Isomerism
It is possible for organic molecules with the same molecular formula to have different structures
Definition- Structural isomers: same molecular formula different structures (or structural formulae)
There are three types of structural isomerism •Chain isomerism •Position isomerism •Functional group isomerism
Chain isomerism: Compounds with the same molecular formula but different structures of the carbon skeleton
These isomers arise because of the carbon chains can be branched. For example, there are two isomers
of butane, C4H10. In one of them, the carbon atoms lie in a "straight chain" whereas in the other the chain is branched H H H H H H H H C C C H H C C C C H H H H H H H H C H
butane H methyl propane
There are three isomers of pentane C5H12 H
H H H H H H H H H H C H H H H C C C C H H C C C C C H H C C C H H H H H H H H H H C H H H H C H H pentane H 2-methylbutane 2,2-dimethylpropane
False isomers
Do not draw "false" isomers which are just twisted versions of the original molecule. Twisting the molecule into a different shape does not make a different isomer. Isomers are only formed if a bond would have to be broken and reassembled into the different structure
H H H H H H H H H C H H H C C C C H H C C C H
H H H C C H H H H H H C H H H H C H
These are all exactly the same compound. H
N Goalby chemrevise.org 1 Number of Possible Chain Isomers for Selected Alkanes
Molecular Formula Number of possible isomers
C4H10 2
C5H12 3
C6H14 5
C7H16 9
C8H18 18
C9H20 35
C10H22 75
C15H32 4,347
C20H42 336,319
C30H62 4,111,846,763
Position isomers: Compounds with the same molecular formula but different structures due to different positions of the same functional group on the same carbon skeleton
H H H H H H
H C C C H 1-bromopropane H C C C H 2-bromopropane
Br H H H Br H
Functional group isomers: Compounds with the same molecular formula but with atoms arranges to give different functional groups
H H H H
H C O C H Methoxymethane: an ether H C C O H ethanol: an alcohol H H H H H H H H C H C C H
Cyclohexane- cyclo alkane CH3CH2CH2CH2CH=CH2 hexene- alkene H C C H C H H H H Aldehydes and ketones of the same chain length would be classed as functional group isomers- e.g. Propanal and propanone (both C3H6O)
H H O H O H
H C C C H C C C H
H H H H H
N Goalby chemrevise.org 2 Structural Isomer Questions
1) a) Define what a structural isomer is. b) Define what a molecular formula is.
2) Draw all the structural isomers of hexane and name them. What type of structural isomers are these isomers?
3) a) Draw the displayed formulae and name all the straight-chain isomers with the molecular formula C4H8Br2 b) Name the type of structural isomerism that these isomers show and explain why
4) a) Name and draw the displayed and skeletal formulae of a functional group isomer of pent -1-ene b) Name and draw the displayed and skeletal formulae of a positional isomer of pent -1-ene c) Name and draw the displayed and skeletal formulae of a chain isomer of pent -1-ene
5) The following is the structure of methoxyethane H H H H C C O C H H H H a) Draw the displayed formulae and name a functional group isomer of methoxyethane b) Draw the displayed formulae and name a positional isomer of the compound you have drawn in part 5a
6) Draw the skeletal formulas of the structural isomers of the different alcohols with the molecular formula
C4H10O. Name the isomers.
7) Deduce the number of structural isomers for Hexane C6H14
8) Name the compound on the right and draw its skeletal formula. Deduce how many other position isomers of the compound on the right can be formed.
9) Compound Y is shown below. It is a member of a homologous series of hydrocarbons. H H H H C C C C H H H H (a) Write the general formula of the homologous series that contains Y. (b) Name a process used to obtain a sample of Y from a mixture containing other members of the same homologous series. (c) Name and draw the skeletal formula of a position isomer of Y. (d) Name and draw the skeletal formula of a functional group isomer of Y.
