Original: In organic and inorganic chemistry , nucleophilic substitution is a fundamental class of substitution reaction in which an '' electron rich '' nucleophile selectively bonds with or attacks the positive or partially positive charge of an atom attached to a group or atom called the leaving group ; the positive or partially positive atom is referred to as an electrophile .
Simplified: It is a type of substitution , or replacing , reaction . It happens when one nucleophile is replaced by another positive carbon atom .