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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: DannyBoi on February 28, 2011, 07:26:37 PM

Title: Acid Catalyzed reaction mechanisms?
Post by: DannyBoi on February 28, 2011, 07:26:37 PM

Hi,

I'm having trouble doing the arrow pushing for this question:

(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fimg837.imageshack.us%2Fimg837%2F9595%2F85803943.png&hash=e653e69666c5c589abb1d17b76d76045664204d4)


For A, I push one of the two electron pairs on the hydroxy group to an H in H3O and then let the H3O turn to water, but I don't understand how to move the methyl group to the adjacent carbon? Any clarification is appreciated

Title: Re: Acid Catalyzed reaction mechanisms?
Post by: MissPhosgene on February 28, 2011, 08:13:24 PM

For b, can you do alkyl shifts?

Title: Re: Acid Catalyzed reaction mechanisms?
Post by: DannyBoi on February 28, 2011, 08:38:02 PM

Hi again MissPhosgene!

We've not discussed such a concept so I don't think it's an option for this question.

Title: Re: Acid Catalyzed reaction mechanisms?
Post by: MissPhosgene on February 28, 2011, 09:50:59 PM

Oh! Ha, I actually meant a not b. It's really similar to a hydride shift, except an alkyl group moves.

http://science.uvu.edu/ochem/index.php/alphabetical/a-b/12-akyl-shift/

If you google alkyl shift, a bunch of stuff comes up.

Title: Re: Acid Catalyzed reaction mechanisms?
Post by: discodermolide on March 01, 2011, 12:43:41 PM

Quote from: DannyBoi on February 28, 2011, 07:26:37 PM

Hi,

I'm having trouble doing the arrow pushing for this question:

(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fimg837.imageshack.us%2Fimg837%2F9595%2F85803943.png&hash=e653e69666c5c589abb1d17b76d76045664204d4)


For A, I push one of the two electron pairs on the hydroxy group to an H in H3O and then let the H3O turn to water, but I don't understand how to move the methyl group to the adjacent carbon? Any clarification is appreciated

Try forming a cyclopropyl carbonium ion, which then migrates the methyl group to the adjacent carbon atom

Title: Re: Acid Catalyzed reaction mechanisms?
Post by: voidSetup on March 01, 2011, 10:28:45 PM

I think usually if there is an alkyl/hydride shift you need a carbocation intermediate first. So how could that form once the hydroxy group is protonated?

Title: Re: Acid Catalyzed reaction mechanisms?
Post by: discodermolide on March 02, 2011, 02:03:44 AM

Quote from: voidSetup on March 01, 2011, 10:28:45 PM

I think usually if there is an alkyl/hydride shift you need a carbocation intermediate first. So how could that form once the hydroxy group is protonated?

a non-classical carbonium ion is formed, causing the elimination of water.

Title: Re: Acid Catalyzed reaction mechanisms?
Post by: spirochete on March 02, 2011, 02:15:46 AM

Quote from: discodermolide on March 02, 2011, 02:03:44 AM

Quote from: voidSetup on March 01, 2011, 10:28:45 PM

I think usually if there is an alkyl/hydride shift you need a carbocation intermediate first. So how could that form once the hydroxy group is protonated?

a non-classical carbonium ion is formed, causing the elimination of water.

Please don't confuse the poor kid!  With the questions he's asking we don't need to invoke any fancy non classical conbonium ions, even if they do form.  At the very least we can rationalize everything fine with classical (normal carbocations.

To answer OP's question:  Protonation of a hydroxy group means that water can by pushed out of the molecule as a good leaving group.  Now you have a carbocation that can do all kinds of fun things that you know how to push arrows for.  Just draw hydride or alkyl group shifts, then do an E1 mechanism to give you all those products.