nucleophilically attack

Meaning: A nucleophile ("nucleus-loving") is electron-rich, with a lone pair, a negative charge, or π electrons. To attack nucleophilically is to use that pair to build a bond to an electrophile. In curved-arrow drawings, the arrow starts at the lone pair or negative charge and ends at the electron-poor atom. Carbons bonded to O, N, or halogens carry a partial positive charge (δ+) and are common targets. At a carbonyl carbon, the nucleophile adds, the π electrons shift onto oxygen, and a tetrahedral intermediate forms. At an sp³ carbon bearing a leaving group, SN2 attack comes from the side opposite the leaving group and inverts the stereocenter.

On the MCAT: Expect questions asking which atom is attacked, what product forms when a Grignard reagent or hydride adds to an aldehyde or ketone, or how nucleophilic acyl substitution proceeds through a tetrahedral intermediate. Reactivity of acid derivatives generally runs acid chloride > anhydride > ester > amide. For SN2, you predict faster rates with less hindered carbons and strong nucleophiles.

Common confusion: Students reverse the arrow or attack the wrong atom. The nucleophile is the electron donor and bonds to the electrophile. A base removes a proton, while a nucleophile bonds to an atom other than hydrogen, usually carbon.