N Goalby chemrevise.org 3 Stereoisomerism There are two types of stereoisomerism: Definition: Stereoisomers have the same structural formulae geometrical (E- Z isomerism) and optical but have a different spatial arrangement of atoms isomerism
Isomerism
Structural isomerism Stereoisomerism
E-Z stereoisomerism Optical isomerism
E-Z stereoisomerism Alkenes can exhibit a type of isomerism called E-Z stereoisomerism. This occurs because of restricted rotation that occurs around a double bond
Single carbon-carbon covalent bonds can easily rotate
Cl Cl Cl H Cl H H C C H H C C Cl H C C H H H H H H Cl
All three of these structures are the same
C=C double bonds have restricted rotation, due to position of the pi bond, so the groups on either end of the bond are fixed in one position. It is not easy to flip between the two.
Cl H Cl Cl C C C C H Cl H H C-C C-C pi sigma This produces two possibilities. The two structures cannot bond bond interchange easily so the atoms in the two molecules occupy different positions in space.
E-Z stereoisomers arise when: (a) There is restricted rotation around the C=C double bond. (b) There are two different groups/atoms attached both ends of the restricted double bond
E-Z stereoisomers: H H H H H H Same structure NOT E- two different groups H H Z stereoisomers : two C C C C attached either end of C C C C identical groups attached H H H H the restricted double H H H H to one end of the bond restricted double bond.
N Goalby chemrevise.org 4 H H two different groups H H H C C H attached either end of the restricted double H C C H But-1-ene C C bond- leads to EZ C C H H H H isomers H H H H two identical groups attached to Z- but-2-ene H one end of the restricted double H These are two bond – no E-Z isomers C H isomers as the lack H of rotation around But-1-ene is a structural isomer of But-2- C C the double bonds H ene but does not show E-Z isomerism means one cannot H C H be switched to the H other E -but-2-ene
Naming E-Z stereoisomers Cahn–Ingold–Prelog (CIP) priority rules. On both sides of the double bond determine the priority group
1. Compare the atomic number (Ar) of the atoms directly attached to each side of the double bond; the priority group having the atom of higher atomic number priority Cl Br receives higher priority. C C
H Cl
Priority Priority Cl H group Cl Cl group side 1 side 2 C C C C Z-1,2-dichloroethene H H H Cl E-1,2-dichloroethene If the priority atom is on the same side of the If the priority atom is on the opposite side of double bond it is labelled Z from the german the double bond it is labelled E from the zusammen (The Zame Zide!) german entgegen (The Epposite side!)
2. If there is are groups of atoms instead of single atoms attached to the carbon of the double bond then consider the sum of the H C atomic numbers of the entire group. The 3 CH3 priority bigger group is the priority C C priority H CH2 H3C The CH3CH2 group is bigger than the CH3 so it is the priority group
N Goalby chemrevise.org 5 cis-trans isomerism
In some text books you will find this type of E-Z isomerism referred to as geometric isomerism and cis- trans isomerism. cis-trans isomerism can be considered as a special case of EIZ isomerism in which two of the substituent groups are the same. It has been superseded by E/Z isomerism because the Cahn–Ingold–Prelog (CIP) priority rules allow all alkenes to be considered and not just the ones where the groups are the same .
H H H H H C C H C H H C C C C H H H H H H C H H Z- but-2-ene E- but-2-ene Can also be called Can also be called Cis- but-2-ene trans- but-2-ene
Cis means “on this side of” in latin trans means “across” in latin
The effect of EZ stereoisomerism on physical properties
E-Z stereoisomers can have differing melting and boiling points. δ- δ- Cl δ- Cl Cl H
δ+ C C δ+ δ+ C C δ+
H H H Cl δ- Z-1,2-dichloroethene Boiling point =60oC E-1,2-dichloroethene o This molecule is polar. The polar C-Cl bonds are on Boiling point =48 C the same side of the molecule. One side of the This molecule is non- polar. The polar C-Cl molecule is slightly negative. bonds are on opposite sides of the molecule. The intermolecular forces are both London forces The dipoles cancel out. and permanent dipole-dipole attractions. The intermolecular forces are is only London forces so lower boiling point.
