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Where do bonds form?

Posted: Sun Mar 01, 2015 3:06 pm
by Glenda Marshall DIS 3M
I am slightly confused how to know where bonds form in organic reaction mechanisms. For example, if we look at the reaction between propene, CH3CHCH2, and hydrogen bromide, HBr, how do you know which carbon the hydrogen bonds with once the sigma bond between H and Br breaks? The answer key shows it bonding with the carbon on the right, but I don't know how they got that. Thank you!

Re: Where do bonds form?

Posted: Sun Mar 01, 2015 3:29 pm
by Chem_Mod
Adding H to the end carbon results in a secondary carbocation as the intermediate, meaning that the carbon bearing the positive charge is attached to two other carbons. Adding H to the central carbon instead results in a primary carbocation.

A secondary carbocation is more stable, since more electrons from neighboring carbons can flow in to stabilize the positive charge. So the mechanism that happens is the one which forms the secondary carbocation, which is when the H is attached to the end carbon.

This concept of adding HX across unsymmetrical alkenes is called Markovnikov's Rule, which says:
1. the "H" adds to whichever carbon that already has more H
2. the "X" adds to whichever carbon that is more substituted (has less H)

Re: Where do bonds form?

Posted: Wed Mar 04, 2015 8:53 pm
by Justin Le 2I
So in this case, does the Br add onto the central C?

Re: Where do bonds form?

Posted: Wed Mar 04, 2015 10:22 pm
by Chem_Mod
Yes, the end product will have Br on the more substituted carbon (center and not the end)