Lecture 13-Organic chemistry III: Isomerism

Question 1

What is isomerism?

Answer 1

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

Question 2

How do structural / constitutional isomers differ?

Answer 2

In their bonding sequence / molecular skeleton

Question 3

How do stereoisomers differ?

Answer 3

Only in the arrangement of the atoms in space

Question 4

What are structural isomers?

Answer 4

Different compounds with different physical + chemical properties

Question 5

Name the types of structural isomers?

Answer 5

Chain / skeletal isomerism
Position isomerism
Functional isomerism
Tautomerism

Question 6

What is tautomerism?

Answer 6

(The movement of bonds + a proton)
pairs of molecules readily converted by reaction that includes the transfer of a H atom from one part of the molecule to another involving the switch of formation of single to double bond

Question 7

What is keto-enol?

Answer 7

2 isomers are in equilibrium based on stability (99% vs 1) allowing carbonyls to react at the C next to the carbonyl group

Question 8

What is pyranose?

Answer 8

Hemiacetal

Question 9

What is steroisomerism?

Answer 9

Same structural formula ,but different 3D arrangement of atoms in space

Question 10

What are the 2 types of stereoisomerism?

Answer 10

Geometric -cis/trans E/Z

Optical isomerism

Question 11

How does geometric isomerism occur?

Answer 11

Results from restricted rotation around a bond ,because of single sigma bonds

Question 12

Why can’t double pi bonds have geometric isomerism?

Answer 12

The rotation breaks the bond = not allowed without energy input

Question 13

What does geometric isomerism require?

Answer 13

Different groups at each end of this bond

Different physical + chemical properties

Question 14

What is a cis isomer?

Answer 14

2 alkyl groups on same side of double bond

Question 15

What is a trans isomer?

Answer 15

2 alkyl groups on the opposite side of the double bond

Question 16

What ability do optical isomers have?

Answer 16

To rotate plane polarised light in opposite direction

Question 17

What isomer rotates the plane of polarised light to the RIGHT?

Answer 17

dextrorotatory

d-isomer / +-isomer

Question 18

What isomer rotates the plane of polarised light to the LEFT?

Answer 18

levorotatory
l-isomer
(-)-isomer

Question 19

What molecule has no plane symmetry?

Answer 19

chiral

Question 20

What is a chiral centre / asymmetric carbon atom?

Answer 20

A C atom bonded to 4 different substituents lacks a plane of symmetry

Question 21

Do compounds without chiral centres show optical activity?

Answer 21

Not usually

Question 22

Optical isomers of compounds with 1 chiral centre will show ……. and are ……?

Answer 22

optical activity ……. chemically identical

Question 23

Will compounds with more than 1 chiral centre show optical activity?

Answer 23

Depends on whether they are non-superimposable on their mirror/superimposable images

Question 24

What are enantiomers?

Answer 24

the non-superimposable mirror image forms of a chiral molecule

Question 25

What are meso compounds?

Answer 25

They contain a chiral C ,but not chiral overall

Question 26

What are the systematic nomenclature for chiral centres?

Answer 26

R and S
R= dashed line
S= solid line

Question 27

Enantiomers represent 2 optical isomers how?

Answer 27

(+) and (-)

Question 28

Why do enantiomers have opposite rotatory powers?

Answer 28

Due to the opposite arrangements of groups around each asymmetric C atom

Question 29

Describe the physical properties of enantiomers?

Answer 29

identical ,except in their interaction with plane of polarised light

Question 30

Do enantiomers interact similarly or differently with other chiral molecules?

Answer 30

Differently

Question 31

Generally are enantiomers not interconverted under ordinary conditions

Answer 31

Yes

Question 32

What is a racemic mixture?

Answer 32

A 1:1 mixture of the 2 enantiomers of a compound

Question 33

Describe what’s in a racemic mixture?

Answer 33

• Equal quantities = d- and l- enantiomers
• Notation = d,l or +/=
• No optical activity
• The mixture may have different mp / bp from the enantiomers

Question 34

A structure with n chiral centres has?

Answer 34

2n possible isomers

Question 35

What are diastereomers?

Answer 35

Stereoisomers that are NOT mirror images of each other

Question 36

Diastereomers are most common in….

Answer 36

Molecules with 2/+ chiral compounds where AT LEAST 1 ,but not all differ

Question 37

2 diastereomers have different ….

Answer 37

physical properties e.g. mp , bp + stability

Question 38

Diastereomers have different chemical reactivity with ,,,,,

Answer 38

both achiral + chiral reagents

Question 39

What is the importance of enantiomers?

Answer 39

Proteins/receptors/enzymes made up of L-AA -chiral environment (differentiate between isomers)

Question 40

D-alanine

Answer 40

Component of bacterial cell wall not found in mammalian proteins
-Protects bacteria from protease

Question 41

S-Tirofiban

R-Tirofiban

Answer 41

Anti-platelet drug

1000 fold less active

Question 42

S-thalidomide

R-thalidomide

Answer 42

Anti-angiogenic,teratogen

Sedative

Question 43

Compare the polarised light for enantiomers + diastereomers?

Answer 43

E = equal amounts
D = different amounts

Question 44

Compare the physical,chemical and NMR spectroscopy for enantiomers + diasteremoers?

Answer 44

E = identical
D = different

Question 45

Are enantiomers or diastereomers able to separate by physical methods (achiral)?

Answer 45

Diastereomers

Question 46

Do enantiomers react differently with chiral molecules?

Answer 46

Yes -allows separation via chiral techniques

Question 47

Do diastereomers react differently with chiral molecules or achiral molecules?

Answer 47

BOTH

Question 48

Enantiomers form what type of mixture?

Answer 48

1:1 racemate