Edexcel A level Chemistry Topic 17:
Organic Chemistry II 2024 Exam
Questions and Answers
What is optical isomerism? - Answer>>a form of
stereoisomerism
-optical isomers: non-superimposable mirror images of each
other.
What prerequisites are required for optical isomerism to arise? -
Answer>>1. a chiral centre (carbon atom, sometimes nitrogen,
with 4 different groups/atoms attached to it)
2. mirror images are non superimposable
3. no internal plane of symmetry (due to chirality)
What are enantiomers? - Answer>>isomers that are mirror
images of each other
- another word for optical isomers
What properties of enantiomers are similar or different? -
Answer>>- same chemical properties
- similar physical properties
- completely different biological properties
e.g. thalidomide, limonene
What is plane polarised light? - Answer>>light that passes
through a sheet of Polaroid in a single direction
How do optical isomers affect plane polarised light? -
Answer>>- one optical isomer rotates plane polarised light
anticlockwise whereas the other isomer rotates plane polarised
light in a clockwise direction
, How to draw optical isomers - Answer>>- chiral centre should
lead to tetrahedral geometry
- so bond angle of 104.5
Define a racemic mixture - Answer>>- 50:50 mix of each
enantiomer of a chiral compound
- doesn't rotate plane polarised light, the two enantiomers cancel
each other's light rotating effect
Why can you react two achiral things together and get a racemic
mixture of a chiral product? - Answer>>- because when two
molecules react, there's often an equal chance of forming each of
the enantiomers
- e.g. with butane and chlorine,equal chance of each of the H
atoms being replaced with the Cl
How does a polarimeter work? - Answer>>- light oscillating all
directions
- have a polarised sheet in a fixed position (in a polarimeter tube)
- light passing this sheet is now plane polarised light
- second polaroid sheet which rotates
- place sample between first and second polarising sheet
- angle of rotation made by optical isomer is detected by the
detector.
SN1 mechanism - Answer>>- 2 steps
- first step is reversible, second step isn't
- start with a single enantiomer reactant
- STEP 1: a group breaks off, leaving a planar ion
- STEP 2: planar ion can be attacked from either side
- means the products will be a racemic mixture of two optical
isomers of each other.
- so doesn't rotate plane polarised light.
Organic Chemistry II 2024 Exam
Questions and Answers
What is optical isomerism? - Answer>>a form of
stereoisomerism
-optical isomers: non-superimposable mirror images of each
other.
What prerequisites are required for optical isomerism to arise? -
Answer>>1. a chiral centre (carbon atom, sometimes nitrogen,
with 4 different groups/atoms attached to it)
2. mirror images are non superimposable
3. no internal plane of symmetry (due to chirality)
What are enantiomers? - Answer>>isomers that are mirror
images of each other
- another word for optical isomers
What properties of enantiomers are similar or different? -
Answer>>- same chemical properties
- similar physical properties
- completely different biological properties
e.g. thalidomide, limonene
What is plane polarised light? - Answer>>light that passes
through a sheet of Polaroid in a single direction
How do optical isomers affect plane polarised light? -
Answer>>- one optical isomer rotates plane polarised light
anticlockwise whereas the other isomer rotates plane polarised
light in a clockwise direction
, How to draw optical isomers - Answer>>- chiral centre should
lead to tetrahedral geometry
- so bond angle of 104.5
Define a racemic mixture - Answer>>- 50:50 mix of each
enantiomer of a chiral compound
- doesn't rotate plane polarised light, the two enantiomers cancel
each other's light rotating effect
Why can you react two achiral things together and get a racemic
mixture of a chiral product? - Answer>>- because when two
molecules react, there's often an equal chance of forming each of
the enantiomers
- e.g. with butane and chlorine,equal chance of each of the H
atoms being replaced with the Cl
How does a polarimeter work? - Answer>>- light oscillating all
directions
- have a polarised sheet in a fixed position (in a polarimeter tube)
- light passing this sheet is now plane polarised light
- second polaroid sheet which rotates
- place sample between first and second polarising sheet
- angle of rotation made by optical isomer is detected by the
detector.
SN1 mechanism - Answer>>- 2 steps
- first step is reversible, second step isn't
- start with a single enantiomer reactant
- STEP 1: a group breaks off, leaving a planar ion
- STEP 2: planar ion can be attacked from either side
- means the products will be a racemic mixture of two optical
isomers of each other.
- so doesn't rotate plane polarised light.