N Goalby chemrevise.org 6 E-Z Isomerism Questions 1. State the meaning of the term stereoisomers
2. Identify the feature of the double bond in but-2-ene that causes it to form two EZ stereoisomers
3. Draw stuctural formulae of the E-Z isomers of the following compounds a) 3-methylpent-2-ene. b) but-2-ene c) 3-methylpent-2-ene d) pent-3-en-2-ol e) hex-3-ene f) pent-2-ene
4. Name the following including the correct E-Z letter at the start of the name
H3C CH3 C C H CH2 H3C
5. Compound A has the molecular formula C5H10O. It has a branched carbon chain and exists as a pair of E-Z Stereoisomers. Draw the structures and name the two isomers. H H
6 Using the carbon skeleton like it is drawn on the right, H C C Draw the 3 stereoisomers of 1,4-dichlorobuta-1,3-diene. C C H H H
7. Draw the two E-Z stereoisomers of hex-4-en-2-one.
8. Explain why the following weak unsaturated acid CH3(CH)4COOH shows EZ isomerism. Draw the structures of each of the E-Z isomers of this acid.
9. How many E-Z stereoisomers are there of the molecule shown below, including the molecule shown?
O
OH
N Goalby chemrevise.org 7 Optical Isomerism
Optical isomerism occurs in carbon compounds with 4 different groups of atoms attached to a carbon (called an asymmetric carbon).
H H H H A carbon atom that has four different groups attached is H C C C C H called a chiral (asymmetric) carbon atom H H O H
H
This causes two different isomers that are not superimposable to be formed. They are mirror images
H H H H
H H H H H H H H H H H
mirror H Mirror images Non superimposable
H H
If the compound does not have 4 different groups then when it is rotated it will be H H H H superimposable
H H not isomers (same compound)
Butan-2-ol has four different groups around a carbon and so forms optical isomers
OH OH
C C CH H3C H5C2 3 C2H5 H H
Two compounds that are optical isomers of each other are called enantiomers.
N Goalby chemrevise.org 8 Optical isomers have similar physical and chemical properties, but they rotate plane polarised light in different directions.
One enantiomer rotates it in one direction and the other enantiomer rotates it by the same amount in the opposite direction.
One optical isomer will rotate light clockwise (+)(called dextrorotatory). The other will rotate it anticlockwise(-)(called laevorotatory). -ve enantiomer +ve enantiomer Anticlockwise clockwise rotation rotation
Polarimeter The rotation of light by a chiral compound can be observed by using a polarimeter. The light source will produce unpolarised light which is light travelling in all planes. If this light is passed through a polarising filter then plane polarised light will be formed, which is light that only travels in one plane. When this passes through the sample of the chiral substance the plane polarised light will rotated in to a different plane.
Polarising Light filter source Angle of rotation of plane polarised light α
Unpolarised light Plane polarised light Sample solution of chiral Plane substance polarised light Analysing after rotation filter
Method for using a polarimeter •first put water in sample tube and rotate analysing filter to block out light •replace water with sample. If the sample is chiral light will be seen •rotate analysing filter again to block out light. The amount the analyser is rotated is the amount the light has been rotated.
N Goalby chemrevise.org 9 R/S Nomenclature Different systems of nomenclature are is existence for optical isomers. D/L or +/- are commonly used, but both have been superseded by the more informative R/S system. H 4 1) Using the Cahn–Ingold–Prelog (CIP) priority rules, used in EZ isomerism, assign a priority number for each of the four groups. The ones with the higher atomic number A have C r 2 3 the higher priority. HOOC CH3 OH 1 (highest priority O biggest Ar ) OH1 2) Redraw the molecule with the atom with the lowest H priority pointing into the plane of the page (in this case H) 2 C 3 HOOC CH3
3) Imagine moving from substituent 1 to 2 to 3. If moving OH1 in a clockwise direction then assign the compound the R H letter. If moving anticlockwise the assign the compound the 3 This is an S S letter 2 C enantiomer HOOC CH3
N Goalby chemrevise.org 10 Racemic mixtures
A mixture containing a 50/50 mixture of the two isomers (enantiomers) is described as being a racemate or racemic mixture.
