Bromine swings both ways when coupling with double bonds

Here's a little chemical example of my favorite dictum: polarization followed by attack, followed by depolarization.

In certain solvents, molecular bromine, Br₂, cracks apart into plus and minus ions: bromide and bromonium (note the cool use of the "ium" suffix for the positively charged ion):

Br—Br  —>  Br^-  +  Br⁺

Now Br₂ will add to ethylene to make ethylene dibromide, a useful pesticide, but the bromine and ethylene can't simply couple because it's forbidden. Instead, bromonium ion approaches ethylene, sucking at and polarizing her most available electrons (recall I deduced that ethylene was female here). Bromonium's negatively charged doppelgänger (umpolung!) then backside attacks the polarized ethylene, after which her electrons collapse into bromonium's clutches and the partners relax. Here's a visual:

Umpolung: The Strange Case of Dr. Proton and Mr. Hydride

Umpolung is a chemistry term meaning "reversal of polarity." This is a useful trick, to change something from having a plus charge to having a minus charge. Hydrogen is the simplest example and shows how the same element behaves differently depending on whether it is cloaked in electrons or not.

The promiscuous proton, H⁺, flits from base to base in water. But hydride (written as H^-), laden with two electrons, usually seeks an electrophile with which to couple irreversibly. If proton and hydride get together, they make little H₂'s.

Hydrogen usually appears on the upper far left side of the Periodic Table sitting above Li, Na, K,.., emphasizing its usual cationic (H⁺) character.  But umpolung explains why it's sometimes useful to think of hydrogen as belonging on the upper right side (next to He), sitting above the halogens, F, Cl, Br,.., emphasizing its anionic H^- = hydride ~ halide character.