17:Organic Chemistry II
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What is a stereoisomer?
Compounds with the same molecular and structural formula but differ because their groups occupy different positions in space
What are the two types of stereoisomers?
• geometric isomerism • optical isomerism
What is an optical isomer?
Isomers with a chiral centre which are non-superimposable mirror images of each other
What does it mean if two isomers are enantiomers?
They are non-superimposable mirror images of each other.
What does it mean for a carbon atom to be a chiral centre?
It has four different groups attached to it (making it an asymmetric carbon atom)
What is a racemic mixture?
A substance containing equal parts of each enantiomer, so any polarisation in light is cancelled out
What are the two types of Nucleophilic Substitution?
Sn1 - unimolecular nucleophilic substitution Sn2 - bimolecular nucleophilic substitution
How many steps is an Sn1 reaction?
Two steps
How many species are involved in the rate determining step in Sn1 reactions?
One
Why does Sn1 reactions mainly happen with tertiary halogenoalkanes?
Due to the formation of a tertiary or secondary carbocation, which is more stable
How can a tetrahedral shaped molecule form a racemic mixture when it undergoes an Sn1 reaction?
The carbocation formed is a planar structure, this means that the nucleophilic attack can happen from either side forming a racemic mixture
How many steps are involved in an Sn2 reaction?
One step
How many species are involved in the rate determining step in Sn2 reaction?
Two
How does Sn2 reaction work?
A high energy transition state is formed (very unstable)
What type of halogenoalkanes undergo Sn2 reactions?
Primary (and sometimes secondary) halogenoalkane
What happens to the shape of the molecule when the nucleophile attacks?
There is an inversion of shape
Why is the electron density more stable in a C=C bond than a C≡O bond?
The (C≡O) bond is polar so the electron density is greater near the oxygen atom than near the carbon atom
How do you find the chiral centre?
1) Find the carbon with four different groups surrounding it 2) Then we need to draw the molecule using the tetrahedral shape
How do you draw the tetrahedral shape?
Two normal lines (can’t be opposite sides) One wedged line One dotted line
What is the method used to identify optically active isomers?
Optically active isomers will rotate plane polarised light.
What is a racemic mixtures?
When we have an equal amount of each enantiomer -> racemates don’t rotate plane polarised light because the two enantiomers cancel each other out
How is a racemic mixture made?
Made by reacting achiral substances together, when the molecule reacts there is an even chance of forming each enantiomer
What are the steps of Sₙ1 mechanism to make enantiomers?
1) The X attracts the pair of elections in the C-X bond, so it becomes 𝜹- 2) The X leaves and leave a carbocation 3) The lone pair from the Y is attracted to the carbocation and forms a bond Because it’s planar it can attack from above or below
What are the steps of Sₙ2 mechanism to make enantiomers?
In an Sn2 mechanism both products react in one step and the nucleophile always attacks the opposite side of the leaving group