCTIONAL GROUPFUNCTIONAL GROUP
molecules have same molecular formulamolecules have different functional groupsmolecules have different chemical propertiesmolecules have different physical properties
ALCOHOLS and ETHERS
ALDEHYDES and KETONES
ACIDS and ESTERS
MORE DETAILS FOLLOW
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 bondingto hydrogen bonding. low boiling pointget higher boiling point insoluble in waterand solubility in water
Chemical properties Lewis base InertWide range of reactions
STRUCTURAL ISOMERISM – STRUCTURAL ISOMERISM – FUNCTIONAL GROUPFUNCTIONAL GROUP
ALDEHYDES and KETONES
Name PROPANAL PROPANONE
Classification ALDEHYDE KETONE
Functional Group R-CHO R-CO-R
Physical properties polar C=O bond gives polar C=O bond gives dipole-dipole interaction dipole-dipole interaction
Chemical properties easily oxidised to acids of undergo oxidation undersame number of carbons extreme conditions only
reduced to 1° alcohols reduced to 1° alcohols
STRUCTURAL ISOMERISM – STRUCTURAL ISOMERISM – FUNCTIONAL GROUPFUNCTIONAL GROUP
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 bondingto hydrogen bonding. insoluble in waterget higher boiling pointand solubility in water
Chemical properties acidic fairly unreactivereacts with alcohols hydrolysed to acids
STRUCTURAL ISOMERISM – STRUCTURAL ISOMERISM – FUNCTIONAL GROUPFUNCTIONAL GROUP
STEREOISOMERISMSTEREOISOMERISM
Molecules have the SAME MOLECULAR FORMULA but 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
GEOMETRICAL ISOMERISM IN ALKENESGEOMETRICAL ISOMERISM IN ALKENES
Introduction
• an example of stereoisomersim• found in some, but not all, alkenes• occurs due to the RESTRICTED ROTATION OF C=C bonds• get two forms....
CISGroups/atoms are on the
SAME SIDE of the double bond
TRANSGroups/atoms are on OPPOSITE SIDES across the double bond
Isomers - have different physical properties - e.g. boiling points, density- have similar chemical properties - in most cases
GEOMETRICAL ISOMERISMGEOMETRICAL ISOMERISM
RESTRICTED ROTATION OF C=C BONDS
Single covalent bonds can easily rotate. What appears to be a different structure in an alkane is not. 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
GEOMETRICAL ISOMERISMGEOMETRICAL ISOMERISM
RESTRICTED ROTATION OF C=C BONDS
C=C bonds have restricted rotation so 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
GEOMETRICAL ISOMERISMGEOMETRICAL 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
GEOMETRICAL ISOMERISMGEOMETRICAL ISOMERISM
Isomerism in butene
There are 3 structural isomers of C4H8 that are alkenes*. Of these ONLY ONE exhibits geometrical isomerism.
BUT-1-ENE 2-METHYLPROPENEtrans BUT-2-ENEcis BUT-2-ENE
* YOU CAN GET ALKANES WITH FORMULA C4H8 IF THE CARBON ATOMS ARE IN A RING
OPTICAL ISOMERISMOPTICAL 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
OPTICAL ISOMERISMOPTICAL ISOMERISM
CONTENTSCONTENTS
SPOTTING CHIRAL CENTRES
Look at each carbon atom in the chain and see what is attached to it. For a chiral centreyou 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
CH3CH2CHClCH3
CH3CH2CH2CH2Cl C 3 H’s around it NOT chiralC 2 H’s around it NOT chiralC 2 H’s around it NOT chiralC 2 H’s around it NOT chiral
C 3 H’s around it NOT chiralC 2 H’s around it NOT chiralC H, CH3, Cl,C2H5 around it CHIRALC 3 H’s around it NOT chiral
(CH3)3CCl C 3 H’s around it NOT chiralC 3 CH3’s around it NOT chiral
1-chlorobutane
2-chlorobutane
2-chloro-2-methylpropanane
(CH3)2CHCH2Cl C 3 H’s around it NOT chiralC 2 CH3’s around it NOT chiralC 2 H’s around it NOT chiral1-chloro-2-methylpropanane
OPTICAL ISOMERISMOPTICAL 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
OPTICAL ISOMERISMOPTICAL ISOMERISM
What is a non-superimposable mirror image?
Animation doesn’t work in old versions of Powerpoint
OPTICAL ISOMERS - OPTICAL ISOMERS - DIFFERENCEDIFFERENCE
• 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 LAEVOROTATORYd or + form l or - form
OPTICAL ISOMERS - OPTICAL ISOMERS - DIFFERENCEDIFFERENCE
• 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 LAEVOROTATORYd or + form l or - form
Racemate a 50-50 mixture of the two enantiomers (dl) or (±) is a racemic mixture. The opposite optical effects of each isomer cancel each other out
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
OPTICAL ISOMERISMOPTICAL ISOMERISM
The polarimeter
If the light appears to have turned to the right turned to the left DEXTROROTATORY LAEVOROTATORY
A Light source produces light vibrating in all directions
B Polarising filter only allows through light vibrating in one direction
C Plane polarised light passes through sample
D If substance is optically active it rotates the plane polarised light
E Analysing filter is turned so that light reaches a maximum
F Direction of rotation is measured coming towards the observer
A B
C DE
F
OPTICAL ISOMERISMOPTICAL 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
OPTICAL ISOMERISMOPTICAL 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
OPTICAL ISOMERISMOPTICAL 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 reaction produces a mixture of the two optical isomers because both modes of attack are possible
OPTICAL ISOMERISMOPTICAL ISOMERISM
Synthesis of 2-hydroxypropanoic acid (lactic acid)
LACTIC ACID can be formed from ethanal in a two stage process.
1. Nucleophilic addition of hydrogen cyanide to ethanal
2 Hydrolysis of the nitrile group
HCN H+ / H2O
OPTICAL ISOMERISMOPTICAL ISOMERISM
Synthesis of 2-hydroxypropanoic acid (lactic acid)
LACTIC ACID can be formed from ethanal in a two stage process.
1. Nucleophilic addition of hydrogen cyanide to ethanal
2 Hydrolysis of the nitrile group
HCN H+ / H2O
During the first stage, the nucleophilic CN- ion can attack from below, or above, the aldehyde.
A mixture of the two enantiomers is formed.
OPTICAL ISOMERISMOPTICAL ISOMERISM
Synthesis of 2-hydroxypropanoic acid (lactic acid)
LACTIC ACID can be formed from ethanal in a two stage process.
1. Nucleophilic addition of hydrogen cyanide to ethanal
2 Hydrolysis of the nitrile group
HCN H+ / H2O
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 mixture of the two lactic acid forms.
OPTICAL ISOMERISM - OPTICAL ISOMERISM - THALIDOMIDETHALIDOMIDE
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, THALIDOMIDE is 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.
REVISION CHECKREVISION 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
Recall and understand the importance of optical activity in natural product chemistry
CAN YOU DO ALL OF THESE? CAN YOU DO ALL OF THESE? YES YES NONO
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WELL DONE!WELL DONE!Try some past paper questionsTry some past paper questions
© 2004 JONATHAN HOPTON & KNOCKHARDY PUBLISHING
ISOMERISMISOMERISM
The EndThe End