Reactions and Reaction Mechanisms



Reactions in organic chemistry are divided into many different types, including acid-base, nucleophilic addition, nucleophilic substitution, electrophilic addition, electrophilic substitution, and free radical reactions. Reactions are classified by the type of reaction that occurs, such as elimination or substitution. Each type of reaction consists of one or more mechanistic steps. Each classification of reaction will have the same or similar mechanistic steps. For example, a substitution reaction will follow a similar mechanistic pathway regardless of the reactant and reagent involved in the reaction. All reactions involve breaking bonds and forming new bonds. Curved arrows are a directional shorthand that indicate the flow of electrons from electron-rich sites to electron-poor sites. Mechanistic steps include proton transfer, nucleophilic attack, loss of a leaving group, and rearrangement. Some mechanistic steps are reversible.

At A Glance

  • There are many types of organic reactions: acid-base reactions, nucleophilic addition and substitution reactions, electrophilic addition and substitution reactions, and radical reactions. There are many parts to a reaction, including starting materials, products, reaction arrows, reagents, solvents, and conditions.
  • Acid-base reactions involve the reaction of an acid with a base, which involves a proton transfer (Brønsted-Lowry acid-base reaction) or electron pair transfer (Lewis acid-base reaction).
  • Addition and elimination reactions are two types of reactions that involve an alkene or alkyne. Addition reactions add a group in place of an alkene or alkyne, and elimination reactions remove a group to form an alkene or alkyne.
  • Radical reactions involve homolytic cleavage, in which a bond breaks and each atom gets one electron.
  • Mechanisms show how a reaction proceeds through a series of fundamental steps. Curved arrows show the movement of electrons in these steps. The arrows always move from nucleophile to electrophile, never electrophile to nucleophile.
  • Reaction mechanisms show the mechanistic steps required to convert reactants into products.
  • There are four main steps in mechanisms: proton transfer, nucleophilic attack, loss of leaving group, and rearrangement.
  • Mechanisms are combinations of the four main mechanistic steps to show how reactants are converted into products. Many mechanisms are reversible, meaning they can go from reactants to products or from products to reactants.