3.3.7-Optical isomerism (PAPER 2)

Question 1

What is optical isomerism?

Answer 1

A form of stereoisomerism- where isomers have same structural formula but different arrangement of atoms in space. Isomers are mirror images of eachother and have chiral carbon atom.

Question 2

What is a chiral molecule? enantiomers

Answer 2

One with 4 different groups attached to a C atom. These groups can be arranged in 2 different ways forming 2 different molecules- enantiomers.

Question 3

What are enantiomers?

Answer 3

Mirror images of eachother which are non-superimposable- regardless of how you turn them they will not overlap.

Question 4

How to draw a molecule as an enantiomer?

Answer 4

Find the chiral centre- the C atom with 4 different groups surrounding it

Draw this using the tetrahedral 3D shape

Draw the mirror image to show both enantiomers- a reflection on the right with functional groups inverted

Question 5

What are optically active enantiomers?

Answer 5

These rotate plane polarised light- which is a method of detecting optically active compounds.

Question 6

Describe plane polarised light

Answer 6

Standard light oscillates in all directions so we pass the light through a polaroid filter to produce plain polarised light, this light only oscillates in one direction.

Question 7

How to use plane polarised light to determine enantiomers?

Answer 7

Optically active compounds rotate the plane polarised- one enantiomer rotates the light clockwise, the other rotates it anticlockwise by the same degree.

Question 8

What is a racemic mixture?

Answer 8

An equal amount of each enantiomer.

Question 9

Why do racemates not rotate plane polarised light- Why are they optically inactive?

Answer 9

The 2 enantiomers rotate light equally in opposite directions so cancel out.

Question 10

How to make a racemic mixture of a chiral product?

Answer 10

Reacting achiral (not chiral) substances together, making an even chance of forming each enantiomer.

Question 11

Draw the 2 possible enantiomers when butane reacts with bromine

Answer 11

2 different structures formed where different H has been replaced by Br.

     H H3C  C   Br
    C2H5

     Br H3C  C   H
    C2H5

Question 12

Which molecules can make racemic products?

Answer 12

Molecules with planar profiles such as double bonds in C=C and C=O

Question 13

Draw the reaction between propanal (C3H6O) and acidified potassium cyanide KCN- attack on carbonyl group (C=O)

Answer 13

Draw the C3H6O molecule, with aldehyde group

C=O part is planar so CN- ion can attack delta positive C from either above or below forming 2 different enantiomers.

Even chance of nucleophile attacking from top and bottom: 50/50 mixture of both enantiomers so racemic mixture of products.

OH  H  C   CN            
C2H5

  OH NC  C  H 
   C2H5