[mohit.shurtugal]

Suppose i have multiple groups attached to a benzene ring..all of which may show resonance...like say a nitro group,a halogen group and a carboxylic group..then which group will actually resonate with the benzene ring...note that all the groups are conjugated with the ring individually...
please explain in detail

[mohit.shurtugal]

i searched all around for this but couldnt get dependable answers..say even if i have two chlorine groups attached to a benzene ring...then what??...
all books speak about simple resonance...none about this

[azmanam]

remember that resonance structures are not isolated structures.  The real structure is a weighted average of all possible resonance structures.  Resonance structures are a bookkeeping mechanism for us to depict where the electron density is centered in various points throughout a resonance-stabilized molecule.  That said, the electron cloud for a multi-substituted benzene ring will be spread out over all substituents that are capable of resonance.  All groups will engage in resonance at the same time.

[mohit.shurtugal]

thats correct...but say i am supposed to predict where a nucleophile will possibly attack the compound...then ill need to draw the canonical forms and then finally the hybrid to find the positive spot bearing atom..how do i go about with that?

[Smrt guy]

The ring will attack an electrophile, not be attacked by a nucleophile (at least in most cases).  Typically the most electron donating substituent creates the biggest directing effect, but mixtures can happen.

[mohit.shurtugal]

Am sorry but i dont understand..

[azmanam]

it will depend on their placement and spacial relationship to each other around the ring.  please post the full name or drawn structure of the benzene ring in question and list the reagents involved in the reaction

[mohit.shurtugal]

its not for a reaction....
i have just mentioned a compund as an example....

say..

1 chloro 3 nitro 5 amino benzene..

[azmanam]

do you know if those substituents are ortho/para or meta directors?  they all will participate in resonance.  when you have a mix of groups on a benzene ring, you have to decide which groups are stronger, which positions are blocked, and whether o/p or m will dominate.  start by deciding which groups are o/p or m directors and choose the best substitution position individually for each substituent. that is, where will the chloro group most likely direct a reagent?  where is the nitro group most likely to direct a reagent, and where is the amino gorup most likely to direct a reagent?  do more than one group direct to the same position?  that's likely the substitution position

[mohit.shurtugal]

right..i can do that...and predict electrophilic centres and nucleophilic centres...

i hope i wont be asked to draw the resonance structures of such a complicated compound???...

also

i have posted another another question in this forum regarding the heat of combustions of geometric isomers...its a multiple choice quest and i narrowed down to two options but cant figure out any further...would be very grateful if anyone could help me with that as i have already wasted hours on google over it

[g-bones]

The ring will attack an electrophile, not be attacked by a nucleophile (at least in most cases).  Typically the most electron donating substituent creates the biggest directing effect, but mixtures can happen.

Careful, If the halide he was talking about was a fluorine or even chlorine and the nitro or carboxylic acid is ortho/para to this halide, a nucleophile could attack in SnAr.  But overall it is important to note that resonance forms do not exist exclusively, each one represents the molecule in part.  hope this helps