Electrophilic Aromatic Substitution (part II)

(a.k.a "What's the deal with ortho, meta, and para directors?")

Some examples of “ortho/para directors”
the OH group in phenol

the Br in bromobenzene

the CH3 group in toluene

 -NR2 , -OR , alkyl, aryl, akenyl groups are o/p directors and activate the ring.
 -F , -Cl , -Br , -I are o/p directors, but deactivate the ring toward electrophilic aromatic substitution.

The incoming substituent will be directed to the para position, rather than ortho, because of steric strain.  If the para position is already occupied, the ortho/para director will direct the incoming group to the ortho position. 

For example:

Para position is occupied... …so Cl is directed to add to ortho position


 

Some examples of "meta directors"

the acyl group in benzaldehyde

the NO2 group in nitrobenzene

acyl , -CN , -SO3H , CF3 , and -NO2 are meta directors and deactivate the ring
toward electrophilic aromatic substitution.

 

Ways to remember which groups are ortho/para directing and which are meta directing:

I.  Lone pairs/alkyls
Groups with a lone pair at the first atom of connection are ortho/para directors.

Alkyl groups are also ortho/para directors.

II.  Electronegativity
Groups (-XY) where X is more electronegative than Y are ortho/para directors:

 

Groups (-XY) where Y is more electronegative than X, or where X bears a formal + charge, are meta directors.

These directing effects are explained by the stability of the intermediate carbocation

Consider the intermediate when E+ attacks at ortho, meta, and para positions on aniline (NH2 is a known o/p director.)
ortho attack:

especially STABLE (octets)              

meta attack:

no special stability for any of these intermediates
para attack:

especially STABLE (octets)             

NH2 directs incoming groups ortho and para because of the especially stable carbocations generated in the ortho and para routes.

 

Consider the intermediate when E+ attacks at ortho, meta, and para positions on nitrobenzene (NO2 is a known meta director.)
ortho attack:

especially UNstable                                   

meta attack:

no special stability or instability for any of these intermediates
para attack:

especially UNstable                                   

NO2 directs incoming groups meta to avoid the especially UNstable carbocations generated in the ortho and para routes.

Summary

Groups that donate electron density (alkyls, those with a lone pair at the point of attachment, those with less electronegative atoms connected to the atom of attachment) stabilize the carbocation intermediate that is formed during ortho and para electrophilic aromatic substitution, so these positions are favored.  There is no extra stability associated with the carbocation intermediate that is formed during meta substitution, so this position is  not favored.

Groups that withdraw electron density (nitro, cyano (CN), acyls--groups with more electronegative atoms connected to the atom of attachment, and/or a positive charge at the first point of attachment) destabilize the carbocation intermediate that is formed during ortho and para electrophilic aromatic substitution, so these positions are disfavored.  There is no extra instability associated with the carbocation intermediate that is formed during meta substitution, so this position is favored.

 

Multiple Substituents
If multiple substituents contradict each other regarding position of the next incoming substituent, the o,p directors “win,” since they are the substituents (generally) that stabilize the electrophilic aromatic substitution process.  While halogens are o,p directors their electronegativity makes them “deactivators” towards electrophilic aromatic substitution.

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