Knight content L11-15

Question 1

What is the requirement for chirality

Answer 1

a molecule must have a non-superimposable mirror image

Question 2

What are the 3 types of chirality

Answer 2

Central chirality
Axial chirality
Planar chirality

Question 3

What does central chirality arises due to due to

Answer 3

central chirality arises due to the presence of one or more chiral centres

Question 4

What does axial chirality arise due to

Answer 4

axial chirality arises due to the presence of a chiral axis that can also be thought of as helicity

Question 5

What does planar chirality arises due to

Answer 5

planar chirality arises due to the presence of a chiral plane

Question 6

In what circumstance is allenes chiral And why

Answer 6

The two systems are othogonal and as a result the substituents on the two ends of the alkene are in different planes. Alkenes are chiral provided the R groups on the outer carbons are not the same R1R2C=C=CR3R4 - chiral

Question 7

Describe compounds with cumulative double bonds

Answer 7

if there is an odd number of double bonds the may possess cis-trans isomerism, if there is an even number of double bonds then they may possess chirality

Question 8

What are allenes

Answer 8

Allenes refers to a type of molecule which has two contiguous touching C=C

Question 9

Describe what happens when we replace on or both of the double bonds in an allene with a ring

Answer 9

One or both of the double bonds in an allene can be replaced by a ring and the resulting exo-alkylidene monocyclic or spiro-bicyclic compounds can exhibit chirality

Question 10

What is atropisomerism

Answer 10

Compounds which are chiral only due to hindered rotation around a single bond are called atropisomeric

Question 11

How are atropisomeric enantiomers interconverted

Answer 11

The enantiomers can be interconverted without the need to break a bond and afe therefore conformations rather than configurations

Question 12

What sort of substituents increases the barrier to rotation

Answer 12

Bulky orthodoxy-substituents increase the barrier to rotation

Question 13

Describe how we assign the RS nomenclature for the stereo axes

Answer 13

View the molecule along the stereoaxis. All groups attached to the front of the axis have a higher priority than all groups attached to the rear of the axis.

Question 14

Give examples of molecules which possess at stereo-plane

Answer 14

Cyclophanes and small trans-cycloalkenes

Question 15

Describe how helices are assigned

Answer 15

Helices are assigned as p if the sense of the twist is clockwise along fhe axis or M is the sense of fhe twists is anti-clockwise to the twist

Question 16

Define homotopic

Answer 16

Homotopic if making a change to one of the groups then separately to the other group gives two structures which are identical

Question 17

Define enantiotopic

Answer 17

Enantiotopic if making a change to first one of the groups, then to the other give two structures which are enatiomers of one another

Question 18

Define diastereotopic

Answer 18

Diastereotopic if making a change to first one of the groups then to another gives two structures which are diastereoisomers of one another

Question 19

What does the term heterotopic mean

Answer 19

The term heterotropic is used to describe groups of identical constitution which are not homotopic, ie either enantiotopic or diastereotopic

Question 20

What is a prochiral centre

Answer 20

A prochiral centre is an atom which becomes a chiral centre when one of two heterotropic groups is changed to a different group or when a new group is added to one of the heterotropic faces

Question 21

How do we assign the pro-R and pro-S nomenclature for heterotropic groups on a prochiral centre

Answer 21

Replace the heterotopic group with one of slightly higher priority (eg by isotopic substitution) and then use cahn ingold priority rules as usual

Question 22

When do we use the nomenclature Re and Si

Answer 22

For an sp2 hybridised prochiral centre Re and Si are assigned according to priority

Question 23

How do we assign Re and Si

Answer 23

Assign the priorities of the 3 groups, if the order of the priorities is clockwise then the top face is Re. If the three priorities is anti-clockwise then the top face is Si

Question 24

What is a prostereogenic centre

Answer 24

An atom which is attached to different heterotopic ligands whether or not the atom becomes a chiral centre when one of the heterotopic ligands is changed to a different group

Question 25

Describe the properties diastereotopic groups and faces

Answer 25

different chemical and physical properties. This is often seen both in chemical reactions and in the NMR spectra of compounds which possess diastereotopic groups