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Chemistry (Semester 1) > L13: Isomerism > Flashcards

L13: Isomerism Flashcards

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1
Q

Define isomerism

A

Molecules which have the same molecular formula, but different arrangement of their atoms

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2
Q

What is the difference between structural isomers and stereoisomers?

A

Structural isomers differ in their bonding sequence.

Stereoisomers differ only in the arrangement of their atoms in space.

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3
Q

What are the 4 types of structural isomer?

A

1) Chain or skeletal isomerism
2) Position isomerism (position of the FG)
3) Functional isomerism (identity of the FG)
4) Tautomerism (movement of bonds and a proton (H))

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4
Q

What are the two types of stereoisomerism?

A

1) Geometric (cis/trans)

2) Optical

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5
Q

True or false: Trans isomer has two alkyl groups on opposite sides of the double bond

A

True

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6
Q

Fill in the gap:

In a cis isomer, two alkyl groups are on the ______ side of the double bond

A

same

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7
Q

What type of isomer rotates the plane of polarised light to the right (clockwise)?

A

Dextrorotary (d-isomer or +)

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8
Q

What type of isomer rotates the plane of polarised light to the left (anticlockwise)?

A

Laevorotary (l-isomer or -)

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9
Q

If an isomer can rotate the plane of polarised light, what type of isomerism is displayed?

A

Optical

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10
Q

True or false: Compounds with more than one chiral centre will show optical activity

A

False. They may or may not depending on whether they are chiral (non-superimposable) or achiral (superimposable)

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11
Q

True or false: Compounds without chiral centres do not normally show optical activity

A

True

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12
Q

True or false: Compounds with one chiral centre may or may not show optical activity

A

False. Compounds with one chiral centre will always show optical activity

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13
Q

What is a meso compound?

A

A molecule with two or more stereogenic centres, but the molecule itself can be superimposed on its mirror image

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14
Q

What are two types of tautomer?

A

1) Keto-enol

2) Ring-chain

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15
Q

How do you form an enol?

A

Take a ketone C=O, bond a hydrogen to the O and move the double bond to either the immediate left or right of the O-H to make a C=C bond

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16
Q

When drawing out a keto-enol isomer, where do you position the double bond?

A

It doesn’t matter unless there is another double bond to which it can be conjugated

17
Q

How do you form a ring-chain isomer?

A

Take a C-OH from one end of an open chain, and take a bond from a C=O on the other end to cyclise the molecule with a C-O-C bond. The H moves to the O that is missing a bond

18
Q

True or false: Keto-enol and ring-chain reactions are reversible

A

True

19
Q

True or false: For geometrical isomerism, the molecule must have a double bond and it must have two different groups either side of the C=C

A

True

20
Q

How can polarised light tell you if a molecule is an optical isomer?

A

If the molecules rotate the plane of polarised light, the molecules are optical isomers

21
Q

True or false: Compounds can be identified by the amount they rotate the plane of polarised light

A

True. Samples of the same molecule will rotate the plane of polarised light the same amount, assuming they are pure.

22
Q

What are enantiomers?

A

Optical isomers with 2+ chiral centres, both in opposing configuration

23
Q

True or false: Enantiomers have different physical properties

A

False. Enantiomers have identical physical properties in all respects, except in their interaction with plane of polarised light

24
Q

What are diastereomers?

A

Stereoisomers that are non-superimposable and non-mirror images. They have opposite configurations at some chiral centres (but not all of them)

25
Q

True or false: Diastereomers have different physical properties

A

True

26
Q

Where are entantiomers found in biology?

A

Bacterial cell wall - protect bacteria from proteases

27
Q

Fill in the gap:

Enantiomers have _______ physical and chemical properties

A

identical

28
Q

True or false: Diastereomers may rotate the plane of polarised light but by different amounts

A

True

29
Q

True or false: Diastereomers rotate the plane of polarised light by equal amounts in opposite directions

A

False. Enantiomers rotate the plane of polarised light by equal amounts in opposite directions

30
Q

True or false: Enantiomers are impossible to separate by normal physical methods

A

True

31
Q

Fill in the gap:

Diastereomers have ________ physical and chemical properties

A

different

32
Q

True or false: Both enantiomers and diastereomers differ in taste and smell

A

True

33
Q

What is a 1:1 mixture of enantiomers called?

A

A racemate

34
Q

Can enantiomers be distinguished by NMR spectroscopy?

A

No

35
Q

Can diastereomers be distinguished by NMR spectroscopy?

A

Yes

36
Q

How many isomers can you expect to find for diastereomers?

A

2^n

n = number of chiral centres

Chemistry (Semester 1) (10 decks)

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