Pericyclic reactions-cyclo addition and Diels Alder reactions .

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.

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,

( 4+4) type cyclo addition reaction:

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.  

Click here: Pericyclic reaction.