[gallionk]

Suppose I was given a reactant (C₁₂H₂₀O reacting with Br₂ and CH₂Cl₂ solvent to get a product with the molecular formula (C₁₂H₁₉OBR). I know how the mechanism for this reaction works. However, why once the bromonium ion is formed the intramolecular attack of the Other nucleophile is favored?

[orgopete]

There can be several factors including concentration and entropy (think intramolecular reactions). For the question being asked, I used to ask which is a stronger base or which can attack a proton better, Br(-) or H2O?

Br(-) + H3O(+)  HBr + H2O

In which direction would the equilibrium lie? Although this is not the same as nucleophilicity, it does hint at this property.