Optical isomerism

Question 1

asymmetric molecule

Answer 1

Asymmetric = carbon atom is bonded to 4 different groups

An asymmetric carbon atom is chiral and gives rise to optical isomers (enantiomers), which exist as non super-imposable mirror images and differ in their effect on plane polarised light.

Question 2

symmetric molecule

Answer 2

Symmetric = carbon atom bonded to less than 4 different groups or has 2 or more of the same group

Question 3

optical isomerism

Answer 3

Optical isomerism is a form of stereoisomerism and occurs as a result of chirality in molecules, limited to molecules with a single chiral centre.

Optical isomers are two compounds which contain the same number and kinds of atoms, and bonds (i.e., the connectivity between atoms is the same), and different spatial arrangements of the atoms, but which have non-superimposable mirror images. Each non-superimposable mirror image structure is called an enantiomer.

Question 4

racemate

Answer 4

A mixture of equal amounts of enantiomers is called a racemic mixture (racemate)

Question 5

chiral molecules

Answer 5

-asymmetric carbon molecule

-can have a different chiral centre other than carbon but would not be known as a chiral carbon molecule

Question 6

drawing optical isomers

Answer 6

1) draw mirror plane
2) draw central atom
3) draw on atom closest to mirror plane
4) repeat step for rest o atoms
5) check all bonds

Question 7

enantiomers

Answer 7

-non-superimposible mirror images of eachother

Question 8

drawing enantiomers

Answer 8

1) Draw the displayed formula

2) identify the chiral centre

3) Draw 3D depiction of the molecule

4) Draw the mirror image of the molecule

Question 9

polarised light

Answer 9

Light is a wave and from fronton it would look like a straight line

Light waves arranged at the same angle = this is called plane polarised light

Shine plane polarised light through a filter

Unpolarised light through filter = forms plane polarised light

Question 10

rotation of plane polarised light

Answer 10

Solutions of enantimoers rotate plane polarised light where one enantimer rotates light clockwise and the other anticlockwise by the same amount

Solutions of non-chiral molecules don’t rotate plane polarised light

Optically active solutions rotate plane polarised light

Question 11

50:50 mix

Answer 11

50:50 mix of enantimors doesn’t rotate plane polarsed light = optically inactive. This mixture is called racemates

Question 12

what do chiral centres lead to

Answer 12

2 possible mirror images that are mirror images of eachother

Question 13

2 isomers can form/racemates form via

Answer 13

nucelophilic addition reaction

nucelophilles attack carbonyl group

Question 14

chiral atom

Answer 14

A carbon atom that has four different groups attached is called a chiral(asymmetric) carbon atom

These four groups are arranged
tetrahedrally around the carbon.

Question 15

non-superimposible

Answer 15

This causes two
different isomers that
are not superimposable
to be formed. They are
mirror images

Two compounds that are optical isomers of
each other are called enantiomers.

Question 16

nucelophilic addition

Answer 16

Nucleophilic addition of HCN to
aldehydes and ketones (unsymmetrical)
when the trigonal planar carbonyl group
is approached equally from both sides by
the CN- attacking species: results in the
formation of a racemate (equal amounts
of both enantiomers)

Question 17

electrophilic addition

Answer 17

A racemate can also be formed in the reaction of the
electrophilic addition of HBr to an unsymmetrical alkene

Question 18

describe how you would distinguish between separate samples of the 2 entantimoers

Answer 18

-use plane polarised light
-would rotate in opposite directions

Question 19

explain how a structural feature of this enzyme enables it to catalyse hydrolysis of isomer G but not isomer F

Answer 19

enzyme has an active site
enzyme has the correct stereochemistry for isomer G

Question 20

explain why the reaction produces a racemic mixture

Answer 20

-planar carbonyl group
-attack each site with equal probability

Question 21

explain the meaning of a racemic mixture

Answer 21

equal mixture of enantiomers

Question 22

state the relationship between 2 chiral molecules with the same structural formula

Answer 22

non-superimposible mirror images of eachother

Question 23

molecules with planar profiles such as double bonds

Answer 23

make racemic products

e.g propanal and acidified potassium cyanide

attack carbonyl group of unsymmetrical ketones and aldehydes
as C=O is planar the CN- ion can attack either from above or below forming 2 different enantiomers