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A-Level Organic Chemistry AQA (Paper 2) > Isomerism > Flashcards

Isomerism Flashcards

1
Q

What are isomers?

A

•Compounds with the same molecular formula but have different molecular structures or a different arrangement of atoms in space.

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2
Q

What are structural isomers?

A

•Molecules with the same molecular formula but a different structural formula.

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3
Q

What are stereoisomers?

A

•Molecules with the same molecular and structural formulae but a different arrangement of the atoms in space.

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4
Q

What are the types of structural isomerism? (3)

A
  • Chain isomerism.
  • Position isomerism.
  • Functional group isomerism.
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5
Q

What is chain isomerism?

A

•The carbon chain of molecules with the same molecular formulae can vary causing isomerism.

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6
Q

What are the chain isomers of C4H10?

A

•Butane and methylpropane.

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7
Q

What is position isomerism?

A

•Functional groups can be in different positions on the carbon chain resulting in isomers.

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8
Q

What are the position isomers of C3H7OH?

A

•Propan-1-ol and propan-2-ol.

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9
Q

What is functional group isomerism?

A

•Some compounds with the same molecular formulae can contain different functional groups.

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10
Q

What are the functional group isomers of C3H6O?

A

•Propanal and propanone.

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11
Q

What are the functional group isomers of C3H6O2?

A

•Propanoic acid, methyl ethanoate and ethyl methanoate.

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12
Q

What are the types of stereoisomerism? (2)

A
  • E/Z isomerism/geometric isomerism.

* Optical isomerism.

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13
Q

What is E/Z isomerism? (2)

A
  • The two stereoisomers have the same structural formula but the bonds are arranged differently in space.
  • It occurs only around the C=C double bonds when the both of the carbons in the C=C bond have two different groups attached.
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14
Q

What is E isomerism?

A

•Highest priorities are opposite the double bond.

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15
Q

What is Z isomerism?

A

•Highest priorities are on the same side of the double bond.

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16
Q

What are the rules for prioritising groups in E/Z isomerism? (2)

A
  • Cahn-Ingold Priority (CIP) rules- the higher the atomic number (proton number) of the group attached to the Cs attached to the C=C, the higher the priority
  • If they are the same, look at the atoms attached.
17
Q

What is optical isomerism? (3)

A
  • Occurs when there are four different substituents attached to one carbon atom.
  • There are two isomers that are non-superimposable mirror images of one another, but are not identical - they differ in the way they rotate the plane of polarised light (clockwise (+ive isomer) or anticlockwise (-ive isomer)).
  • E.g. bromochlorofluoromethane.
18
Q

What does it mean to be non-superimposable?

A

•When molecules cannot be placed on top of one another and give the same molecule.

19
Q

Does propan-2-ol exhibit optical isomerism? If so, sketch the two optical isomers.

A

•No.

20
Q

Does 2-chlorobutane exhibit optical isomerism? If so, sketch the two optical isomers.

A

•Yes.

21
Q

Does 2-methylbutanoic acid exhibit optical isomerism? If so, sketch the two optical isomers.

A

•Yes.

22
Q

Does butan-2-ol exhibit optical isomerism? If so, sketch the two optical isomers.

A

•Yes.

23
Q

Does butanone exhibit optical isomerism? If so, sketch the two optical isomers.

A

•No.

24
Q

What are enantiomers?

A

•Two compounds that are optical isomers of each other.

25
Q

What is the chiral centre carbon/asymmetric carbon? (2)

A
  • The carbon bonded to four different groups.

* Formulated using ‘*’.

26
Q

What happens when light passes through a polaroid filter? (2)

A
  • All the vibrations are cut out except those in one plane, e.g. the vertical plane.
  • The light now polarised will be affected differently by different optical isomers of the same substance.
27
Q

How can optical rotation be measured? (4)

A
  • Using a polarimeter.
  • 1). Polarised light is passed through two solutions of the same concentration, each containing a different optical isomer of the same substance.
  • 2). One solution will rotate the plane of polarisation through a particular angle, clockwise (+ive isomer).
  • 3). The other will rotate the plane of polarisation by the same angle but anticlockwise (-ive isomer).
28
Q

What is a racemic mixture or racemate? (3)

A
  • A 50:50 mixture of two optical isomers/enantiomers; a 50:50 chance of being attacked from above and below by a nucleophile.
  • Not optically active because the effects of the two isomers cancel out.
  • Each enantiomer rotates a plane of polarised light in opposite directions = no rotation, they cancel out.
29
Q

Describe the process of the synthesis of 2-hydroxypropanoic acid (lactic acid). (4)

A
  • 1). Hydrogen cyanide is added across the C=O bond to form 2-hydroxypropanenitrile (nucleophilic addition reaction, the nucleophile is the CN^- ion).
  • 2). The nitrile group is converted into a carboxylic acid- 2-hydroxypropanenitrile is reacted with water and acidified with a dilute solution of HCl (hydrolysis reaction).
  • 2-hydroxypropanoic acid produced has a chiral centre, it has not been affected by the hydrolysis reaction which only involves the -CN group.
  • You still have a racemic mixture of the two optical isomers.
30
Q

Draw the nucleophilic addition reaction mechanism for the formation of 2-hydroxyproanenitrile. (4)

A
  • 1). :CN^-, arrow going from the lone pair to the carbon with delta positive. (Ethanal)
  • The arrow goes from the double bond to the oxygen with delta negative.
  • 2). Compound with O:^-, arrow from the lone pair goes to the H^+ ion.
  • 3). 2-hydroxyproanenitrile
31
Q

When one optical isomer is an effective drug and the other is inactive, what are the three options for this? (3)

A
  • 1). Separate the two isomers- this may be difficult and expensive as optical isomers have very similar properties.
  • 2). Sell the mixture as a drug- wasteful as half is inactive.
  • 3). Design an alternative synthesis of the drug that makes only the required isomer.

A-Level Organic Chemistry AQA (Paper 2) (17 decks)

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