A new twist on addition: two halogens, anti
So far in this rung the double bond has acted as a nucleophile, reaching out with its electron-rich pi bond to grab a proton and form a carbocation — that is the heart of electrophilic addition. Now picture rolling that same alkene into bromine, Br2. A bromine molecule has no charge and no obvious electrophilic end, yet it reacts almost instantly, decolourising the orange liquid as it goes. The product is a *vicinal dihalide*: one bromine on each of the two former double-bond carbons, as in CH2=CH2 + Br2 -> BrCH2CH2Br. The interesting part is not the product but how the alkene pulls an electrophile out of a molecule that did not appear to have one.
Here is the trick. As the electron-rich pi bond approaches Br2, it pushes the near bromine's electrons away down the Br–Br bond, *inducing* a temporary dipole — the near bromine turns slightly positive, the far one slightly negative. The pi electrons attack that near, now-electrophilic bromine; the far bromine leaves as bromide. So the alkene manufactures its own electrophile on contact. But instead of forming an open carbocation, something cleverer happens: the bromine that just bonded still has lone pairs, and it swings one of them over to the *other* carbon, bridging across the top like a tent. That three-membered bridged ring, carrying a positive charge on bromine, is the bromonium ion — a halonium ion.
The bridge forces anti addition — and explains halohydrins
That tent-shaped bromonium is the key to everything that follows. The bromide ion that left earlier now comes back as a nucleophile and attacks one of the two ring carbons from *underneath* — the only side it can reach, because the bromine bridge is sitting on top blocking the other face. It is a backside attack, the same inside-out, umbrella-in-a-gust motion you met in SN2, and it opens the ring. The result is geometrically locked: the two bromines end up on *opposite* faces of the former double bond. We call this anti addition, and it is the signature of any reaction that goes through a bridged ion. Open-carbocation additions cannot guarantee this, because a free, flat cation can be hit from either side.
Now swap the solvent for water and a beautiful variation appears. The bromonium ion still forms, but now the most abundant nucleophile around is water, not bromide. Water attacks the bridge and you get a halohydrin: a bromine on one carbon and an OH on the next. Two honest details fall straight out of the mechanism. First, the addition is still anti, for the same backside-attack reason. Second, water does not attack at random — it attacks the *more substituted* carbon of the bridge. Why? Because that carbon carries more of the positive charge (it would be the better carbocation), so the bridge is weaker and more open there, inviting the nucleophile. The result looks Markovnikov-like — OH on the more substituted carbon — but the cause is the lopsided bridge, not a free cation.
halogenation (anti addition via a bromonium bridge):
Br(+) Br
/ \ Br(-) |
C === C ---> C --- C -----> C --- C two Br, opposite faces
(alkene) (bridged |
ion) Br
halohydrin (water as the nucleophile, in H2O solvent):
Br2 / H2O : C=C -> Br on one carbon, OH on the MORE substituted carbon
(still anti; Markovnikov-like, but from the lopsided bridge)Hydrogenation: flattening the bond, syn
The opposite extreme is to add the smallest possible thing — two plain hydrogens — and erase the double bond entirely. Bubble hydrogen gas, H2, over an alkene and nothing happens: the H–H bond is strong and unreactive. Add a finely divided metal such as palladium, platinum, or nickel and the reaction runs smoothly at room temperature, turning the alkene into the matching alkane. This is catalytic hydrogenation, and the metal is doing real chemical work, an everyday case of transition-metal catalysis. The catalyst's surface grips both the H2 (splitting it into two metal-bound hydrogen atoms) and the alkene, holding them close and weakening their bonds — lowering the energy hill rather than changing where the reaction ends up.
Hydrogenation is also a tool with a hidden ruler. Because adding H2 to a more stable alkene releases less heat than adding it to a less stable one, the *heat of hydrogenation* is a clean experimental measure of alkene stability — it is one of the classic numbers behind the stability trends from guide one. On a triple bond the story is richer: an alkyne can be hydrogenated all the way down to the alkane over an ordinary catalyst, or stopped cleanly at the *cis*-alkene with a deliberately poisoned, hobbled catalyst (Lindlar's), because syn delivery of two hydrogens to a triple bond gives the cis double bond. That controllable half-reduction is a small triumph: one reagent choice decides whether you keep a double bond and which geometry it has.
Oxidation: cutting the bond to read the structure
The last family does something the others cannot: it *cuts the molecule apart at the double bond*. Ozonolysis treats an alkene with ozone, O3, then a mild workup, and snaps the carbon-carbon double bond completely, capping each end with an oxygen. Every C=C becomes two C=O. So a single alkene becomes two carbonyl pieces — aldehydes from carbons that carried a hydrogen, ketones from carbons that carried two other carbons. The brilliance is that this runs *backwards* as a detective tool: if you cleave an unknown and get one acetone and one formaldehyde, you can reassemble the original double bond by mentally fusing the two C=O carbons back into a C=C. Ozonolysis turns a structure question into a much easier one about fragments.
The gentler oxidation does not cut at all — it decorates. Dihydroxylation with cold, dilute potassium permanganate (KMnO4) or with osmium tetroxide (OsO4) adds *two* OH groups, one to each double-bond carbon, giving a 1,2-diol (a *glycol*). And here is the elegant part: both oxygens are delivered together from the same bulky metal-oxygen reagent that bridges across one face, so dihydroxylation is a syn addition — the two OH groups land on the *same* side. Notice we now have the full set: bromine adds two halogens anti, hydrogenation adds two H syn, and dihydroxylation adds two OH syn. Same alkene, three different reagents, three different three-dimensional outcomes.
One double bond, a whole toolkit
Step back and the point of this rung snaps into focus. A single carbon-carbon double bond is not one reaction but a junction box: feed it HX and you get Markovnikov hydrohalogenation; feed it water under the right conditions and you get an alcohol; feed it Br2 and you get anti dihalide; in water, a halohydrin; over a metal with H2, the plain alkane; with O3 or OsO4, oxidised pieces. The molecule you build depends entirely on which reagent you reach for. Mapping reagent to product, and product back to starting material, is the everyday craft of synthesis — and ozonolysis especially is half of structure elucidation, the art of reasoning a molecule's skeleton out of its reactions.
Two threads tie the whole toolkit together, and both are worth carrying forward. The first is *regiochemistry* — when the two added groups are different (HX, halohydrin), which carbon gets which? The honest answer is always "follow the charge": whichever pathway puts positive charge on the more stable, more substituted carbon wins, whether that charge lives in an open carbocation or in the lopsided lean of a bridge. The second is *stereochemistry* — anti for a bridged ion, syn for a same-face delivery — which tells you not just what atoms are present but how they sit in space. Read every new alkene reaction through these two questions and the toolkit stops being a list to memorise and becomes a small, predictable logic.