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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: NewtoAtoms on July 20, 2008, 06:46:14 PM

Title: 1,2 addition and 1,4 addition- electrophilic addition reactions of diens.
Post by: NewtoAtoms on July 20, 2008, 06:46:14 PM

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

Title: Re: 1,2 addition and 1,4 addition- electrophilic addition reactions of diens.
Post by: macman104 on July 20, 2008, 07:23:39 PM

You can't just add the double bond anywhere, you have to follow resonance.  Check out the following link.
http://www.chem.sc.edu/faculty/handy/334Notes/Chapter22.doc

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.