Knight content L11-15
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What is the requirement for chirality
a molecule must have a non-superimposable mirror image
What are the 3 types of chirality
Central chirality Axial chirality Planar chirality
What does central chirality arises due to due to
central chirality arises due to the presence of one or more chiral centres
What does axial chirality arise due to
axial chirality arises due to the presence of a chiral axis that can also be thought of as helicity
What does planar chirality arises due to
planar chirality arises due to the presence of a chiral plane
In what circumstance is allenes chiral And why
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
Describe compounds with cumulative double bonds
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
What are allenes
Allenes refers to a type of molecule which has two contiguous touching C=C
Describe what happens when we replace on or both of the double bonds in an allene with a ring
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
What is atropisomerism
Compounds which are chiral only due to hindered rotation around a single bond are called atropisomeric
How are atropisomeric enantiomers interconverted
The enantiomers can be interconverted without the need to break a bond and afe therefore conformations rather than configurations
What sort of substituents increases the barrier to rotation
Bulky orthodoxy-substituents increase the barrier to rotation
Describe how we assign the RS nomenclature for the stereo axes
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.
Give examples of molecules which possess at stereo-plane
Cyclophanes and small trans-cycloalkenes
Describe how helices are assigned
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
Define homotopic
Homotopic if making a change to one of the groups then separately to the other group gives two structures which are identical
Define enantiotopic
Enantiotopic if making a change to first one of the groups, then to the other give two structures which are enatiomers of one another
Define diastereotopic
Diastereotopic if making a change to first one of the groups then to another gives two structures which are diastereoisomers of one another
What does the term heterotopic mean
The term heterotropic is used to describe groups of identical constitution which are not homotopic, ie either enantiotopic or diastereotopic
What is a prochiral centre
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
How do we assign the pro-R and pro-S nomenclature for heterotropic groups on a prochiral centre
Replace the heterotopic group with one of slightly higher priority (eg by isotopic substitution) and then use cahn ingold priority rules as usual
When do we use the nomenclature Re and Si
For an sp2 hybridised prochiral centre Re and Si are assigned according to priority
How do we assign Re and Si
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
What is a prostereogenic centre
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