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Author Topic: 1,2 addition and 1,4 addition- electrophilic addition reactions of diens.  (Read 14692 times)

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Hello Organic Chemists. 

Please consider the following reaction:

          CH3                                   Br  CH3                       CH3
           l            HBr                       l   l                                                     l
H2C=C-C-CH2  ------>             H3C-C-C=CH2        +   H3C-C=C-CH2Br
       l                                           l                                                     l
       CH3                                      CH3                                                 CH3

2,3-dimethyl,1,3-budadiene       3-bromo-2,3-dimethyl-1-butene           1-bromo,2-3-dimethyl 2-butene
                                            PRODUCT 1,2 addition                         PRODUCT 1,4 addition

I understand that for product 1,2 addition the H and Br are added to adjacent carbons.
However for the product 1,4 addition.  How do I know where to place the double bond?!?!?  Really I could place it pretty much anywhere as long as I respect the same amount of H atoms as in the beginning reagant. 

Can anyone explain where I decide to place the double bond in the 1,4 addition product???

Thank you so much!!!

New to Organic Chemistry


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You can't just add the double bond anywhere, you have to follow resonance.  Check out the following link.

Read through the first few pages and you'll see how the reaction procedes.

Remember, first you add hydrogen, to form a carbocation (which is allylic), then it resonates, and you can add to the other end of the molecule.

Also, you forgot a double bond in your diagram.

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