Identify the reagents shown in bold in the following equations as nucleophiles or electrophiles:
$(a) \ CH_{3}COOH + \mathbf{HO^{-}} \to CH_{3}COO^{-} + H_{2}O$
$(b) \ CH_{3}COCH_{3} + \mathbf{CN^{-}} \to (CH_{3})_{2}C(CN)OH$
$(c) \ C_{6}H_{6} + \mathbf{CH_{3}C^{+}=O} \to C_{6}H_{5}COCH_{3} + H^{+}$

Electrophiles are electron-deficient species and can accept an electron pair. Nucleophiles are electron-rich species and can donate an electron pair.
$(a) \ CH_{3}COOH + HO^{-} \to CH_{3}COO^{-} + H_{2}O$
Here,$HO^{-}$ acts as a nucleophile because it is an electron-rich species.
$(b) \ CH_{3}COCH_{3} + CN^{-} \to (CH_{3})_{2}C(CN)OH$
Here,$CN^{-}$ acts as a nucleophile because it is an electron-rich species.
$(c) \ C_{6}H_{6} + CH_{3}C^{+}=O \to C_{6}H_{5}COCH_{3} + H^{+}$
Here,$CH_{3}C^{+}=O$ (acylium ion) acts as an electrophile because it is an electron-deficient species.