Definition of pericyclic reactions
There is a large body of chemical reactions of olefinic compounds which do not involve ionic or free
radical intermediates and which are remarkably insensitive to the presence or
absence of solvents and catalysts.
Many of these reactions are characterized by the making and breaking of two or more bonds in a single concerted step through cyclic transition-states where in all first order bonding changes occur.
Because of their cyclic transition-states, they are known as pericyclic reactions. A pericyclic reaction may occur thermally or photo chemically.
Many of these reactions are characterized by the making and breaking of two or more bonds in a single concerted step through cyclic transition-states where in all first order bonding changes occur.
Because of their cyclic transition-states, they are known as pericyclic reactions. A pericyclic reaction may occur thermally or photo chemically.
What is cyclo addition reaction ?
A thermal or photo
chemical addition or coupling reaction between double bonds in two separate
molecules in which the two units get joined by two sigma bond created from the two original pi bonds to form a cyclic
product via a cyclic transition state is called cyclo addition reaction.
For example, cyclo addition reaction between cyclo pentadiene
and substituted ethylene. Another example of cyclo addition reaction is photo chemically dimerisation of ethylene.
Mechanism of (4+2) cyclo addition :
Frontier molecular
orbital ( FMO ) method.A cyclo addition
reaction between butadiene and ethylene is the example of (4+2) type of cyclo addition reaction.The
mechanism of this cyclo addition reaction are as
follows,
Woodward-Hoffmann selection rules for cyclo addition reaction are as follows,
Types of cyclo addition reaction:
There are two types of cyclo addition reaction, (I)
Exo-cyclo addition reaction and (II) Endo-cyclo addition reaction.
Exo-cyclo addition reaction :
If the smaller side of a reactant molecule lies under the larger side of another reactant molecule during the formation of the product molecule, the cyclo addition is called exo-cyclo addition reaction.
For example, reaction between cyclo pentadiene ( diene) and maleic anhydride ( dienophile ) .
Exo-compound thermo dynamically controlled product, thus
with increasing the temperature of the reaction favours the formation of the
exo-compound.
Endo cyclo addition reaction:
If the larger side of reactant molecules lies under the larger side of another reactant molecule during the formation of the product molecules, the cyclo addition is called endo cyclo addition reaction.
For example, reaction between cyclo pentadiene ( diene) and maleic anhydride ( dienophile ) .
Endo-compound kinetically controlled product.
What is Diels Alder reaction ?
This is an example of (4+2) type of cyclo addition reaction.
The reversible and thermal (4+2) cyclo
addition reaction between a diene and a
dienophile in which the dienophile adds 1,4
to the diene to produces a six membered cyclic product or adduct, is known as
Diels Alder reaction.Few example Diels Alder reaction are as follows,
Photo chemical reaction of anthracene molecule is a most common example of (4+4) type of cyclo addition reaction. The reaction are as follows,
Summary
- What is pericyclic reaction ?
- What is cyclo addition reaction ? Type of cyclo addition reaction .
- What is Diels Alder reaction ?
- Wood ward Hoff mann rules for cyclo addition reaction.
No comments:
Post a Comment