NJC Isomerism Tutorial Answer

2Isomerism TutorialProperties of isomers1Two organic isomers are known to contain the same functional group. Assess the following statements about the two molecules.(a)They have the same empirical formula. true / may be true / false(b)They have the same boiling point.true / may be true / falseEnantiomers have same boiling point, however structural isomers & cis-trans isomers have different boiling point. (c)They have the same chemical properties.true / may be true / falseIsomers with same functional group same the same chemical properties. (d)They have the same solubility in water.true / may be true / falseE.g. cis-trans isomers with polar groups may have different polarity (e.g. trans 1,2-chloroethene is non-polar while cis 1,2-dichloroehthene is polar) and could have different solubility in polar solvent.Constitutional Isomerism (Structural Isomerism)2How many isomers of dichlorobenzene are there? (benzene: )A2B 3C 4D5 Cl Cl Cl Cl Cl ClCl Cl Cl Cl1,2-dichlorobenzene 1,4-dichlorobenzene 1,3-dichlorobenzene The 3 isomers are:

3A halogenoalkane has the molecular formula C3H5Cl3. Which are the possible names of the isomers of this compound? 11,1,1-trichloropropane21,2,2-trichloropropane32,2,3-trichloropropaneA 1, 2 and 3 are correctB1 and 2 only are correctC2 and 3 only are correctD1 only is correct 1,1,1-trichloropropaneClClCl ClClCl ClCl ClClCl Cl1,1,2-trichloropropane1,2,2-trichloropropane1,2,3-trichloropropaneCis-Trans Isomerism4How many structural and cis-trans isomers are there in dichloropropene,C3H4Cl2?A3B 5C 6D7 1,1-dichloropropeneClClClClClClcis-1,2-dichloropropenetrans-1,3-dichloropropene ClCltrans-1,2-dichloropropeneClcis-1,3-dichloropropeneCl Cl2,3-dichloropropeneCl3,3-dichloropropeneCl Cl

5–Farnesene is a constituent of the natural wax found on apples. It is also responsible for the characteristic odour of green apples.–FarneseneHow many cis-trans isomers does this molecule have?A2B 4C 8D16 each C of C=C (C3, C4 and C5, C6) has 2 different groups attached no of cis-trans isomers = 22 = 46Low fat sunflower spreads which are high in polyunsaturated contain esters of linoleic acid.CH3(CH2)4CH=CHCH2CH=CH(CH2)7CO2HOn the lid of a brand of spread, it is claimed that the spread contains virtually no trans fatty acid. Which isomer does not contain a trans linkage that could be present in the spread?ACCCH3(CH2)4HCCH2C(CH2)7CO2HHHH CCCH3(CH2)4HCCH2C(CH2)7CO2HHHtranstransHBCCHCCH2C(CH2)7CO2HHHHCH3(CH2)4 CCHCCH2C(CH2)7CO2HHHHCH3(CH2)4transcisCCCCH3(CH2)4HCC(CH2)7CO2HHCH2HH CCCH3(CH2)4HCC(CH2)7CO2HHCH2HHciscisDCCHCC(CH2)7CO2HHCH2CH3(CH2)4HH transcisCCHCC(CH2)7CO2HHCH2CH3(CH2)4HH 12 56891011

Optical Isomerism (Enantiomerism)7The drug cortisone has the formula shown. O CH3O CH3COHOCH2OH * OCH3OCH3COHOCH2OH*****cortisoneHow many chiral centers are present in each cortisone molecule?A3B 4C 5D68What is the maximum number of stereoisomers for the molecule shown below? CH3CH3 CH3 CH(OH)CH3 O H3C O CH3 * * * * ** CH3CH3 CH3 CH(OH)CH3 O H3C O CH3 ** * (*: chiral C)A 28B29C 210D. 211Note : C=C in a ring cannot exhibit cis-trans isomerism[Ans Q2 – 8: BBDBCDB][N22/1/18]9The drug ibuprofen has two enantiomers. One enantiomer has the desired pharmacological activity while the other is inactive. Which statement gives correct reasons for this difference?1The biological receptors that the drugs bond with are chiral2The enantiomers are structural isomers with different chemical properties.3The enantiomers have many different physical properties.A1,2 and 3B1 and 2 onlyC1 onlyD2 and 3 only1. As the receptors are chiral and specific in shape, they can only bind to the drug which can fit the receptors. Hence, only one of the two enantiomers of ibuprofen that fits the receptors is able to bind successfully to it.2. Enantiomers are stereoisomers, not structural isomers.3. Enantiomers have similar physical properties and chemical properties (except for interactions with another chiral molecule)

