Is water a better nucleophile than bromide?

Question

I am a student of biochemistry. My organic chemistry professor was giving a lecture and he talked about halohydrin formation reactions. At one point, he said that "water attacks instead of bromide because water is a better nucleophile than bromide". He repeated that five times or so. A classmate argued that, because water is the solvent in the reaction and is in greater proportion, it will attack faster. But that still doesn't mean water is a better nucleophile, right? Bromide itself has a negative net charge, while water has no net charge, only 2 free electrons. The question is, was my professor wrong or is there something I'm missing? Thank you very much.

Answer 1

In terms of the halohydrin reaction, your friend is correct. The water molecule will attack first because it is the solvent in the reaction, and with water being the solvent, that means there are many more molecules of water than there are bromide ion. If you think in terms of Avogadro's number, a mole of bromide ion contains 6.022 * 10^23 ions of bromide, whereas, if you're using water as a solvent, you can't quantify the number of water molecules in the solvent compared to the number of bromide ions.

Therefore, a water molecule will more than likely attack first, leading to addition of water and a bromine atom. However, in terms of nucleophilicity, a bromide ion is probably a better nucleophile than a water molecule because the bromide ion has a negative charge, and negatively charged species tend to be better nucleophiles in organic chemistry.