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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: smatik on March 26, 2013, 07:49:36 AM

Title: which is more reactive towards nuceoplhillic substituion?
Post by: smatik on March 26, 2013, 07:49:36 AM
I used to check the small amount of Δ+ charge on the carbonyl carbon for deciding the reactivity. If we have CC(=O)Cl and CC(=O)O. I know that O is more electronegative than Cl and hence will increase the amount Δ+ on carbonyl carbon more than in the case of Cl also small OH molecule will provide less hinderance than large Cl. So according to me the answer should be CC(=O)O but the correct answer is CC(=O)Cl.Can you please explain me how? Thanks
Title: Re: which is more reactive towards nuceoplhillic substituion?
Post by: Sunil Simha on March 26, 2013, 08:12:43 AM
Have you considered the role of resonance delocalization of electrons from Cl and O?
Title: Re: which is more reactive towards nuceoplhillic substituion?
Post by: smatik on March 26, 2013, 08:22:21 AM
hmm cl will be a good donor is it O???
Title: Re: which is more reactive towards nuceoplhillic substituion?
Post by: Sunil Simha on March 26, 2013, 08:30:58 AM
If you remember why Cl is ortho-para directing though it is deactivating (during electrophilic aryl substitution), maybe the same logic applies here too...
Title: Re: which is more reactive towards nuceoplhillic substituion?
Post by: smatik on March 26, 2013, 08:50:08 AM
well thats because chlorine has three lone pairs of electron which go into resonance with delocalized pi electrons of benzene which increase the amount of Δ- charge on ortho and para positions but in this case if chlorine does so its nucleophillic reactivity of carbonyl carbon will decrease.
BUT Phenol is more ortho and para directing than chlorobenzene(Due to resonance) which means the Δ- charge developed in case of ch3cooh is more than that in case of ch3cocl and hence carbonyl carbon of ch3cocl becomes more favorite site for a nucleophillic attack. Thanks for the discussion..very useful and i got my answer
Title: Re: which is more reactive towards nuceoplhillic substituion?
Post by: Sunil Simha on March 26, 2013, 10:01:01 AM
Well in addition to what you have said Cl has a stronger inductive effect than resonance effect and hence the net result is pulling away of electron toward itself, unlike oxygen.