** Organic molecules also undergo substitution, elimination, and addition reactions..
(1) Substitution Reactions
** Substitution is a reaction in which an atom or a group of atoms is replaced by another atom or group of atoms.
** In a general substitution reaction, Y replaces Z on a carbon atom.
** Substitution reactions involve σ bonds: one δ bond breaks and another forms at the same carbon atom.
** The most common examples of substitution occur when Z is hydrogen or a heteroatom that is more electronegative than carbon.
Examples of Elimination Reactions
 Nucleophilic substitution at an sp3 hybridized carbon atom
** Elimination is a reaction in which elements of the starting material are “lost” and a π bond is formed.
** In an elimination reaction, two groups X and Y are removed from a starting material. Two σ bonds are broken, and a π bond is formed between adjacent atoms.
** The most common examples of elimination occur when X = H and Y is a heteroatom more electronegative than carbon.
(3) Addition Reactions
** Addition is a reaction in which elements are added to a starting material.
** In an addition reaction, new groups X and Y are added to a starting material. A π bond is broken and two σ bonds are formed.
**Addition and elimination reactions are exactly opposite. A π bond is formed in elimination reactions, whereas a π bond is broken in addition reactions.
Examples of Addition Reactions
 Electrophilic addition to carbon–carbon multiple bonds