Synthesis Problem

[sfpublic]

Hi, I just had a test question for a synthesis problem (you have methoxybenzene and you want 1-methoxy-4-propylbenzene) and it was considered completely wrong, I was wondering why:

 

Methoxybenzene ---> (1) + FeBr3, Br2 (2) + Mg, ether (3) +1-bromopropane, ether --> 1-methoxy-4-propylbenzene

 

The right answer is (1) + AlCl3, 1-chloro-1-oxopropane (2) H20 (3) Zn(Hg), aq HCl.

 

I understand why the right answer is right, but I don't understand how my answer is completely wrong, and I got zero points for it.

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anyone? any help would be great, thanks!

[Kaeroll]

Hi there.

 

Your answer is not completely wrong at all. If I understand your description correctly (I'm not familiar with 'oxopropane' but I'm guessing you mean a three-carbon acyl chloride), your method is a simple Friedel-Crafts alkylation. It's not a reaction I've ever gotten on with too well, so I'm not sure why you're using Mg/ether, but I trust you have your reasons.

 

The problem with the alkylation reaction is that the MeO is an electron releasing substituent, directing to the p/o positions (or 2 and 4, if you prefer). The product of a single alkylation (likely at the p position for steric reasons) is still activated to further reaction - the alkyl substituent is (debatably) mildly electron releasing too, but in any case certainly does not deactivate the ring. As such, you would struggle to stop the reaction at a single substitution. (Tight stoichiometric control might achieve this, but it's unnecessary faffing about).

 

As you are aware, the 'correct' method is the more efficient acylation followed by reduction. The acylated product is deactivated to further reaction, giving the desired product.

 

Your question highlights an important point in synthetic chemistry - there are often several different ways to obtain a desired product, each with pros and cons. I think it was a bit harsh of your tutor to give you nil points for a reasonable, albeit less practical, alternative. It may be worth discussing this with your tutor and getting their point of view.

 

I have a question of my own now, something I've never properly understood about these reactions - the choice of Lewis acid. Why FeBr3 for alkylation, yet AlCl3 for acylation?

 

I hope this is of some help.

 

Kaeroll