Unit 4.1 Stereoisomerism (og)
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What are the 2 types of isomerism?
• Structural isomerism • Stereoisomerism
Define structural isomerism
Same molecular formula but different structural formulae
Define stereoisomerism
Same molecular formula but atoms occupy different positions in space Atoms arranged in 3D space
What are the 3 types of structural isomerism?
• Chain isomerism (CH3 above or below or summin) • Position isomerism (Functional group different places) • Functional group isomerism (Functional group within, also in diff places)
What are the 2 types of stereoisomerism?
• Geometrical isomerism • Optical isomerism
What is geometrical isomerism
• Occurs due to the restricted rotation of C=C double bonds • 2 forms • E and Z (CIS and TRANS)
What is optical isomerism
• Occurs when molecules have a chiral centre • Get two non-superimposable mirror images 4 different groups of atoms attached
So u obviously, should know how to tell what isomerism is taking place. However, how to actually name them, yano?
I’ll look at book…. right???
What molecules can have geometrical isomerism?
The ones with the pi-bond (C=C double bond) Cuz like, restricted rotation And so 2 different forms: CIS (Z) & TRANS (E)
How do u tell if geometrical isomerism exist?
• Pi-bond n shi • Two different atoms/groups attached
What molecules can have optical isomerism?
Appears to be 4 different groups attached… so….?
What are the 2 different forms of optical isomerism?
• Optical isomers • Enantiomers
When does optical isomerism occur?
• When molecules have a chiral centre
What consists of a chiral centre?
• Asymmetric carbon atom (4 different atoms/groups arranged tetrahedrally around)
Explain thalidomide scandal (ADHD edition)
• Thalidomide marketed as mild sleeping pill • Unfortunately caused babies born with malformed limbs D: • Pair of thalidomide enantiomers: 50:50 ratio (equimolar ratio) • Turns out it had teratogenic deformities despite only one of them causing it • That’s cuz molecule interconverts between isomers • Pure sample of only one isomer = IMPOSSIBLE
Define equimolar ratio?
• 50:50 mixture • of enantiomers • aka racemic mixture
How would u spot chiral centres?
Remember: - 4 different atoms/groups arranged tetrahedrally around it So, 4 different atoms/groups around a single atom. Can be found in like an alkane or summin Skeletal formulas too
Explain molecules containing chiral carbons having two forms?
• They are non-superimposable mirror images of each other (Can’t stack the molecules on top of each other)
U might have to: 1. Draw a displayed formula of such 2. Spot a chiral molecules 3. Re-draw the molecule in 3D 4. And its mirror image
Okay?
How do u get differences in optical isomers?
• Differ in their reaction to plane-polarised light • Polarimeter type shi • Rotation = differences for sure = cuz of brightness
State the model answer of polarimeter outcomes (optical isomers differences)
• A single enantiomer will rotate plane polarised light in a single direction • Either to the left or the right • The other enantiomer would rotate the light in the opposite direction
What it means if the light appears to have turned to the right?
It’s DEXTROROTATORY (d) or (+) form
What it means if the light appears to have turned to the left?
It’s LAEVOROTATORY (l) or (-) form
Why is there no net effect on the rotation of plane polarised light?
The 2 directions they rotate the light is cancelled out