Maui3
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FC-alkylation Positioning
I am gonna do a FC-alkylation soon with ethyl bromide, 1,3-benzodioxole (1,2-methylenedioxybenzene) and AlCl3 as catalyst. I am a bit confused as to
what position on the benzodioxole that the ethyl-group is introduced. Also, I read somewhere that for the FC-alkylation of toluene the positioning of
the ethyl group can depend on the temperature of the reaction.
It might be my bad searching-skills, but I also couldn't find a lot of info on this - so I'd appreciate if somebody could help clarify it for me.
Thanks.
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Keras
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FC is a nucleophilic reaction, it forms a carbanion which will attack on the most electron rich carbon of the aromatic cycle. With 1,3-benzodioxole
there is no obvious such point, because oxygens somehow cancel each other influence – each free carbon is both favoured and unfavoured. Because of
steric hindrance, I would say that attack at ortho- positions w/r to any of the oxygens is unlikely, so that leaves you with the para/meta positions.
Also, for reasons of steric hindrance, it is unlikely that you have more than one alkylation.
My tuppence, though.
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DraconicAcid
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FC is electrophilic addition, and will form a carbocation that will add to the most electron-rich carbon of the aromatic ring.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Maui3
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Thank you for the clarification! But this still depends on temperature, right?
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Keras
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Not that I am aware of. But you may be right, increasing the temperature might lead to a higher level of multiple alkylation.
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Dr.Bob
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Lower temp can favor the monoreaction in FC as well as pushing the reaction mostly to the lowest transition state product. I have had to cool some
reactions to below 0C to get primarily one product over another possible one, but then the reaction took 2 days. What fun that was.
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Keras
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Although the primary means of reducing multiple alkylation is to use a large excess of the aromatic compound, in order to ensure that the chances that
the alkyl moiety has to encounter an already alkylated molecule are low.
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Dr.Bob
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In our case, the aromatic was the rare/expensive part, so we had to go the other way, which makes everything tougher. But your point is often the
easiest way. I have done that as well, but not much, sadly, as much easier.
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