In the reaction of 1-butene with bromine water, the species that acts as the nucleophile is c) a bromide ion.
The reasoning behind this is that nucleophiles are species that donate electron pairs to form chemical bonds. When bromine water, which contains bromine (Br2) and water (H2O), interacts with 1-butene, the bromine molecule undergoes polarization. The double bond in 1-butene can attack one of the bromine atoms, leading to the formation of a cyclic bromonium ion intermediate.
Subsequently, the bromide ion (Br–) from the dissociation of bromine in water acts as a nucleophile, attacking the more substituted carbon of the intermediary bromonium ion, facilitating the addition reaction that ultimately leads to vicinal dibromide.
In contrast, elemental bromine (a) is not a nucleophile; rather, it is the electrophile in this reaction. A hydronium ion (b) typically acts in acid-base reactions, while water (d), while neutral, does not possess the same nucleophilic strength as the bromide ion in this context. Therefore, the correct answer is the bromide ion.