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Reduction of a ketone using zinc in acetic acid

  1. Apr 23, 2012 #1
    I'm gonna be giving a 1st year student chemistry tutoring so I'm going over some of his past exam papers to make sure I know all the stuff but I'm already at a question I can't do:
    http://img571.imageshack.us/img571/5716/26705030.png [Broken]
    So from the formula, I can see that the degree of unsaturation for the compound is 2. Theres at least one C=C bond in there but the oxygen could either be a carbonyl group or an alcohol. Seeing as it reacts with H2SO4, I assume its an alcohol and the reaction it undergoes is an acid catalysed dehydration of the -OH group. This will yield the alkene. Then ozonolysis breaks the alkene into 2 ketones or aldehydes. Its the final part that has me confused. I don't know what this zinc reduction is and how it could convert ketones or aldehydes into a cyclic ketone.
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Apr 24, 2012 #2

    chemisttree

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    See here.
    Also, ozonolysis doesn't produce the ketone directly. The trioxolane it produces requires a reductive workup.
     
    Last edited: Apr 24, 2012
  4. Apr 24, 2012 #3
    Thanks. Ozonolysis is an even more useful reaction than I thought, I didn't know you can get different products depending on the workup.
     
  5. Apr 24, 2012 #4

    chemisttree

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    So, do you have an idea about the structures of A,B and the starting material?
     
  6. Apr 25, 2012 #5
    Yeah, the starting compound is two cyclopentane rings bridged by a sigma bond. There is a hydroxyl group attached to one of the bridge carbons. The acid catalysed dehydration either turns the bridge into a double bond (ozonolysis of this product yields 2 molecules of cyclopentanone) or else turns a neighbouring bond into a double bond (ozonolysis of this one yields a molecule with an aldehyde at one end of the chain, a ketone in the middle and a cyclopentane ring at the other end of the chain). Thats a tricky question for a 1st year exam paper in my opinion. The rest of the questions were fairly straightforward, but that one would have definitely caught me out in first year. I tried explaining this to the guy I was tutoring but he didn't know about acid catalysed dehydrations and was only vaguely familiar with ozonolysis reactions so I just taught him about ozonolysis reactions and moved onto the next question.
     
    Last edited: Apr 25, 2012
  7. Apr 25, 2012 #6

    chemisttree

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    That's what I thought as well and you're right about that being a tricky question for first semester O-chem.
     
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