An electrophile is a species that accepts a pair of electrons to form a new covalent bond.
This is also the definition of a Lewis acid!
The corresponding reactions would be
# underbrace("NH"_3)_(color(blue)("electrophile/acid"))+ underbrace ("B"^-)_(color(blue)("nucleophile/base")) → "H"_2"N"^"-" +"H-B"#
#underbrace ("NH"_3)_(color(blue)("electrophile/acid")) + underbrace ("B"^"-")_(color(blue)("nucleophile/base")) → "H"_2"N"^"-" +"HB"#
#underbrace("CH"_3"OH")_(color(blue)("electrophile/acid")) + underbrace("B"^"-")_(color(blue)("nucleophile/base")) → "CH"_3"O"^"-" + "HB"#
But in the first step in the addition of #"Br"_2# to alkenes, the #"Br"_2# acts as an electrophile acid.
#underbrace("H"_2"C=CH"_2) _(color(blue)("electrophile/acid")) + underbrace("Br"_2)_(color(blue)("nucleophile/base")) → "H"_2stackrel(+)("C")"-CH"_2"Br" + "Br"^-#