Optical Isomerism. Types of isomerism. chemrevise.org 20/08/2013. N Goalby Chemrevise.org. Isomerism. Structural isomerism.

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1 ptical Isomerism N Goalby hemrevise.org Types of isomerism Isomerism Structural isomerism Stereoisomerism Geometric isomerism ptical isomerism 1

2 ptical Isomerism ptical isomerism occurs in carbon compounds with 4 different groups of atoms attached to a carbon (called an asymmetric carbon). mirror Mirror images Non superimposable ompounds that show optical isomerism are called chiral compounds. ptical Isomers an be rotated and are superimposable not isomers (same compound) 2

3 hiral arbons These four groups are arranged tetrahedrally around the carbon This causes two different isomers that are not superimposable to be formed. They are mirror images The central asymmetric carbon is called the chiral centre. Light source ptical isomers have similar physical and chemical properties, but they rotate plane polarised light in different directions. Unpolarised light Polarising filter Plane polarised light Sample solution of chiral substance Plane polarised light after rotation Angle of rotation of plane polarised light α Analysing filter ne optical isomer will rotate light clockwise and the other isomer will rotate light in an anti-clockwise direction. 3

4 These stereoisomers are called enantiomers. 3 N 3 N mirror ne optical isomer will rotate plane-polarised light clockwise (+)(called dextrorotatory). The other will rotate it anticlockwise(-)(called laevorotatory). A mixture of equal amounts of the two optical isomers will not rotate plane-polarised light. The mixture is called a racemic mixture or a racemate Method for using polarimeter ief method first put water in sample tube and rotate analyser to block out light replace water with sample. If the sample is chiral light will be seen rotate analyser again to block out light. The amount the analyser is rotated is the amount the light has been rotated. 4

5 hemical Reactions and ptical Isomers Formation of a racemate: A racemate will often be formed in a reaction when a triagonal planar reactant or intermediate is approached from both sides by an attacking species e.g. the reaction of ethanal and N N: 3 :N The carbonyl group is planar and can be attacked from both sides hemical Reactions and ptical Isomers N: 3 :N N 3 There is an equal chance of either enantiomer forming so a racemate forms. No optical activity is seen 3 N 5

6 Addition of to But-1-ene If the alkene is unsymmetrical, addition of hydrogen bromide can lead to isomeric products. 3 δ + δ : : The bromide can attack this planar carbocation from both sides leading to a racemate Nucleophilic substitution mechanism omoethane [Primary haloalkane] + aqueous hydroxide ions δ+ δ This is the mechanism we learnt in module 2. There are, however, two major types of nucleophilic substitution and the products formed by the different mechanisms can show different optical activity. The two mechanisms are called SN1 and SN2 EXTRA 6

7 Nucleophilic substitution mechanism, S N 1 2-bromobutane [Secondary haloalkane] + aqueous hydroxide ions 3 δ+ δ The breaks off leaving a planar carbocation intermediate The - ion can then attack from either side resulting in different enantiomers and a racemate forms or EXTRA Nucleophilic substitution mechanism, S N 2 3 omoethane [Primary haloalkane] + aqueous hydroxide ions δ+ δ In the SN 2 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. The animation on the next slide shows this happening. 3 EXTRA 7

8 : If the reactant was chiral then during the reaction the opposite enantiomer would form Ibuprofen Drug action and optical isomers Drug action may be determined by the stereochemistry of the molecule. Different optical isomers may have very different effects

9 9 Thalidomide ne enantiomer of thalidomide causes birth defects in unborn children whilst the other had useful sedative problems. Unfortunately is was given in a racemic mixture when first used. N N N N Where is hiral carbon? R thalidomide (dangerous drug) S thalidomide (effective drug)