[kidd101]

Could anyone explain me why when propene reacts with HCL, only one product forms, 2-chloropropane (not 1-chloroporopane)
deeper explanation than "markovnikov rule is obeyed."


thank you,
Mark

[azmanam]

To hell with Markovnikov's rule.  It's a useful mnemonic, but doesn't tell you anything about the mechanism.

The first step of the mechanism is an acid/base reaction between an alkene and a protic acid.  The pi electrons in the alkene act as a base, and the proton on HCl acts as an acid.  The electrons attack the proton and pick up the proton to form a carbocation and a chloride ion.

So the question is, which carbocation is formed?  A primary carbocation could be formed, or a secondary carbocation could be formed.  alkyl groups are electron donating, and alkyl groups help stabilize positive charge.  The primary carbocation has only one alkyl group stabilizing the positive charge.  The secondary carbocation has two alkyl groups stabilizing the positive charge.  The secondary carbocation is more stable than the primary carbocation because two electron donating groups are helping to stabilize the positive charge.

The second step involves attack of the chloride ion on the carbocation.  Because the secondary carbocation forms, the chloride attacks the secondary carbocation, and the primary product of the reaction is 2-chloropropane.

see here:
http://www.cem.msu.edu/~reusch/VirtTxtJml/addene1.htm#add1b


edit: let me clarify a bit.  the net result is that the proton ends up on the less substituted carbon atom, and the other part (chloride here) ends up on the more substituted carbon atom.  This is the definition of Markovnikov's rule.  Addition to alkenes will follow this pattern.  Not because it's a rule.  It's not like the alkene sits around thinking 'Ok, reacting with HCl... better follow Markovnikov's rule.  Let's let chloride attach to the more substituted carbon atom.'  No.  The electrons flow logically, if you know the logic.