10(a) What is meant by the term ‘optical isomerism’? Optical isomers (enantiomers) are molecules with the same structural formula which are not superimposable mirror images of each other. These molecules lack both a point and plane symmetries.(b) What conditions must be fulfilled for a compound to exhibit optical activity?- has at least 1 chiral C (C with 4 different atoms / groups of atoms attached)- is not a racemic mixture (mixture containing equal amount of the non-superimposable mirror images (enantiomers) or molecule does not have an internal mirror plane / a plane of symmetry (c) Draw the structural formula of the alkane with the lowest Mr that can exhibit optical isomerism. CH2CH2CH3C H CH2CH3CH3 3-methylhexane ** or CHCH3C H CH2CH3CH3 2,3-dimethylpentane CH3 ** Application Questions11A hydrocarbon Q consists of 86% carbon by mass.(a)Calculate the empirical formula of Q. C HMass in 100g /g8614Amount /mol 7.1714simplest ratio11.95  2The empirical formula is CH2(b)5.0 g of Q was found to occupy 2.0 dm3 at 0 oC and 1 atm. Deduce the molecular formula of Q.Mr = PVmRT = = 56.0Let MF of Q be (CH2)n 14n = 56.0n= 4Molecular Formula of Q is C4H8.(c)Q exists as a pair of cis-trans isomers. Draw the structural formulae for the isomers.Q is but-2-ene (each C of C=C bond has 2 different groups attached)(d)P is a structural isomer of Q and does not show cis-trans isomerism. Suggest a structure for P.P: alkene or cycloalkane

12The effect of plane-polarised light on tartaric acid, HO2CCH(OH)CH(OH)CO2H, was investigated by Louis Pasteur.CO2HCO2HOHHOHHtartaric acidPasteur identified three different types of tartaric acid molecule.- molecule A rotated plane-polarised light to the right.- molecule B rotated plane-polarised light to the left.- molecule C has no effect on plane-polarised light.(a)Label each chiral centre in tartaric acid with an asterisk (*). C CO2H C CO2H OH OHH H * * (b)Suggest an explanation for the observations that Pasteur made.MoleculeExplanationACOOHCCHOHHOHCOOH HOOCCCHOHHOHHOOCA & BBoth chiral centres in Molecule A rotate plane of polarised light in the same direction. The rotation effect does not cancel out. Molecule B is the enantiomer of Molecule A. It rotates plane of polarized light in the opposite direction as Molecule A. (Note: It is impossible to tell from the structure the direction in which plane-polarised light is rotated. We can only do this by direct measurement using a polarimeter.)BCThere is a plane of symmetry in Molecule C. The 2 chiral centers in Molecule C have opposite chirality. The rotation of plane-polarised light by each half cancels each other out; Molecule C does not rotate plane polarised light and is known as a meso compound.HOOCCCH HOHOHHOOC

13Glyceraldehyde as shown below is chiral and exists as two enantiomers, D-glyceraldehyde, and L-glyceraldehyde. HOOOHglyceraldehyde(a)Identify the chiral centre using *. HO OOH glyceraldehydeH* (b)Given that sample I contains only the D-isomer which rotates the plane of plane-polarised light in the clockwise direction. Complete the table below using the given information. samplenumberrelative amount of D-isomer in samplerelative amount of L-isomer in sampleabout the sampleIs sample optically active?direction of rotation of plane-polarised lightI1000pure D-isomeryesclockwiseII7525excess D-isomeryesclockwiseIII5050racemic mixtureno-IV2575excess L isomeryesanticlockwiseV0100pure L isomeryesanticlockwise(c)The samples of glyceraldehyde were analysed for optical activity. Only one sample was found to be optically inactive. Identify the optically inactive sample and account for its optical activity.Samples III is optically inactive as it is a racemic mixture (50% of each isomer). The equal but opposite rotation of plane-polarized light by each enantiomer cancels out completely. The racemic mixture is optically inactive.

14Compound X can exhibit stereoisomerism. CH3 C C CH C CH3 H H Cl CH2 CompoundX (A) (B) (a)What is the maximum number of stereoisomers that X can have? Explain your answer