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The mechanism fixes the faces an addition uses: halogens add anti through a bridged halonium ion, catalytic hydrogen adds syn from a metal surface, and additions through open carbocations show no preference; anti addition is stereospecific.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
You will decide whether an addition is syn or anti, predict cis or trans ring products, order the bromonium mechanism, count new stereocentres, and predict how alkene geometry decides a bromination product.
You know an addition turns a C=C into a C–C with two new groups, and you can follow stereocentres through a mechanism. The two new groups can arrive on the same face or opposite faces, and the mechanism decides which.
Syn addition puts both groups on the same face of the former double bond; anti addition, on opposite faces. A bromonium ion is a three-membered ring of two carbons and a bromine. A stereospecific reaction gives a different stereoisomer from each stereoisomer of the starting material.
Bromine: anti. The pi bond attacks Br2, and the attacked bromine bridges both carbons as a bromonium ion, on one face of the former double bond. That face is now blocked, so bromide must attack from the other face, opening the bridge. The two bromines end up anti: cyclohexene gives trans-1,2-dibromocyclohexane.
Catalytic hydrogenation: syn. H2 and the alkene both sit on the surface of a metal such as palladium or platinum, and the two hydrogens are delivered from that surface to the same face: 1,2-dimethylcyclohexene gives cis-1,2-dimethylcyclohexane.
HBr: no preference. It goes through an open, flat carbocation, which bromide can reach from either face.
Another way: table
Addition stereochemistry.
| Reagent | Via | Result |
|---|---|---|
| Br2, Cl2 | bridged halonium ion | anti |
| H2 on Pd or Pt | metal surface | syn |
| HBr, H2O/H+ | open carbocation | no preference |
Because anti addition is fixed, the geometry of the alkene carries into the product. (E)-but-2-ene with Br2 gives meso-2,3-dibromobutane; (Z)-but-2-ene gives the (2R,3R)/(2S,3S) pair. Each starting isomer gives its own product: the reaction is stereospecific. Chemists use this to set two stereocentres at once, choosing the alkene geometry to choose the outcome.
Addition stereochemistry is unrelated to the reaction pathway. The pathway decides it.
Bromine adds syn because both atoms come from one molecule. The bridge forces anti.
Hydrogenation gives trans products. It gives cis.
E and Z alkenes give the same bromination product. Each gives its own.
Chlorine forms a bridged chloronium ion on one face.
The bridge blocks that face.
Chloride attacks from the opposite face.
Anti addition.
So the product is trans-1,2-dichlorocyclopentane.
On a ring, anti means trans.
Syn or anti?
Syn: both hydrogens from the metal surface.
How do the two new hydrogens end up?
On the same face of the ring, cis to each other.
An alkene adds hydrogen, H2, on a palladium surface. Do the two new groups arrive on the same face, opposite faces, or either?
Match each ring addition to its product.
| trans-1,2-dibromocyclohexane | cis-1,2-dimethylcyclohexane | trans-1,2-dichlorocyclopentane | |
|---|---|---|---|
| cyclohexene + Br2 | |||
| 1,2-dimethylcyclohexene + H2, Pd | |||
| cyclopentene + Cl2 |
Put the steps of bromine adding to cyclohexene in order.
Number the steps in order (write the number in the box):
Bromine adds to cyclohexene to give 1,2-dibromocyclohexane. How many new stereocentres does the addition create?
Answer:
Bromine adds anti to (E)-but-2-ene. The product, 2,3-dibromobutane, can be the meso form or the (2R,3R)/(2S,3S) pair. Which forms?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
For each addition, say whether it is anti, syn, or has no preference.
| stereochemistry | |
|---|---|
| Br2 | |
| H2 on platinum | |
| HCl |
You can predict the stereochemistry of an addition. Tell someone why bromine adds to cyclohexene to give the trans product. Next: additions to triple bonds.
8. Your turn: 1-methylcyclohexene with H2 on platinum, step 3