Electron pair transfer in a stable molecule

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Electron pair transfer in a stable molecule

Postby Angela_Park_1F » Wed Mar 08, 2017 12:42 pm

Hi, I know that electrons pairs will move towards electron deficient molecules, or electrophiles. But in the case where a molecule is stable on its own, without addition or subtraction of electrons, why do NU and E reactions still occur? For example in CH3CH=CHCH3, the molecule has a neutral charge, yet the lone pair in the double bond gets transferred to a lone H molecule. Why would electrons want to leave somewhere that is already stabilized?

Cliff Danza 3F
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Re: Electron pair transfer in a stable molecule

Postby Cliff Danza 3F » Wed Mar 08, 2017 1:43 pm

The example you mention is the electrophilic addition reaction with HBr and Butene. This reaction occurs because the Br atom is more electronegative than the H atom, so it draws electrons toward the Br atom, which leaves H with a delta positive. The double bond in butene is an electron rich region due to the overlapping pi bonds, and so if HBr collides with butene with the H atom hitting the double bond, the electrons from the double bond transfer to the H atom, forming a bond with one C, and the electrons from the HBR molecule transfer to the Br atom making it Br-. Then, because the double bond no longer exists, one C will be delta positive (3 bonds). The Br- atom will then form a bond with with this C+, and the charge form this new 2Bromobutane molecule is neutral again.

So, the reason the stable Butene molecule transfers its double-bond electrons is an energetic collision with HBr -- which would need to be at least the Activation Energy value.

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