Formation of chlorohydrins from alkenes using water and Cl2

by James

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.

{ 9 comments… read them below or add one }


what is the mechanism of the last reaction



The mechanism is exactly the same, except the oxygen is attached to the rest of the molecule.


Ryan McCauslin



James Ashenhurst

I use both misspellings of the name. : – )



could you by any chance explain the mechanism of the 3 rxn? trying to get it and I’m not sure if the water that just attacked leaves or if the HCl adds an H to the other OH and then that leaves


T Fizzle

Agree – the mechanism of the 3rd rxn is not clear



I drew a picture of the mechanism of this reaction, see here:


Andrew Geller

it is a *chloronium ion, not a bromonium ion, correct? just a typo, but worth fixing :)



yes, thank you! Fixed.


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