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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: destofworlds on July 14, 2017, 01:14:40 PM

Title: Another Mechanism Problem
Post by: destofworlds on July 14, 2017, 01:14:40 PM

The question requires me to show the step by step mechanism of the reaction in the top half of the image.

The bottom half is what I have come up with as an answer.  Is it at all right?  I feel like it isn't correct because of C4 getting those 5 bonds.  The C4-C5 bond shifting to become a C3-C5 bond also feels very wrong.

If this is, as I suspect, very wrong; could someone please point me in the right direction?

Thanks very much for any help/advice you may be able to give.

Title: Re: Another Mechanism Problem
Post by: Babcock_Hall on July 14, 2017, 03:38:10 PM

Can your starting material be protonated?

Title: Re: Another Mechanism Problem
Post by: destofworlds on July 14, 2017, 04:07:49 PM

Perhaps something like this?  Am I close?

Title: Re: Another Mechanism Problem
Post by: Babcock_Hall on July 14, 2017, 05:06:59 PM

Step 3 does not make any sense.  You are pushing electrons onto a carbon that already has two bonds to hydrogens and two bonds to carbons.

Title: Re: Another Mechanism Problem
Post by: destofworlds on July 14, 2017, 05:13:59 PM

I'm not sure what I can do instead?  Can I protonate the double bonded oxygen first or something?

Title: Re: Another Mechanism Problem
Post by: Babcock_Hall on July 15, 2017, 05:18:12 PM

I would try that.

Title: Re: Another Mechanism Problem
Post by: destofworlds on July 15, 2017, 05:37:20 PM

Ok, thank you very much for all the help

Title: Re: Another Mechanism Problem
Post by: Babcock_Hall on July 17, 2017, 07:49:40 AM

My suggestion is to protonate the oxygen and draw reasonable resonance structures.  Can you show us that?

Title: Re: Another Mechanism Problem
Post by: clarkstill on July 17, 2017, 12:11:26 PM

Beware of carbon atoms with 5 bonds (AKA 'texas carbon') when you draw your mechanisms... if you find yourself with an intermediate that has one, it is undoubtedly wrong. Be especially careful with the presence of implicit hydrogens on your structures - they can't just disappear!

Babcock Hall is right - protonate the oxygen and think about what resonance forms that leads to. When you encounter a molecule like this and you don't know where to protonate, remember that the lone pairs are the most 'available' electrons for protonation, so it's more likely that these are protonated than one of the alkenes.

Title: Re: Another Mechanism Problem
Post by: Susuman on July 18, 2017, 09:58:01 AM

What did you add to get a benzene? How did the new bond is created? I'm dealing with a similar problem...

Title: Re: Another Mechanism Problem
Post by: Babcock_Hall on July 18, 2017, 10:14:13 AM

susuman,

If you want help with a problem, start a new thread and show us your attempt.

Title: Re: Another Mechanism Problem
Post by: destofworlds on July 18, 2017, 07:55:47 PM

I turned in my problems yesterday and I got 9.5/10 points on this problem. I somehow made the error of not adding the double bonds to the benzene ring at the end (I am still kicking myself for that).  The image is what I turned in, apart from that I fixed my careless error.  Thank you all very much for the assistance on this.