Electrophilic addition of alkenes

[aim1732]

In reaction of alkenes with bromine water the electrophile Br⁺ has the first chance to attack(form a ring cabocation-like species). But the second anti attack(of the nucleophile) is attributed to water instead of Br^-. Can somebody tell me why? Because water is stronger nucleophile? And why should that be? Or perhaps it could be because it is more abundant being the solvent?

 

[DDTea]

The main reason is that there is much more water than there is Br-, so a reaction between the bromonium ion and water is more likely than Br- attacking the bromonium ring.

More than that, though: the Br- is solvated and surrounded by water molecules, which both distributes its charge density over a larger area and makes it a much poorer nucleophile. This makes it even less likely to be able to attack the bromonium ion.

 

[aim1732]

One more thing-which of them is actually the stronger nucleophile?

 

[DDTea]

That's difficult to say because you're comparing atoms in different periodic families, specifically oxygen (the "business end" of the OH(-) group) and Br-. You'd have to evaluate them on the basis of their polarizability (Br- is more polarizable) as well as their size (OH(-) is a lot smaller), but also the solvent (in this case, water; solvents play a huge role in reactions with nucleophiles).

In general, and I don't know how satisfying of an answer this will be to you: nucleophilicity is based in kinetics, so if you want to determine which one is a "stronger nucleophile," you'd have to see which one has a faster reaction rate under some specified conditions.

 

[Yanick]

You can think of water being a better nucleophile because the pKa of hydronium (conjugate acid of water) is -1.7 or 0, depending on who you ask. Whereas the pKa of HBr is -8 or -9. However, as stated above, nucleophilicty/electrophilicity is determined experimentally and the stronger base (weaker conjugate acid) is not always the better nucleophile. If you get stuck on an exam though, this kind of analysis is helpful and you'll probably get the answer correct more often than not.

Otherwise you can think of it this way. Rate depends on concentrations of reacting species. In the flask, once the bromonium intermediate is formed, you have 2 reactions competing. Bromide adding or water adding. Molarity of water, when in aqueous solution, is ~55M. Water wins pretty much by "getting there faster." There is water everywhere, its solvating all ions and when an opportunity arises water will attack.

So its not really the case of better or worse nucleophile, except for the fact that water CAN act as a nucleophile whereas something like carbon tetrachloride cannot and therefore you will get addition of bromide to the bromonium intermediate if CCl4 is used as solvent.

 

[aim1732]

Yes this is something that has always confused me. Nucleophilicity and basicity are not always parallel. One reason I have read is the one mentioned: basicity is an equilibrium characteristic and the nucleophilicity a kinetic one. Are there others?