Question about Exercise 4.25

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Jinghui Song 2J
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Joined: Wed Sep 21, 2016 2:59 pm

Question about Exercise 4.25

Postby Jinghui Song 2J » Wed Mar 01, 2017 9:42 am

In question 4.25, it asked us to use curved arrows to indicate electron movement in the reaction.
It is a reaction between C2H4 and Br2. I don't understand why one of the Br atom is negatively charged while the other is positively charged, because I don't think there is any electronegativity difference between those two Br atoms.
Can anyone help answer this question?
Thank you in advance!

Andrew Nguyen 3G
Posts: 22
Joined: Wed Sep 21, 2016 2:59 pm

Re: Question about Exercise 4.25

Postby Andrew Nguyen 3G » Wed Mar 01, 2017 9:57 am

There is no negativity difference between the two atoms.

However, ethene has an electron rich double bond. The pi bonds extend outwards from the molecule and effects Br2 upon collision. When the Br2 molecule approaches the electron rich area, it causes "induced polarization". The dispersion forces of the surrounding electron rich area forces a sudden polarization in Br2 as the nucleophile side is attracted to it. This only occurs during collision.

This is further explained in the course reader on page 82.

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