[pesona3f]

whats the difference between inductive effect and resonance effect??

i read this sentence in mcmurry organic chem book
'note again that halogens, alkoxy, hydroxyl and amino groups withdraw electron inductively and donate electron by resonance'

from what i know, hydroxyl and amino (NH2) is activator and make benzene ring more reactive toward electrophilic substitution.. if it withdraw electron inductively, doesn't it make the benzene ring less reactive???

[azmanam]

yes, because halogens are electronegative, they deactivate the benzene ring... but because they have lone pairs (like oxygen and nitrogen) then can donate those lone pairs into the benzene ring to activate the ring.  it's a paradox.

in the end, resonance wins - but electronegativity mitigates resonance - and halogens are slightly activating groups.  they're o,p directors.

[pesona3f]

thankz, thats really give me an idea 

[Dolphinsiu]

Inductive effect will help benzene to be activated or deactivated. If an electron donating group is attached to benzene (in monosubstituted way), ortho or para directing is favored for electrophilic addition since carbocation formed in the intermediate is more stabilized. If an electron withdrawing group is attached to benzene, meta directing is favored for the same reason.

Resonance effect just enhance the stability of benzene when electrophilic attack is suffered. The more the canonical form the compound bears, the more stable is the compound towards electrophilic attack. That is why sometimes only ortho directing is favored.

[aldoxime_amine]

in the end, resonance wins - but electronegativity mitigates resonance - and halogens are slightly activating groups.  they're o,p directors.

Are you sure?

My teacher emphasised quite often that halogens are weak deactivators, yet o-p directors the reason being their high electronegativity predominating over their resonance effect. For in case of -Cl,-Br,-I, overlap is 3p-2p,4p-2p and 5p-2p respectively in the C-X linkage in aryl halides.
-F is just too electronegative.

[pesona3f]

alkoxy, amine and hydroxy also inductively electron withdrawing group..

from what i understand when i read your post Dolphinsiu, the prority given to the inductive effect to determine either EWG ot EDG... in that case, OH phenol will have meta director....

[azmanam]

Are you sure?

you're right.  on further review (http://www.cem.msu.edu/~reusch/VirtTxtJml/benzrx1.htm#benz1), halogens are weakly deactivating - in that the rate is slower than unsubstituted benzene (that's what activating/deactivating refer to... rate of reaction relative to benzene-H₆).  However, they're still o,p-directors.

So when we say 'in the end...' it depends on what 'end' we mean.  For rates, induction wins and the arylhalide reacts slower than benzene.  but for substitution, resonance wins and the arylhalide is an o,p-director.

[lancenti]

It seems the first question was never really answered.

Inductive Effect is an effect achieved through a sigma bond.

Resonance Effect is an effect achieved through a pi bond.

This should help you decide on the significance of each, depending on the reaction mechanism in question.

Example
Methyl Groups are Electron Donating via Inductive Effect

Phenol Groups are Electron Donating via Resonance Effect