3.3.7 - Optical Isomerism Flashcards

1
Q

What is a chiral (or asymmetric) carbon?

A

Carbon atom that 4 different groups has attached to it

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2
Q

What are optical isomers called?

A

enantiomers

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3
Q

Enantiomers are _______ ____ ____

A

non-superimposed mirror images

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4
Q

Draw 2-hydroxypropanoic acid and indentify its chiral centre

A
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5
Q

Draw the 2 enantiomers of 2-hydroxypropanoic acid

A
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6
Q

Plane-polarised light only vibrates in ___ direction

A

one

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7
Q

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

A

They rotate plane-polarised light

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8
Q

Describe what happens when enantiomers rotate plane-polarised light

A
  • One enantiomer rotates it in a clockwise direction
  • & other rotates it in an anticlockwise direction
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9
Q

What does a racemic mixture contain?

A

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

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10
Q

Racemates don’t show any ____ ______

A

optical activity

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11
Q

Why don’t racemates show any optical activity?

A

2 enantiomers cancel each other’s light-rotating effect

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12
Q

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

A
  • React 2 achiral things together
  • ∵ when 2 molecules react there’s an equal chance of forming each of enantiomers
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13
Q

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

A
  • 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
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14
Q

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

A

It’s difficult and expensive

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15
Q

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

A

planar (flat)

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16
Q

When is a racemate formed in a reaction mechanism?

A

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)

17
Q

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

A
18
Q

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

A

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

19
Q

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

A

Make a product that doesn’t have a chiral centre

20
Q

Name another mechanism where a racemate can be formed

A

electrophilic addition of HBr to an unsymmetrical alkene