What Alkene Bromination Really Looks Like

Cyclopentene + Br₂: watch a bromine bridge form on one face of the ring, then bromide attack from the other face. Alkene bromination (electrophilic addition), step by step on a computed reaction path. 🧪 Explore it in 3D (rotate it, step through it, see the orbitals): https://justbasicchemistry.com/bromin... The C=C double bond's π electrons reach out and grab one bromine of Br₂. At the same moment that bromine swings a lone pair onto the other carbon and the Br–Br bond breaks, with both electrons going to the other bromine. The result is a bromonium ion, a three-membered ring with the positive bromine bridging both carbons, plus a bromide ion. The bridge blocks one face of the ring, so the bromide can only attack a carbon from the back. The ring opens and the two bromines end up on opposite faces: trans-1,2-dibromocyclopentane. The pink curly arrows and yellow electron pairs show where the electrons go, just like the arrows you draw in organic chemistry. 🎨 Atoms ⚫ Carbon (C) ⚪ Hydrogen (H) 🟤 Bromine (Br), the big dark red-brown ones (The faint molecules in the background are solvent.) 🧪 Molecules • Cyclopentene, C₅H₈ (the alkene) • Bromine, Br₂ • Bromonium ion (the three-membered ring intermediate) and bromide ion, Br⁻ • trans-1,2-Dibromocyclopentane (product, formed as a 50:50 mix of mirror images) ⏱️ What to watch for 0:00 Cyclopentene and a Br₂ molecule 0:02 Step 1: the C=C π bond attacks one bromine, the Br–Br bond breaks 0:04 The bromonium ion: bromine bridging both carbons (+), with bromide (−) nearby 0:11 Step 2: bromide attacks from the back face and the bridge opens 0:14 trans-1,2-Dibromocyclopentane: one bromine on each face of the ring Why trans: with cycloalkenes only the trans dibromide forms (OpenStax Organic Chemistry, §8.2). If an open carbocation formed instead of the bridged bromonium ion, bromide could attack from either face and some cis product would appear. It doesn't. Going deeper: in dichloromethane, the usual solvent, the rate depends on the Br₂ concentration squared. A second Br₂ helps pull the leaving bromide away as tribromide, Br₃⁻ (Bellucci & Chiappe, J. Org. Chem. 1993). Our movie uses a single Br₂ to keep it clear. The motion of the atoms was computed with quantum chemistry (GFN2-xTB with an implicit solvent model) by driving exactly the bonds the mechanism says change and letting every other atom respond. It isn't a transition-state calculation. The curly arrows and electron pairs are drawn on top to show the electron flow. Music: original. #chemistry #organicchemistry #bromination #alkene #electrophilicaddition #bromonium #stereochemistry #science #STEM #orgo #chemistrystudent