Formation of chlorohydrins from alkenes using water and Cl2
Description: Alkenes treated with chlorine (Cl2) in the presence of water will form chlorohydrins. The stereochemistry of the products is anti.
Notes: Note that this reaction is Markovnikoff selective (OH ends up on most substituted carbon of the alkene) and the OH and Cl groups have opposite stereochemistry [where possible]
Notes:Note how in examples 1 and 2 the oxygen is attached to the most substituted carbon (Markovnikoff). Note the trans stereochemistry in example 1. Note that the third example this is an example of an intramolecular reaction.
Mechanism: The alkene attacks Cl2 to form a chloronium ion (Step 1, arrows A and B) which is attacked on the more substituted carbon by water (Step 2, arrows C and D). The positively charged oxygen is then deprotonated to give the neutral alcohol.
Notes: Note that it’s actually more reasonable to show water acting as the base in step 3 (it’s a stronger base). Also, although not shown here the oxygen and chlorine should be trans to each other since the chloronium ion undergoes backside attack by the water.