Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: victor on May 05, 2006, 05:06:05 AM
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I'm having difficulties with this kind of reaction. Actually this is a question from my chemistry book and it said that the product (o-t-butyl anysol) can't be achieved by only doing Friedel-Crafts alkylation:
Ar-OCH3 + t-butylchloride --//--> o-t-butylanysol
then the book said that I have to do bromination first, before applying the t-butyl cation, so there will be two steps:
1. Ar-OCH3 + Br2 --FeBr3--> p-bromoanysol
2. p-bromoanysol + t-butylchloride --AlCl3--> o-t-butylanysol
My question is, why do I have to do that 2-step way while I think that the one-step way is also possible?? any reason for this?
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i am not clear about the question. nor i understood what happened to the bromine atom at the para position after applying tert-butyl cation.
but anyway, one possible explanation for the requirement of the two steps could be as follows.
the tert-butyl group is bulky and wont favour the ortho position due to steric hindarance. so you have to force it. for that bromination is suggested by the text book. ones you brominate the para position is locked and the tert-butyl cation cannot go anywhere but the ortho position.
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hmm...if it's like that, then why do you use bromination process?? while you know that Br is ortho,para directing group. I think it's better for me to do a nitration process because NO2 group is meta directing.
So, for the first product you'll get: p-nitroanysol. From the structure, we can see that methoxide is o,p-directing group while NO2 which is located in the para position is meta-directing group.
So, if it's like that, the place for the last group, which is t-butyl cation is specific only in the ortho side of methoxy group. Beside, applying bromination also will stop the further reaction isn't it?? Because of Br's high electronegativity...:lol:
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you are right. but then you didnt tell me what happened to the bromine atom after the second step? is it still there or you do one more step to remove the bromine atom introduced in the first step? bcos the expected product is o-t-butyl-anisole.
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Um, in the first product, p-bromoanisole is obtained and then, alkylation is done for the second process making a o-t-butylanisole....means that Br group is leaving and get attached to H atom which is released by t-butyl cation group...
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puzzling. let me discuss with my friends. i hope this question is real... will be back soon.
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Scheme attached
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the scheme is fine. infact i too was thinking that there would be a third step to remove the halogen. but then then question is we have to get the product in two steps, not three.
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What about a selective lithiation at o-position of anisole by n-butyllithium.
Two step synthesis is also possible from phenol.
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@AWK
Thanks for your replies, the scheme is correct (like in my book even though the third process is not drawed there), but I wanna ask is, why do you have to add Br group first, instead of directly adding t-butyl group to the anisole??
And if the answer is that t-butyl group is bulky so something must force it to become attached with anisole in ortho position, why must Br group?? How if I change into NO2 group??
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Directly, you can obtain 4-t-butylanisole predominantly.
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one possible reason for the use of bromine could be that, bromine is easy to remove (third step) than the nitro group as suggested by you.
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Ohh...I see now..ok, thanks for all your replies...I really appreciate it...
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Nitro group easy for remove too. First hydrogenation it with nascent H for example with Zn/HCl. The formed amine then diazotation with NaNO2/HCl and the formed diazonium chloride salt may be eliminated with H3PO3...
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Nitro group easy for remove too. First hydrogenation it with nascent H for example with Zn/HCl. The formed amine then diazotation with NaNO2/HCl and the formed diazonium chloride salt may be eliminated with H3PO3...
But nitro group deactivate aromating ring towards an electrophilic substution (though in the case of anisole not completely)
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Indeed not completely :)
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Nitro group easy for remove too. First hydrogenation it with nascent H for example with Zn/HCl. The formed amine then diazotation with NaNO2/HCl and the formed diazonium chloride salt may be eliminated with H3PO3...
But nitro group deactivate aromating ring towards an electrophilic substution (though in the case of anisole not completely)
Hey boss, talking about deactivator, I think that Br group also deactivates aromatic ring because of it's high electronegativity... ;D (this discussion is getting interesting, I never imagine that it would be that interesting... ;D)
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though bromine is electronegative, it is only weakly deactivating compared to nitro group. that may be the reason why bromine is preferred over nitrogroup.
one more reason, though less significant, could be that, the use of mineral acid could cleave the ether linkage. ofcourse, with HCl the rate of hydrolysis will be very less.
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Yup, because Cl is far more electronegative than Br...