Reaction terminology

Here is a list of terms used in the description of chemical reactions, along with an explanation of their correct uses.

  • Catalyst
    A catalyst is a component in a chemical reaction, which increases the reaction rate by changing the reaction mechanism. A catalyst is not used in the reaction and thus is often effective in sub-stoichiometric amounts.

  • Curly arrow
    A curly arrow is a special type of arrow used to denote the movements of the electrons when describing the reaction mechanism of a chemical reaction.

  • Electrophile
    An electrophile is a species which will accept donation of electrons to create a new bond.

  • Intermediate
    An intermediate, or more precisely a reaction intermediate, is a chemical species which is formed from the reactants but continues to react to form the product(s) or other intermediates. Reactions that contain more than one step will also have reaction intermediates. A reaction intermediate represents a local minimum on a reaction energy diagram.

  • Leaving group
    A leaving group is a part of a molecule which breaks away from the substrate during the reaction, taking with it the electron pair that made up the bond which is being broken.

  • Lone-pair
    A lone-pair is a pair of valence electrons which do not partake in bonding.

  • Nucleophile
    A nucleophile is a chemical species which will donate electrons to an electrophile to form a new bond during a chemical reaction.

  • Product
    A product is the outcome of a chemical reaction.

  • Reactant
    A reactant is the species that is transformed into the product in a chemical reaction. A reactant is also referred to as a substrate.

  • Reaction conditions
    The reaction conditions of a reaction include factors like: The temperature at which the reaction should occur; if any particular atmosphere should be used (for example Argon or Nitrogen for air-free conditions); the pressure the reaction should run at (if different from atmospheric pressure); the stirring speed (if important); the reaction time.

  • Reagent
    A reagent is a substance added to a reaction to cause the chemical transformation of the reactant(s) to occur, or to test for the presence of a particular compound. A reagent is consumed during the cause of the reaction, in opposition to a catalyst, which is not consumed.

  • Solvent
    A solvent is a chemical substance that dissolved a compound, which is then the solute. A solvent is usually a liquid, but can also be a gas or a supercritical fluid. Solvents usually do not partake in a chemical reaction.

  • Substrate
    A substrate is a chemical species that is consumed during a chemical reaction to give a product. A substrate can also be referred to as a reactant; however, in catalytic reactions substrate is the correct term.

  • Transition state
    The transition state of a chemical reaction represents the highest energy point on the path from reactants to products. A transition state is not a stable species and cannot be isolated. A transition state should not be confused with an intermediate!

Correct use of certain terms:
One or more reactants (also referred to as the substrate), dissolved in a solvent, are transformed into products, either spontaneously or through the action of a reagent or a catalyst. If the reaction has multiple steps, then intermediates will form after each step, except for the last one which forms the final product. Every reaction step passes through a transition state, which represents the highest energy point on the path from reactants to products.

The reactants are A and B. They react in the presence of a reagent, catalyst, and solvent. An arrow goes from the reactants to the products. The reagent is above the catalyst and they are both above the arrow. The solvent is below the arrow. The products are C and D.