A racemic mixture (a mixture of equal amounts of the two optical isomers) will not rotate plane-polarised light.
Racemate no rotation
Chemical Reactions and Optical Isomers
Formation of a racemate CH3 A racemate will be formed in a reaction H mechanism when a trigonal planar reactant or C -NC: intermediate is approached from both sides by :CN- an attacking species O
H H H3C CN NC CH3 There is an equal chance of C C either enantiomer forming so a racemate forms. No OH OH optical activity is seen.
Mechanism for the reaction (drawn the same for both enantiomers) Nucleophilic addition of HCN to aldehydes δ- - H+ and ketones (unsymmetrical) when the O O : O H trigonal planar carbonyl is approached from + both sides by the HCN attacking species: Cδ results in the formation of a racemate H3C C C2H5 H3C C C2H5 H3C C2H5 :CN- CN CN
A racemate can also be formed in the reaction of the electrophilic addition of HBr to an unsymmetrical alkene
- H H :Br The bromide can H + attack this planar + - C CH2 C δ δ carbocation from C CH2 H Br both sides leading to Br CH2 H3C CH3 H3C Br a racemate CH3 CH3 CH3 H2C CH CH2 CH3 :Br - Major product s 90% If the alkene is H H unsymmetrical, addition of + hydrogen bromide can C C CH2 CH3 CH2 CH2 CH2 CH3 Minor lead to isomeric products. H H product Br 10%
N Goalby chemrevise.org 11 Nucleophilic substitution reactions and optical activity
In the topic of reactions of halogenoalkanes we look at the nucleophilic substitution reactions of halogenoalkanes with aqueous hydroxide ions to form alcohols. The reaction can occur via two different mechanisms called SN1 and SN2. Primary halogenoalkanes tend to undergo the SN2 mechanism and tertiary halogenoalkanes undergo the SN1 mechanism. Secondary halogenoalkanes can undergo either mechanism. The two different mechanisms have a different effect on optical activity of chiral reactants. By observing the differing effects it can give us evidence for which mechanism is occurring.
Nucleophilic substitution mechanism, SN1 H H The OH- ion can then + - attack from either side C resulting in different H C C Br + - 3 :OH enantiomers and a racemate forms CH2CH3 CH3 C2H5 The Br breaks off leaving a planar carbocation intermediate H H
C C H3C C2H5 H5C2 CH3 OH or HO
Nucleophilic substitution mechanism, SN2
H - H CH3 + - H H C C Br 3 HO C Br HO C CH3 - HO: CH2CH3 CH2CH3 CH2CH3
In the SN2 mechanism no intermediates are formed and the reaction occurs via a transition state If the reactant was chiral then during the reaction the opposite enantiomer would form through inversion as shown below. H H HO C Br H HO C Br CH H CH C 3 H3C 3 2 HO C Br
H3CH2C H CH3 CH2CH3 H H3C C Br
- HO C HO: CH3 H3CH2C CH2CH3
N Goalby chemrevise.org 12 Drug action and optical isomers Drug action may be determined by the stereochemistry of the molecule. Different optical isomers may have very different effects
Ibuprofen CH3
H3C HC CH CH CH3
CH2 C C HC CH CH C O HO
Thalidomide H H O O H H O C H H C C H H C C N C C C C C C H N C C O C N H C C C C H C C C H C C C C H O H H C N H O H O H H O Chiral carbon H S thalidomide (effective R thalidomide (dangerous drug) drug)
One enantiomer of thalidomide causes birth defects in unborn children whilst the other had useful sedative problems. It was used as a drug for treat pregnant women with morning sickness. At the time the side effect was not know. Unfortunately it was given in a racemic mixture when first used and many babies were born with birth defects
N Goalby chemrevise.org 13