Stereochemistry and isomerism
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What are constitutional isomers?
They have different connectivity
What are stereoisomers?
Differ in the three dimensional arrangement of the groups
What are the steps for assigning the conformation of alkenes?
• Assign a priority to each part of one end of alkene, one bond at a time looking at atomic number • Repeat on the other side
What is the conformation if the two top priority groups are on opposite sides?
E
What is the conformation if the top two priority groups are on the same side?
Z
How do you expand a pi bond to assign priority?
Dummy atoms are added Eg. C=O would be shown as C is attached to O and O - C
What are enantiomers?
Stereoisomers which are mirror images
What is a stereo/chiral centre?
Carbon with four different groups attached
What is used to assign configuration around a chiral centre?
R or S
What are the steps in assigning R or S?
• Assign priorities in the same way as for an alkene to the four groups around the carbon • Arrange the molecule so that the lowest priority is pointing away from you • Then look at how the priorities 1,2,3 are pointing, clockwise or anticlockwise
What configuration is assigned to anticlockwise groups?
S
What configuration is assigned to clockwise groups?
R
Which way does the R enantiomer rotate plane polarised light?
Anticlockwise Called (-)
Which way does the S enantiomer rotate plane polarised light?
Clockwise Called (+)
How do you measure the rotation of plane polarised light?
Use a polarimeter It tells you the angle through which the light has been rotated is measured and therefore if it is (-) or (+)
What is a racemic mixture?
A 50:50 mixture of enantiomers
What effect does a racemic mixture have on plane polarised light?
It does not rotate it as one enantiomer rotates it clockwise and the other anticlockwise so they cancel each other out
What does chiral mean?
A molecule is chiral if it is not superimposable on its mirror image
What does achiral mean?
A molecule which can be superimposed on top of its mirror image
What are diastereoisomers?
Stereoisomers which are not enantiomers (mirror images of each other)
What are the physical and chemical properties like in diastereoisomers?
They are different