In electrophilic addition to alkenes, which species acts as the electrophile?

• A. The species that accepts an electron pair
• B. The species that donates an electron pair
• C. A radical species
• D. A solvent molecule

Answer: (A)

In electrophilic addition, the key idea is that the electrophile is an electron-pair acceptor. The double bond in the alkene acts as a nucleophile, using its π electrons to attack a species that is electron-poor. The electrophile is typically a positively polarized or electron-deficient partner, such as Br2 in which one bromine becomes Brδ+ and accepts electron density, or a proton (H+). This acceptance of the electron pair by the electrophile opens up the new bond to the alkene, while the leaving part becomes a counterion (like Br−). The other possibilities don’t fit the role: a donor of electron pairs would be the nucleophile, a radical species isn’t required for this classical mechanism, and a solvent molecule is not inherently acting as the electrophile in the standard reaction.