Isomerism 2814 1 ISOMERISM - A general survey STRUCTURAL ISOMERS have the same molecular formula but different structural formulae They occur due to variations in . . . CH 3 the carbon skeleton CHAIN ISOMERISM CH3 CH 2 CH2 CH3 CH3 CH CH3 positions of a functional CH3CH2CH2Cl CH3CHClCH3 group on a chain POSITION ISOMERISM CH3 CH3 CH2 CH3 H C C C C H H H H Cl Cl Cl relative positions Cl on a benzene ring Cl Cl functional group H H H H FUNCTIONAL GROUP H C C OH H C O C H ISOMERISM H H H H Differences between isomers Boiling Point • “straight” chain isomers have higher boiling points than branched chain isomers • the greater the degree of branching the lower the boiling point • branching decreases the effectiveness of intermolecular attractive forces • less energy has to be put in to separate the molecules • boiling points can also vary between isomers containing different functional groups e.g alcohols and ethers - due to dipole-dipole interaction and/or hydrogen bonding. Chemical properties Most isomers show similar chemical properties if the same functional group is present. However, it is best to have a look at each structure and apply any knowledge of the chemical reactions of the compounds in question. Q.1 Draw all the structural isomers of C4H8. How many are i) alkanes and ii) alkenes ? Q.2 Draw all the structural isomers of formula C4H10O. How many are alcohols ? Q.3 Predict the isomer of pentane with the lowest boiling point. 2 2814 Isomerism GEOMETRICAL ISOMERISM Occurrence • a form of stereoisomerism. • found in alkenes, it occurs due to the restricted rotation of C=C double bonds • the two forms are known as CIS and TRANS Q.4 Make a model of but-2-ene and compare it with the examples shown below. What do you have to do to convert your structure to the other form? CH 3 CH 3 CIS groups on the same side of the double bond C C H H CH 3 H TRANS groups are across the double bond C C H CH3 Both molecules have the double bond in the same position but the atoms occupy different positions within space. Quick check • are two similar atoms, or groups of atoms attached to the same end of the C=C ? • if so you will not get geometrical isomers CH 3 Br CH 3 H CH 3 H C C C C C C H H H Br Br H A B C A and B are geometrical isomers, C isn’t. It is a structural isomer of the other two. Q.5 Work out all the possible structural isomers of pentene C5H10 and hexene C6H12 . How many exhibit geometrical isomerism? Properties Geometrical isomers have different physical properties (e.g. boiling point) and sometimes react differently in certain chemical reactions. Q.6 Using molecular models attach four different coloured balls to a central black one. Without looking at the model, repeat the procedure. Is it an exact copy of the first? Can it be stacked colour for colour on top of the first? If not, is there a relationship between the two? Swap any two colours. Is there any change? Isomerism 2814 3 OPTICAL ISOMERISM D D Occurrence • another form of stereoisomerism • occurs when compounds have ... C C non-superimposable mirror images B A A B Existence • the two different forms are known as optical isomers or enantiomers and occur when molecules have a chiral centre. • to find such a centre, look for an asymmetric carbon atom ... one with four different atoms, or groups of atoms arranged tetrahedrally around it. CH3 example C2H5 C OH Br • two forms exist which are non-superimposable mirror images of each other; i.e. you can’t stack one form exactly on top of the other. Difference • isomers differ in their reaction to 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. Polarising filter produces plane polarised light Filter is rotated until the light appears brightest Light source Sample under examination 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 mixture of the two enantiomers (dl) or (±) is known as a racemic mixture • the opposite optical effects of each isomer cancel each other out 4 2814 Isomerism Examples Optical activity is widespread, especially in nature, biochemistry and pharmaceuticals. e.g. COOH COOH H C CH3 H C CH3 NH2 OH 2-aminopropanoic acid 2-hydroxypropanoic acid (Alanine) (Lactic acid) O The drug thalidomide is optically active but only one of the optical isomers is effective. Many years ago N women gave birth to babies with abnormalities caused by taking thalidomide tablets which contained O O N O some of the ‘wrong’ enantiomer. Q.7 How many structural isomers of C6H14 are optically active? How many structural isomers of butanol, C4H9OH, are optically active? Q.8 Which of the following can exist as enantiomers? a) 2-bromopropane b) 2-bromobutane c) 2-bromopentane d) 3-bromopentane e) CH3CH(OH)C2H5 f) CH3CH(OH)CH3 Q.9 Why is there the possibility of enantiomers being formed when butanone undergoes nucleophilic addition with HCN? Do all carbonyl compounds produce a mixture of products with HCN? If not, why not? SUMMARY ISOMERISM STRUCTURAL ISOMERS STEREOISOMERS Same molecular formula - Same structural formula, different different structural formula arrangement of groups in space. GEOMETRICAL ISOMERS OPTICAL ISOMERS Groups fixed in different space as Groups fixed in different space as a a result of the restricted rotation result of the asymmetry of the structure. of double bonds etc. Non-superimposable mirror images of each other.