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Topic: Synthesis Problem with Aromatic Meta and Ortho Directors  (Read 3066 times)

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Offline yzhw

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Synthesis Problem with Aromatic Meta and Ortho Directors
« on: February 10, 2015, 01:52:07 PM »
For the synthesis problem, this is how I worked it out by first adding 4 equivalents of Br2 and FeBr3.  I was wondering does my work makes sense?




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« Last Edit: February 10, 2015, 02:01:25 PM by Dan »

Offline kriggy

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Re: Synthesis Problem with Aromatic Meta and Ortho Directors
« Reply #1 on: February 10, 2015, 02:55:53 PM »
I dont think your bromination will work that way. It could but its higly unlikely and possibly you get black-ish mixture of products which would be almost impossible to separate.
In the next step I fail to see how you add the carbon inbetween MgBr and benzene ring. Not to mention even if it worked in that way you show, having that degree of selectivity of 3 from 4 same substituents would be something very unique. The Grignard reaction is OK.
Is the point of this exercise to start from benzene? It is very impractical starting material

Offline yzhw

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Re: Synthesis Problem with Aromatic Meta and Ortho Directors
« Reply #2 on: February 10, 2015, 06:50:04 PM »
This is a synthesis problem that was assigned in class with benzene as the starting marterial and a benzene with four substituents as the product.  What reagents do you suggest me to add first?

Offline kriggy

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Re: Synthesis Problem with Aromatic Meta and Ortho Directors
« Reply #3 on: February 11, 2015, 03:35:25 AM »
Well Im not sure, I wasnt thinking about it deeply but the 1st thing I would do is to make the 1,3,5-trimethylbenzene. Then you can  make mono bromination (because all the positions being equivalent) and the grignard reaction. Possibly you need to add such a substituent which directs the alkylations into the positions you want and can be removed/changed into the bromine

Offline k1mng

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Re: Synthesis Problem with Aromatic Meta and Ortho Directors
« Reply #4 on: February 11, 2015, 04:36:47 AM »
I'm thinking that the product from your first step would be hard to produce and isolate from the other various brominated benzenes.

Perhaps to install the 3 methyl groups you'd need a ortho- and para-directing substituent. As a thought, halogens can act as these sorts of directing substituents so if you can mono-brominate the benzene ring (which can be relatively easily done using Br2 and FeBr3 in carbon disulfide) however the methyl groups installed on the ring will themselves act as o-/p- directors and lead to all sorts of variably methylated benzenes.. not to mention perhaps an assortment of cross-coupled aromatics. So for me, I'd go for an efficient directing group that can be selectively removed later, like an amine.

Do you know any ways to generate an aniline from benzene? (Hint: there's a nice reactive intermediate that's accessible following a benzene mono-halogenation).

Following this, we need to be able to get rid of the -NH2 group, do you know any procedures for this?

Offline yzhw

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Re: Synthesis Problem with Aromatic Meta and Ortho Directors
« Reply #5 on: February 12, 2015, 01:21:43 AM »
I figured it out...first add the reagent Br2/FeBr3 to get a bromine attached to a benzene.  Then use Grignard to get the big substituent.  Next use Friedel Crafts Alkylation to form the methyls.  Lastly, use H2O/HCl, if elimination occurs on the OH due to Friedel Craft Alkylation...

Offline orgopete

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Re: Synthesis Problem with Aromatic Meta and Ortho Directors
« Reply #6 on: February 12, 2015, 08:52:01 AM »
I think the problem was intended to be more steps and likely involving a diazotization reaction. (It could be done in a very short method, but messy, requires separation, and not likely described in intro ochem texts.)
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