L13: Isomerism Flashcards Preview

Chemistry (Semester 1) > L13: Isomerism > Flashcards

Flashcards in L13: Isomerism Deck (36):
1

Define isomerism

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

2

What is the difference between structural isomers and stereoisomers?

Structural isomers differ in their bonding sequence.

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

3

What are the 4 types of structural isomer?

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))

4

What are the two types of stereoisomerism?

1) Geometric (cis/trans)

2) Optical

5

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

True

6

Fill in the gap:

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

same

7

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

Dextrorotary (d-isomer or +)

8

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

Laevorotary (l-isomer or -)

9

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

Optical

10

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

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

11

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

True

12

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

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

13

What is a meso compound?

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

14

What are two types of tautomer?

1) Keto-enol

2) Ring-chain

15

How do you form an enol?

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

16

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

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

17

How do you form a ring-chain isomer?

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

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

True

19

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

True

20

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

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

21

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

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

22

What are enantiomers?

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

23

True or false: Enantiomers have different physical properties

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

24

What are diastereomers?

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

25

True or false: Diastereomers have different physical properties

True

26

Where are entantiomers found in biology?

Bacterial cell wall - protect bacteria from proteases

27

Fill in the gap:

Enantiomers have _______ physical and chemical properties

identical

28

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

True

29

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

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

30

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

True

31

Fill in the gap:

Diastereomers have ________ physical and chemical properties

different

32

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

True

33

What is a 1:1 mixture of enantiomers called?

A racemate

34

Can enantiomers be distinguished by NMR spectroscopy?

No

35

Can diastereomers be distinguished by NMR spectroscopy?

Yes

36

How many isomers can you expect to find for diastereomers?

2^n

n = number of chiral centres