4.1 Stereoisomerism

Question 1

what are stereoisomers?

Answer 1

stereoisomers are compounds with the same structural formula but different spatial arrangement of atoms

Question 2

how is stereoisomerism different to structural isomerism?

Answer 2

structural isomers have the same molecular formula but different structural formulas, stereoisomers have the same molecular and structural formulas but different spatial arrangement of atoms

Question 3

two main types of stereoisomerism

Answer 3

E-Z isomerism and Optical isomerism

Question 4

whydoes E-Z isomerism occur?

Answer 4

E-Z isomerism occurs due to the restricted rotation around a carbon double bond, if both carbons have different groups attached different isomers are produced, depending on the arrangement of groups around the double bond

Question 5

Name and define the two isomers E-Z isomerism produces

Answer 5

E-isomer - the high priority groups on each side of the double bond are diagonally across from each other
Z-isomer - the high priority groups on each side of the double bond are either both above or both below the carbon double bond.

Question 6

how can you determine the priority of the groups in E-Z isomerism?

Answer 6

of atoms directly bonded to the carbon atoms, on each carbon the atom with the higher atomic number is given the higher priority

Question 7

define optical isomerism

Answer 7

a type of stereoisomerism, optical isomers have the same structure but are non-superimposable mirror images of each other.

Question 8

what is a chiral centre?

Answer 8

a chiral centre is a carbon atom which has four different groups attached to it

Question 9

why os a chiral centre req for optical isomerism?

Answer 9

a chiral centre has four different groups attached to it and these groups can be arranged in different ways to give two different molecules known as optical isomers

Question 10

define enantiomer

Answer 10

two different molecules which are non-superimposable mirror images of each other and arise from optical isomerism

Question 11

explain the optical activity of optical isomers

Answer 11

optical isomers are optically active meaning they rotate in plane polarised light

Question 12

how does the optical activity differ for a pair of enantiomers?

Answer 12

the two enantiomers will rotate plane polarised light in opposite directions

Question 13

what is a racemic mixture?

Answer 13

a racemic mixture contains equal quantities of each enantiomer of an optically active compound

Question 14

what is another term for a racemic mixture?

Answer 14

a racemate

Question 15

what effect does a racemic mixture have on plane polarised light?

Answer 15

a racemic mixture has no effect on plane polarised light as equal quantities of each enantiomer are present, each enantiomer rotates in opposite directions in plane polarised light cancelling each other out

Question 16

why is optical isomerism a problem for the drug industry?

Answer 16

often only one enantiomer is effective due to an enzyme’s active site being 3D, a reaction can be modified to produce a single enantiomer but is difficult and expensive

Question 17

why do reactions involving planar bonds often produce racemates?

Answer 17

planar bonds can be attacked by a nucleophile from either above or below, if the carbon has different groups attached to it, the two enantiomers will be produced depending on direction of attack by nucleophile on the planar bond. A racemic mixture will be produced as there’s an equal chance of forming each of the enantiomers