17. Organic II (OPTICAL ISOMERS)
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What type of isomerism is optical isomerism
Stereoisomerisms
Describe what is meant by a chiral carbon centre
A carbon with 4 different groups occupying the 4 bonds
How do you draw optical isomers
Locate the chiral carbon centre Draw one enantiomer in the tetrahedral shape Then draw the mirror image beside it to get the other enantiomer
What does a solid wedge and dotted wedge show when drawing isomers
Solid wedge - bond coming out of the page towards you Dotted line - bond going into the page away from you
Optical isomers are…
Non superimposable images of each other
What will optical isomers do to plane polarized light (light that has passed through a polarizing filter to only vibrate in one direction)
Optical isomers will rotate the plane polarized light with each enantiomer rotating it in equal directions clockwise or anticlockwise Therefore, optical isomers are said to be optically active
Describe what is meant by a racemic mixture and how it is made
A mixture made up of 50% of both enantiomers Made when 2 a-chiral molecules react as there is a equal chance of either hydrogen atom being replaced Other than this, optical isomers have very similar properties
Are racemates optically active
No - The plane polarised light is rotated in equal and opposite amounts by both enantiomers so there is no overall rotation.
How many enantiomers are formed in SN1 mechanisms
2 - the planar ion can be attacked by nucleophille from either side of the chiral carbon and forms a carbocation intermediate –> the product will be a racemic mixture and therefore NOT optically active
Why are optical isomers produced from SN1 reactions not optically active
There is a trigonal planar shape around the carbon that has lost the halogen group. OH- nucleophile can now attack from either side of that carbon so a racemic mixture of the 2 optical isomers form These 2 optical isomers rotate the plane of polarised light by the same amount but in opposite directions so they are not optically active
How many enantiomers are formed from SN2 mechanism
1 - This optical isomer will rotate the plane of polarised light in the other direction as the reactant - Forms a transition state where the nucleophile (OH-) only one side which is the side opposite to the leaving group Transition state has brackets and negative charge dumbass
How do you work out the number of optical isomers a compound has from the number of its chiral centre/s
2^n Where n = number of chiral centres