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    • Thread Starter

    Draw the structure of the alkene that would form 1,2-dibromo-3-methylbutane when reacted with bromine.

    This is from an exam paper, but I didn't really understand it.

    I came to the conclusions that it's not made by electrophillic addition, because the original molecule has no C=C bond
    Not elimination as no OH- to accept electrons

    Possible nucleophilic substitution, but then the final product does not contain bromine so ruled this out too.

    Please can anyone help explain this reaction and the formation of the product to me!
    The final product is 3-methylbut-1-ene

    It is electrophillic addition. The original molecule is an alkene, which has a C=C bond......

    1,2-dibromo-3-methylbutane is the final product. It is made from the alkene(which you are working out) and bromine water by electrophillic addition

    You have mixed up the product and reactant, 3-methylbut-1-ene + bromine would undergo electrophilic addition and react to form 1,2-dibromo-3methylbutane.

    Marks awarded (this is typically a 5 marker with one mark awarded for naming the mechanism):

    1) Curly arrow from C=C double bond to Br in Br2
    2) Curly arrow from Br-Br bond to Br (partial negative dipole)
    3) Structure of intermediate carbocation (alpha carbon being carbon number 2 in the chain as this is more stable, but the alternative would also give you the mark)
    4) Curly arrow from lone pair on the Br- ion to the alpha carbon
    • Thread Starter

    Oh wow Thank you both so much, I really thought I'd lost all my AS knowledge haha!


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