stereo specific and stereo selective reaction
What is stereo specific reaction ?
A reaction may be stereo specific, if stereo chemically different materials give stereo chemically different products.
For example, the addition of carbene to cis and trans but-2-ene is a cis addition. Thus the addition is stereo specific, that is, each geometrical isomer forms one product, cis isomer gives cis-product and trans isomer gives trans product .
The configuration of the two products are different. In both addition reaction , the carbene used is singlet carbene.
What is stereo selective reaction ?
A stereo selective reaction is that where between two or more possible stereo isomeric product , one is formed predominance. For example, addition of triplet carbene toalkene.
Stereo chemistry of halogen addition to alkenes
Addition of halogen to alkene is usually predominantly trans that is , the addition is stereo selective.
For example the addition of bromine or chlorine to maleic acid gives (+,-) dibromosuccinic acid, which can result only from trans addition.
In this reaction, the step is the formation of a bridged or non classical carbonium ion, followed by attact by the bromide or chloride ion .
The bromide ion attact from behind and along the bonding C – Br+ line , and this results in a walden inversion at this carbon atom.
Since the bromonium ionis symmetrical, each carbon atom is attracted equally well ,and consequently, resulting the formation of a pair of enantiomers and hence the product is a racemic modification.
Another example of
halogen addition to alkenes is addition of
bromine to cis and trans but-2-ene.
The reaction occurs through the formation of a bridged or non classical carbonium ion, followed by attact by the bromide ion.
Evidence for the formation of cyclic bromide ion come from the lower temperature and NMR studies.
The reaction occurs through the formation of a bridged or non classical carbonium ion, followed by attact by the bromide ion.
Evidence for the formation of cyclic bromide ion come from the lower temperature and NMR studies.
Summary
- What is stereo selective reaction ?
- What is stereo specific reaction ?
- Stereo chemistry of halogen addition reaction to alkenes .
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