carbon center

Meaning: A carbon center is a particular carbon atom singled out by its role in a molecule or reaction. An electrophilic carbon center is electron poor, because it is bonded to a more electronegative atom (the carbonyl carbon, a carbon bonded to a halogen) or is a carbocation, so nucleophiles attack it. A nucleophilic carbon center is electron rich, as in a Grignard reagent, an enolate or cyanide. The term can also mean a chiral carbon center, bonded to four different groups.

On the MCAT: Passages ask which carbon a nucleophile attacks and how its substitution changes the mechanism. In SN2, the nucleophile attacks a methyl or primary carbon from the back side and inverts the configuration. In SN1, a tertiary carbocation center is planar and sp², so attack from either face can give a racemic mixture. You may also count stereocenters: n centers give at most 2ⁿ stereoisomers.

Common confusion: A carbon with four single bonds is not automatically a stereocenter; it needs four different groups. Also, a carbon is not fixed as electrophilic or nucleophilic, because its polarity depends on its neighbors. The carbon of C=O is electrophilic, while the carbon of C–MgBr is nucleophilic.