Bromine (Br₂) is brown, and one of the classic tests for alkenes is that they turn a brown aqueous solution of bromine colourless. Alkenes decolourize bromine water: alkenes react with bromine. The product of the reaction is a dibromoalkane, and the reaction on the right shows what happens with the simplest alkene, ethylene (ethene). In order to understand this reaction, and the other similar ones you will meet in this chapterwhere we started talking about reactivity in terms of nucleophiles and electrophiles. As soon as you see a new reaction, you should immediately think to yourself, ‘Which reagent is the nucleophile; which reagent is the electrophile?’ Evidently, neither the alkene nor bromine is charged, but Br2 has a low-energy empty orbital (the Br–Br σ*), and is therefore an electrophile. The Br–Br bond is exceptionally weak, and bromine reacts with many nucleophiles like this.

In the reaction with ethylene, the alkene must be the nucleophile, and its HOMO is the C=C π bond. Other simple alkenes are similarly electron-rich and they typically act as nucleophiles and attack electrophiles.

● Simple, unconjugated alkenes are nucleophilic and react with electrophiles.

When it reacts with Br₂, the alkene’s filled π orbital (the HOMO) will interact with the bro mine’s empty σ* orbital to give a product. But what will that product be? Look at the orbitals involved.

The highest electron density in the π orbital is right in the middle, between the two car bon atoms, so this is where we expect the bromine to attack. The only way the π HOMO can interact in a bonding manner with the σ* LUMO is if the Br₂ approaches end-on—and this is how the product forms. The symmetrical three-membered ring product is called a bromo nium ion.

How shall we draw curly arrows for the formation of the bromonium ion? We have a choice. The simplest way is just to show the middle of the π bond attacking Br–Br, mirroring what we know happens with the orbitals.

But there is a problem with this representation: because only one pair of electrons is moving, we can’t form two new C–Br bonds. We should really then represent the C–Br bonds as partial bonds. Yet the bromonium ion is a real intermediate with two proper C–Br bonds (the box in the margin presents evidence of this). So an alternative way of drawing the arrows is to involve a lone pair on bromine.

We think the first way represents more accurately the key orbital interaction involved, and we shall use that one, but the second is acceptable too. Of course, the final product of the reaction isn’t the bromonium ion. The second step of the reaction follows on at once: the bromonium ion is itself an electrophile, and it reacts with the bromide ion lost from the bromine in the addition step. We can now draw the correct mechanism for the whole reaction, which is termed electrophilic addition to the double bond, because bromine (Br₂) is an electrophile. Overall, the molecule of bromine adds across the double bond of the alkene.

Attack of Br on a bromonium ion is a normal SN₂ substitution—the key orbitals involved are the HOMO of the bromide and the σ* of one of the two carbon–bromine bonds in the strained three-membered ring. As with all SN2 reactions, the nucleophile maintains maximal overlap with the σ* by approaching in line with the leaving group but from the opposite side, result ing in inversion at the carbon that is attacked. The stereochemical outcome of more complicated reactions (discussed below) is important evidence for this overall reaction mechanism. You may wonder why the bromine attacks a carbon atom in the bromonium ion rather than the positively charged bromine atom. Well, in fact, it can do this as well, but the result is just regeneration of bromine and the alkene: the fi rst step of the reaction is reversible.

مواضيع ذات صلة

Enolization is catalysed by acids and bases

Evidence for the equilibration of carbonyl compounds with enols

Tautomerism: formation of enols by proton transfer

Hydration of alkynes

Breaking a double bond completely: periodate cleavage and ozonolysis

Adding two hydroxyl groups: dihydroxylation

Electrophilic addition to alkenes can produce stereoisomers

Hydration of alkynes

Breaking a double bond completely: periodate cleavage and ozonolysis

Adding two hydroxyl groups: dihydroxylation

Electrophilic addition to alkenes can produce stereoisomers

Electrophilic additions to alkenes can be stereospecific