Nucleophile vs Electrophile

[regina_ho_3A]

Is HI a nucleophile or electrophile?

[Jessica_Kim_3I]

I think it depends on the mechanism. HI can act as an electrophile in electrophilic addition reactions. For example, in an addition reaction with alkenes, the pi electrons of the ethene can attack the H+ in HI since HI is partially negative on I and positive on H. H would be electrophilic, and I think since I would not be taking part in the reaction (until in a later step), you would call HI an electrophile in that first step of the reaction.