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Topic: Electrophilic Aromatic Substitutions/ES2  (Read 4371 times)

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Offline SwedishChef

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Electrophilic Aromatic Substitutions/ES2
« on: March 08, 2010, 12:37:25 PM »
Suppose you have a compound such as Benzaldehyde or a ketone-mono-substituted benzene(carbonyl C is attached to the ring).
The goal is to attach a nitro group para to the aforementioned substituent, which is m-directing. I've thought of converting the carbonyl group to an o/p directing alkyl group via Wolff-Kishner Reduction, but not sure if its a reversible mechanism after adding the nitro group(i.e. converting the alkyl group back into its original carbonyl substituent. Plus there's also the issue of a mixture of products since o/p are mechanistically linked, which are usually temperature dependent.
Any suggestions?

Offline Smrt guy

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Re: Electrophilic Aromatic Substitutions/ES2
« Reply #1 on: March 08, 2010, 09:56:50 PM »
What is the directing effect of an acetal/ketal?  I don't recall having seen this listed in directing effects before.  It seems to me, however, that it is more like an alkyl group than anything, so if I had to guess, I would say it is o/p- directing.

Offline orgopete

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Re: Electrophilic Aromatic Substitutions/ES2
« Reply #2 on: March 09, 2010, 09:56:48 AM »
Many textbooks use this example to use a scheme that involves insertion of a directing group, a substitution reaction, and then a later removal of the directing group.

An example is nitration, reduction, acylation, (EAS), hydrolysis, diazotization, and reduction.
Author of a multi-tiered example based workbook for learning organic chemistry mechanisms.

Offline SwedishChef

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Re: Electrophilic Aromatic Substitutions/ES2
« Reply #3 on: March 09, 2010, 11:01:11 AM »
What is the directing effect of an acetal/ketal?  I don't recall having seen this listed in directing effects before.  It seems to me, however, that it is more like an alkyl group than anything, so if I had to guess, I would say it is o/p- directing.

Yes, alkyl substituents are ortho/para directing activators. However, carbonyl substituents are meta-directing deactivators of the benzene ring. I'm wondering if, say for example, you have aromatic compounds such as 1-phenylethanone, benzoic acid, or benzaldehyde, and you want to perform a substitution specifically at the para position of the ring. What is the best way to go about doing this while minimizing the amount of mixtures of unwanted products?

Offline stewie griffin

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Re: Electrophilic Aromatic Substitutions/ES2
« Reply #4 on: March 09, 2010, 12:23:12 PM »
Re: SwedishChef
Smrt guy is simply proposing that rather than completely remove the carbonyl (as you intended via Wolff-Kishner), why not protect it as an acetal (technically a ketal if your starting carbonyl is a ketone). Once protected it's no longer a carbonyl, but essentially an alkyl group. Thus hopefully you will get predominately o/p reaction. Once the SEAr is complete, just deprotect to reveal your starting carbonyl again.

Offline orgopete

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Re: Electrophilic Aromatic Substitutions/ES2
« Reply #5 on: March 09, 2010, 07:11:15 PM »
Re: SwedishChef
Smrt guy is simply proposing that rather than completely remove the carbonyl (as you intended via Wolff-Kishner), why not protect it as an acetal (technically a ketal if your starting carbonyl is a ketone). Once protected it's no longer a carbonyl, but essentially an alkyl group. Thus hopefully you will get predominately o/p reaction. Once the SEAr is complete, just deprotect to reveal your starting carbonyl again.
Of course, you need to avoid doing your electrophilic substitution reaction under acidic conditions.
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Offline stewie griffin

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Re: Electrophilic Aromatic Substitutions/ES2
« Reply #6 on: March 09, 2010, 07:37:03 PM »
Re: SwedishChef
Smrt guy is simply proposing that rather than completely remove the carbonyl (as you intended via Wolff-Kishner), why not protect it as an acetal (technically a ketal if your starting carbonyl is a ketone). Once protected it's no longer a carbonyl, but essentially an alkyl group. Thus hopefully you will get predominately o/p reaction. Once the SEAr is complete, just deprotect to reveal your starting carbonyl again.
Of course, you need to avoid doing your electrophilic substitution reaction under acidic conditions.
Sure. I wasn't saying it would work the way Smrt guy was proposing. I did feel though that SwedishChef was missing what Smrt guy was trying to suggest. SwedishChef clearly indicated that they wanted to get the carbonyl back after the reaction, and their response indicated that they didn't fully understand the protecting group strategy. I would suggest reducing the ketone to the alcohol and then oxidizing it back up at the end. Of course if you have HNO3 around for the SEAr you may run into troubles with formation of a benzyllic carbocation. I've never done it, so I can't say if it works or not.

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