Optical Isomerism Flashcards
What are optical isomers
Mirror Images of each other 🪞
Optical isomerism is a type of what?
Stereoisomerism
What are stereoisomers
Stereoisomers have the same structural formula but have their atoms arranged differently in space
What’s a chiral molecule?
Asymmetric carbon atom that has four different groups attached to it
What are enantiomers/ optical isomers
• Can be arranged in two different ways around the carbon atom so that two different molecules are made
• Chiral molecules
• Mirrored images of each other
• Can’t be superimposed
What does superimposed mean?
• They are the same and you can’t tell them apart
• If molecules can be superimposed, they’re achiral and there’s no optical isomerism
What does achiral mean?
Symmetrical
Asymmetrical
Chiral molecule
How can optical isomers be drawn?
• By locating the chiral centre: carbon atom with four different groups attached
• Draw one enantiomer in a tetrahedral shape, then draw a mirror image beside it
What do optical isomers rotate?
Optical isomers rotate plane-polarised light
Why do optical isomers rotate plane-polarised light?
Because they are optically active
What is the difference in the direction of vibration of normal light and plane-polarised light?
• Normal light- vibrates in all directions
• Plane-polarised light- only vibrates in one direction
How do enantiomers differentiate in their rotation
• One enantiomer rotates in a clockwise direction
• The other enantiomer roasters in an anti-clockwise direction
What is a racemate
• A mixture of enantiomers
• Contains equal quantities of each enantiomer of an optically active compound
Why don’t racemates show any optical activity and how are they formed?
• Racemates don’t show any optical activity- the two enantiomers cancel each other’s light rotating effect
• Racemates are made when chemists react two achiral things together to get a race mix mixture of a chiral product
• There’s an equal chance of forming each of the enantiomers
How is an enantiomer formed with butane and chlorine, give the formula
Check physical flashcards for answers
What other reactions produce Racemates?
Reactions involving planar bonds
What is a planar bond and give examples and where they can be found
• Planar bond is a double bond, flat
• C=O, C=C
• Found in carbonyl groups: aldehydes and unsymmetrical ketones
Describe the reaction between propanal and acidified potassium cyanide
• Involves CN- ion attacking s+ carbon of propanal’s planar C=O group
• CN- ion can attack from two directions, above the plane of the molecule or from below it
• One of the two enantiomers is formed depending on the direction of the nucleophilic attack
• Because C=O bond is planar, there is an equal chance the nucleophile will attack from either directions, an equal amount of enantiomer will be formed
• Leads to a race mixture product
What happens when you react a symmetrical ketone with cyanide
• Makes a product that doesn’t have a chiral centre so it won’t display optical isomerism
What are the issues with creating a single enantiomer
It is is difficult and expensive
Draw the reaction of propanal with acidified potassium cyanide
Check physical flash cards for answers
Explain the meaning of the term stereoisomerism
The property of having stereoisomers which are molecules with the same molecular formula and with their atoms arranged in the same way but with a different orientation of the bonds in space
State how you can distinguish between two enantiomers
Shine (monochromatic) plane-polarised light through a solution of the molecule. The enantiomers will rotate the light in opposite directions
Explain why the enantiomers of the reaction of PENTANAL and acidified potassium cyanide is optically inactive
• Pentanal has a planar C=O bond
• Depending on which direction the CN- ion attacks the s+ carbons from, two different enantiomers can be formed
• The CN- ion is equally likely to attack from either direction
• So equal amounts of each enantiomer are formed, giving an optically inactive racemic mixture
Why are optical isomers called ‘optical’
Because of how they interact with polarised light
