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A-level Chemistry: 3.3.7 Optical Isomerism

Chemistry20 CardsCreated 2 months ago

This set of flashcards covers the concept of chirality, focusing on chiral (asymmetric) carbon atoms, optical isomers (enantiomers), and their properties. It also includes examples like 2-hydroxypropanoic acid, highlighting its chiral center and the behavior of plane-polarized light with enantiomers.

What is a chiral (or asymmetric) carbon?

Carbon atom that 4 different groups has attached to it

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Key Terms

Term
Definition

What is a chiral (or asymmetric) carbon?

Carbon atom that 4 different groups has attached to it

What are optical isomers called?

enantiomers

Enantiomers are _______ ____ ____

non-superimposed mirror images

Draw 2-hydroxypropanoic acid and indentify its chiral centre

Draw the 2 enantiomers of 2-hydroxypropanoic acid

Plane-polarised light only vibrates in ___ direction

one

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TermDefinition

What is a chiral (or asymmetric) carbon?

Carbon atom that 4 different groups has attached to it

What are optical isomers called?

enantiomers

Enantiomers are _______ ____ ____

non-superimposed mirror images

Draw 2-hydroxypropanoic acid and indentify its chiral centre

Draw the 2 enantiomers of 2-hydroxypropanoic acid

Plane-polarised light only vibrates in ___ direction

one

Optical isomers are optically active. What is meant by this?

Describe what happens when enantiomers rotate plane-polarised light

  • One enantiomer rotates it in a clockwise direction

  • & other rotates it in an anticlockwise direction

What does a racemic mixture contain?

Racemic mixture contains equal quantities of each enantiomer of an optically active compound

Racemates don’t show any ____ ______

optical activity

Why don’t racemates show any optical activity?

2 enantiomers cancel each other’s light-rotating effect

How do chemists get a racemic mixture of a chiral product? Explain why this works

  • React 2 achiral things together

  • ∵ when 2 molecules react there’s an equal chance of forming each of enantiomers

Explain how reacting butane and chlorine (2 achiral compounds) yields a racemic mixture of a chiral product

  • Cl atom replaces on the H atoms

  • Either H atom can be replaced so reaction produces mixture of 2 possible enantiomers

  • Each H has 50-50 chance of being replaced = 2 optical isomers formed in equal amounts

What is the disadvantage of using chemical methods to modify a reaction to produce a single enantiomer?

It’s difficult and expensive

Doubles bonds (C=O and C=C) are _____

planar (flat)

When is a racemate formed in a reaction mechanism?

When reactant or intermediate has a trigonal planar group in molecule is approached from both sides by an attacking species

e.g. Nucleophilic addition of HCN to aldehydes and ketones (unsymmetrical)

Draw the 1st stage of the mechanism for the reaction of propanal with acidified potassium cyanide (KCN)

When propanal is reacted with acidified potassium cyanide (KCN), why is there an equal amount of each enantiomer formed?

C=O bond is planar = equal chance that nucleophile will attack from above the plane of the molecule or from below it


Why does the product(s) of nucleophilic addition of HCN to symmetrical ketones not display optical isomerism?

Make a product that doesn’t have a chiral centre

Name another mechanism where a racemate can be formed