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N Goalby Chemrevise.Org 1 Isomerism 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.
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