Orgoman's Ochem questions

[hoabangkhuang]

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Hi everyone, I am confused about these. please explain.

I learned in Organic Chemistry that if it's electron donating groups (activating groups) then it will be ortho, para sub; however, ortho is prefer over para unless there's ring strains or too bulky to have ortho, then para will be prefer. In Dr. Romano's practices, he kept on going this is para direct is the major product. For example on question #15.

#15) Which is the major product?

a benzene with a donating group. So I put Cl on the ortho, but on his explanation he goes... ortho is minor product, whereas, para is the major one.

#12) Benzene ring adding CH3CH2CH2Cl... he goes... on alkylation if u have 3 carbons or more on the linear fashion, u'll get rearranagement products. I seriously don't know this... Can someone please explain.


Thank you.

 

[asckwan]

#15

Para is the major product because of steric hinderance. Ortho has the two groups too close together and they will get in each other's way.

#12

Rearrangement is the draw back of Alkylation. If you want to avoid rearrangement you must use Acylation and then reduce it down with Zn(Hg) and HCl or another reducing agent.

Hope this helped!

 

[armorshell]

Realistically, you should only ever use acylation, because alkylation leads to rearrangement and due to the activating effect of methy groups you often get multiple substitutions...

 

[midwestboy]

rightly stated!

 

[DAT Destroyer]

Hi everyone, I am confused about these. please explain.

I learned in Organic Chemistry that if it's electron donating groups (activating groups) then it will be ortho, para sub; however, ortho is prefer over para unless there's ring strains or too bulky to have ortho, then para will be prefer. In Dr. Romano's practices, he kept on going this is para direct is the major product. For example on question #15.

#15) Which is the major product?

a benzene with a donating group. So I put Cl on the ortho, but on his explanation he goes... ortho is minor product, whereas, para is the major one.

#12) Benzene ring adding CH3CH2CH2Cl... he goes... on alkylation if u have 3 carbons or more on the linear fashion, u'll get rearranagement products. I seriously don't know this... Can someone please explain.


Thank you.

If you add 1-chloropropane to benzene under ALCL3 catalytic conditions, you would get isopropyl benzene as the major product. The reaction if you recall is a carbocation mechanism via the Arenium ion,,,, this means that the reaction is prone to rearrangement. The propyl "carbocation" will rearrange to the more stable isopropyl carbocation,which will then be added to the benzene ring. When faced with two rings, recall one ring is more activating than the other,,,,, The para is almost always preferred over ortho in almost all situations, with a few exceptions such as Toluene sulfonation which I doubt you will see on the DAT.... At any rate, sterically the para will be the major product over the ortho. Any Organic book will reveal the isomeric percentages. If you have any further questions, email me at orgoman.com or call my office and I will be happy to explain it to you on the phone at 1 646 642 6614 Regards, Dr. Jim Romano NYU