optical isomerism

Question 1

optical isomer (or enantiomer)

Answer 1

optically active isomers which are mirror images of each other that are non superimposable and contain chiral carbon centers

Question 2

two subsets of stereoisomerism

Answer 2

geometric isomerism

optical isomerism

Question 3

Chiral carbon

Answer 3

any carbon with 4 DIFFERENT groups attached to it.

Question 4

What is a chiral molecule and what do you usually get asked about?

Answer 4

A molecule that contains a chiral carbon - usually asked to locate the chiral centre of the molecule so look for the chiral carbon.

Question 5

How many optical isomers can you get

Answer 5

only ever two because they have to be mirror images of each other

Question 6

What does it mean if a compound is optically active

Answer 6

compound that can rotate plane polarised light

Question 7

What does a polarising filter do?

Answer 7

Plane polarises light so that it vibrates in the same plane

Question 8

Talk about what actually be happenin once the light has been plane polarised

Answer 8

Pass the plane polarised light through the optically active isomer
the molecules interact with the light
rotating plane polarised light in a particular direction

Question 9

Think of the little picture diagrams I drew and explain them

Answer 9

1) plane polarised light passing through a mixture of one type of optically active enantiomer, molecules interact with the light and the plane of polarised light is rotated CLOCKWISE
2) plane polarised light passing through a mixture of ANOTHER type of optically active enantiomer, molecules interact with the light and the plane of polarised light is rotated ANTICLOCKWISE

Question 10

plane polarised light

Answer 10

• monochromatic light
• light that is travelling in a particular direction
  single wavelength that is one frequency
• all waves vibrating in the same plane

Question 11

normal light

Answer 11

• white light
• unpolarised light so waves travel in all directions
• can only interact with an enantiomer if it is plane polarised light so first it must be passed through a polarising filter

Question 12

what can optical isomers only act on

Answer 12

plane polarised light

because their molecules can only react with a particular plane

Question 13

racemic mixture (racemate)

Answer 13

contains equal quantities of each enantiomer of an optically active compound

50/50 mix of each optically active ennatiomer

Question 14

Cancelling out plane polarised light

Answer 14

one particular enantiomer will rotate a plane of plane polarised light in one direction (eg. clockwise) and the OTHER enantiomer will rotate plane polarised light in the opposite direction (eg. anticlockwise). - rotates light 180 degrees

racemic mixture - no optical activity - both enantiomers cancel out each others light rotating effects.

Question 15

one pure enantiomer output rotation

Answer 15

output light will have rotated 180 degrees

Question 16

Enantiomer uses

Answer 16

cancer drug cis platin
racemic mixtures used to treat patients but often given a double dose to get 50+ 50 = 100 of the useful enantiomer
difficult and expensive to separate racemic mixtures
thalidomide - 1950s - birth defects in pregnant women’s children after they were given it to help with morning sickness

Question 17

see example image of optical isomer of 1-aminoethanol on paper

Answer 17

X

Question 18

Chiral atom

Answer 18

• atoms with 4 different groups attached to it
• asymmetric
• results in formation of two enantiomers

Question 19

enantiomers

Answer 19

two versions of the optical isomer

Question 20

racemic mixture

Answer 20

• equal number of each enantiomer

- does not affect plane polarised light

Question 21

polarimeter

Answer 21

uses plane polarised light to detect racemic mixtures

Question 22

nomenclature for chiral compounds rotating the plane of polarisation

Answer 22

R-clockwise (+) ; S-anticlockwise (-)

?? ASK ABOUT

Question 23

What happens with polarised light in a racemic mixture?

Answer 23

no rotation of polarised light

Question 24

Sn1

Answer 24

• tertiary (and secondary) halogenoalkane hydrolysis
• nucleophile attacks trigonal planar carbocation intermediate from either side with equal probability
• generates racemic mixture; optically inactive

Question 25

Sn2

Answer 25

• primary (and secondary) halogenoalkane hydrolysis
• this reaction inverts stereochemistry (what doe sthis mean)
• if reactant is optically active, so will the products be (same asymmetric arrangements)

Question 26

What does optical activity do

Answer 26

• affects plane polarised light

- provides evidence for reaction mechanisms (Would be Sn2 if product optically active)

Question 27

Why can there be a chiral centre on a ring?

Answer 27

Because although the carbon is attached to the same ring twice, the order of the atoms in the ring is different depending on which way round you go so they count as two different groups.

Question 28

If a molecule has more than one chiral centre, what will be significant about its optical isomers?

Answer 28

It will have more than two optical isomers.

Question 29

Are all naturally occuring amino acids D or L enantiomers?

Answer 29

L-amino acids.

Except glycine which isn’t chiral.

Question 30

If a molecule CAN be superimposed on its mirror image what is it?

Answer 30

Achiral - and it doesn’t have a chiral carbon.