the reaction most often occurs at an aliphatic sp3 carbon center with an electronegative, stable leaving group attached to it - 'x' - frequently a halide atom. the breaking of the c-x bond and the formation of the new c-nu bond occur simultaneously to form a transition state in which the carbon under nucleophilic attack is pentacoordinate, and approximately sp2 hybridized. the nucleophile attacks the carbon at 180° to the leaving group, since this provides the best overlap between the nucleophile's lone pair and the c-x σ* antibonding orbital. the leaving group is then pushed off the opposite side and the product is formed.