ORTHO AND PARA % OF KEY REACTIONS

Dear students,
 In the case of electrophilic substitution over mono substituted benzene,Ph-EDG(ortho,para directing), depending upon statistical ratio the simplest expectation for ortho:para is 2:1.But usually in ortho ,para directing systems,the ortho substitution occurs to lesser extent than expected.
This difference in ortho and para products may be due to many factors, that is, polar effects of thesubstituent,solvent,temperature,size of the substituent, and the entering group i.e steric factors.
Nitration of alkyl benzene under comparable conditions

• Nitration of tolune gives 58% ortho  and 37% para
• Nitration of Ethyl benzen gives 45% ortho and 49% para.
• Nitration of iso propyl benzene gives 30 % ortho and 62% para.
• Nitration of tert butyl benzene gives 16% ortho and 73% para.

Attack on Chlorobenzene by several different electrophiles(increasing in size) 

• If electrophile is chlorine then ortho 39% and para 55%.
• If electrophile is nitro then ortho 30% and para 70%.
• If electrophile is bromo then ortho 11% and 87 %para.
• If electrophile is sulphur trioxide then ortho is 1% and para is 99%.

steric factor is not the sole criteria,in other case electronic factor dominates .

Nitration of halobenzene 

• In flurobenzene ortho is 12% and para 88%
• In chlorobenzene ortho is 30% and para 69%
• In bromobenzene ortho is 37% and para is 62%
• In iodobenzene ortho is 38% and para is 60%.

 Note From  flourine to iodine size increse but para % decrease .