Halogenation
Halogenation - A triangle reaction
Halogenation of alkenes
Alkenes can undergo a halogenation reaction. This is a reaction with Cl2 or Br2 to make vicinal dichlorides or dibromides. This reaction must be run in an inert, non-reactive solvent like CCl4.
If one looks closely, the two halogens end up on neighboring carbon atoms which makes the product a vicinal dihalide. The third reaction, the bromination of cyclopentene, clearly shows the two bromides of the product are on opposite sides of the ring to each other in a trans configuration. This addition where the two groups end up on opposite sides is called anti-addition. This anti-addition occurs because a halonium intermediate is formed in the course of the reaction.
Halonium ions
The mechanisms for the chlorination or bromination of alkenes are identical. Both are triangle reaction. Review that analogy here so this explanation makes sense.
Bromination is shown here. When the alkene reacts with Br2, a triangular bromonium ion is produced with Br- as the leaving group. This Br- does a backside attack at either carbon atom of the bromonium ion. In this triangle reaction, the Br- would attack using pathway (b) more often than (a) since this carbon is more substituted and would have a greater partial positive charge.
Halohydrin formation - a triangle reaction
Before we get into this, if you do not remember the triangle reaction analogy, review it here.
If the solvent used in a bromination or chlorination reaction is not inert but instead is slightly nucleophilic, different products are formed. As an example, a bromohydrin is formed if water is present in the bromination reaction. In this mechanism, a bromonium ion is formed as before. But, if water is the solvent, there are numerous water molecules for every one Br- nucleophile. Water ends up being the nucleophile that attacks. In this triangle reaction, a “+” sign is being held up. So, the weak H2O nucleophile attacks the more substituted carbon atom where more of the + money is held in the pocket under the bags. Once another water molecule cleans it up by deprotonation, it leads to the bromohydrin.
If Cl2 and water are used, a chlorohydrin is formed. These halohydrins are molecules with neighboring halogens and hydroxyl groups.
If Cl2 and water are used, a chlorohydrin is formed. These halohydrins are molecules with neighboring halogens and hydroxyl groups.
Halogenation of alkynes
If an alkyne with two pi-bonds is halogenated, the addition can happen twice.
The first addition of X2 to the alkyne results in a dihaloalkene in a mixture of E and Z isomers.
But, the alkene pi-bond further reacts to form tetrahaloalkanes if there is an excess of halogen.
5. Draw the products of the following reactions.
a)
b)
c)
d)
Answers
5.
a)
b)
c)
d)
