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Homework Help: Electrophilic addition, elimination and stuff like that

  1. Sep 13, 2006 #1

    I've made the following assignments and I'll appreciate it if anybody of you can help me with them and correct me if my answer is wrong. Thanks a lot!

    A) Provide reagents that could be used to effect the transformation of 'Stof A' to each of the following compounds (1-5):

    http://img100.imageshack.us/img100/7635/vraag3vf1.th.gif [Broken]

    I thought the answers are:

    1) HBr : direct electrophilic addition
    2) I don't know
    3) water: direct electrophilic addition
    4) HCl en NaOHwater: first HCl reacts in an electrophilic addition , then OH- does a nucleophillic addition
    5) I don't know (perhaps something with epoxides?)

    B) 1) What other compounds might be formed by the double dehydrohalogenation of a vicinal dibromide?

    http://img148.imageshack.us/img148/6725/vraag4di4.th.gif [Broken]

    My guess isk: cis-dibromobut-2-ene, but I've also read somewhate that NH2- (a strong base) could pluck a H somewhere, and then Na will attach to the molecule on the H's place. Is that possible? Can Na attach to the end of an alkene or to the end of the alkyn?

    2) Can you think of a vicinal dibromide for which formation of an alkyne might be highly unfavorable?

    If C-H and C-Br are syn periplanair in the alkene (the intermediate), then that would be unfavorable, so I thought of:

    http://img99.imageshack.us/img99/1361/vraag4bdu5.th.gif [Broken]

    C) Write a detailed mechanism for each of the following reactions. Explain any relevant regio- or stereochemistry

    http://img100.imageshack.us/img100/7089/vraag5jz7.th.gif [Broken]

    a) regioselective process: the H can attach to the other C in the double bond, but that would not happen cause it would yield a primary instable carbocation.

    b)? Perhaps a polymerization?

    d)? I don't know how this molecule behaves in electrophillic addition.

    e) stereoselective: the other Cl ion can only attack from the backside of the ring.

    f) stereoselective: because the C-O bonds are formed on the side of the alkene's pi bonds. Is that correct?

    g)? I don't know

    D) Hydroboration- oxidation of an alkyne proceeds through the same syn addition mechanism as for an alkene. The C-B bond of the resulting vinyl borane is also replaced with retention of configuration in the oxidation step. Form what you know about the mechanism of hydroboration- oxidation of alkenes, what products will be formed by hydroboration-oxidation of the following compounds? (Hint: recall that enols tautomerize easily to carbonyl compounds!)

    a) 3-hexyne
    b) phenylacetylene
    c) 1-hexyne

    I've never heard of this type of reaction actually..

    Thanks for your help!

    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Sep 14, 2006 #2
    for the second one of part A: if you get an alcohol functional on the carbon you need the ketone on you can oxidize the alcohol into a ketone with sodium dichromate in sulfuric acid. [ie you can go from part 5 of a to part 2 of a] To get that alcohol for part 5 one would need to resonate the double bond into the ring and use an epoxide or a two part reaction of 1) BH3 in THF 2) H2O2. This would create the correct stereochemistry across the bond.
  4. Sep 18, 2006 #3
    Could you please explain how to resonate the double bond into the ring
    in part A5 and how BH3 & H2O2 would react, because I can't find it in my literature.

    To get part A2, isn't a keto-enol tautomerization a possibility?

    Could you please help me with the other questions, cause I really don't have a clue!
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