L13: Isomerism

Question 1

Define isomerism

Answer 1

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

Question 2

What is the difference between structural isomers and stereoisomers?

Answer 2

Structural isomers differ in their bonding sequence.

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

Question 3

What are the 4 types of structural isomer?

Answer 3

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))

Question 4

What are the two types of stereoisomerism?

Answer 4

1) Geometric (cis/trans)

2) Optical

Question 5

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

Answer 5

True

Question 6

Fill in the gap:

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

Answer 6

same

Question 7

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

Answer 7

Dextrorotary (d-isomer or +)

Question 8

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

Answer 8

Laevorotary (l-isomer or -)

Question 9

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

Answer 9

Optical

Question 10

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

Answer 10

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

Question 11

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

Answer 11

True

Question 12

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

Answer 12

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

Question 13

What is a meso compound?

Answer 13

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

Question 14

What are two types of tautomer?

Answer 14

1) Keto-enol

2) Ring-chain

Question 15

How do you form an enol?

Answer 15

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

Question 16

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

Answer 16

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

Question 17

How do you form a ring-chain isomer?

Answer 17

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

Question 18

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

Answer 18

True

Question 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

Answer 19

True

Question 20

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

Answer 20

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

Question 21

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

Answer 21

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

Question 22

What are enantiomers?

Answer 22

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

Question 23

True or false: Enantiomers have different physical properties

Answer 23

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

Question 24

What are diastereomers?

Answer 24

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

Question 25

True or false: Diastereomers have different physical properties

Answer 25

True

Question 26

Where are entantiomers found in biology?

Answer 26

Bacterial cell wall - protect bacteria from proteases

Question 27

# Fill in the gap: Enantiomers have _______ physical and chemical properties

Answer 27

identical

Question 28

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

Answer 28

True

Question 29

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

Answer 29

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

Question 30

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

Answer 30

True

Question 31

# Fill in the gap: Diastereomers have ________ physical and chemical properties

Answer 31

different

Question 32

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

Answer 32

True

Question 33

What is a 1:1 mixture of enantiomers called?

Answer 33

A racemate

Question 34

Can enantiomers be distinguished by NMR spectroscopy?

Answer 34

No

Question 35

Can diastereomers be distinguished by NMR spectroscopy?

Answer 35

Yes

Question 36

How many isomers can you expect to find for diastereomers?

Answer 36

2^n n = number of chiral centres