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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.

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That is not true. As you noted bromine has a negative charge (and also lower electronegativity than oxygen) and is therefore the better nucleophile (even under these aqueous conditions). Halohydrins are not even usual the result of bromination in water. Special conditions are required to prevent too much $\ce{Br-}$ from attacking the bromonium cation (e.g. certain pH, adding additional reagents as $\ce{AgOH}$ or using $\ce{HOBr}$, see the section Addition Reactions Initiated by Electrophilic Halogen in W. Reusch's online Organic Chemistry book). You would usually expect the dihalide product since $\ce{Br-}$ is already close to the bromonium cation as it forms.

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Under many circumstances, I would agree that bromide is a better nucleophile than water. It's negatively charged and, due to its size, bromine is more polarizable than oxygen.

However, in water, the bromide anion is very stable, reducing its nucleophilicity. I suspect that bromide is still more nucleophilic than water (even considering solvent effect), but combined with the large excess of water, it is more likely that water opens the bromonium ion in the halohydrin formation.

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  • $\begingroup$ Do you mean stabilised by hydrogen bonding? Will not water will be more extensively hydrogen bonded? $\endgroup$ – Reeshabh Ranjan Dec 28 '16 at 17:14
  • $\begingroup$ @ReeshabhRanjan Yes, the water molecules are present in a hydrogen-bonded network. However, the concentration effect is indeed overwhelming. The concentration of pure water is around 55.6 M, certainly larger than the [Br-]. Also note that the hydration effect on the Br- may actually be greater as it is in the form of ion-dipole interactions, which are likely stronger than the hydrogen bonding interactions between water molecules. $\endgroup$ – Tan Yong Boon Dec 30 '18 at 4:48
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If the reaction is an alkene reacting with $\ce{Br2}$ in $\ce{H2O}$, the double bond will behave as a nucleophile attacking one of the Br's. Once the Br is attacked it will also keep the carbocation stable until it is attacked by the $\ce{H2O}$ molecule rather than the $\ce{Br-}$. Since there is so much more water in solution the chances of water attacking is more likely than the negative Br attacking.

Once water attacks the carbocation with the lone pair on oxygen it forms a positive charge. Another water molecule will come and pull off a Hydrogen atom forming a Hydroxy. The product will include a Br and Hydroxy group (Halohydrin Formation).

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