Terminology Definitions. NIST....

Chemical Terminology Definitions

This is a list of standard definitions for chemical terminology relevant to aspects of this database. These definitions were adapted from a number of sources, including the "IUPAC Compendium of Chemical Terminology (Gold Book)" by A. D. McNaught and A. Wilkinson (Blackwell Science: 1997).

In the field of chemical kinetics (and other fields) many "disagreements" often occur simply because one person is using a loose colloquial definition of a term, while another is using a more precise, standard definition. This list is an attempt to readily provide the standard definitions and examples.

More terms, definitions, and examples will be added.

Molecular Structure
Constitution	The constitution of a molecular entity is the description of the identity and connectivity (including corresponding bond multiplicities) of the atoms in a molecular entity (omitting any distinction from their spatial arrangement). (IUPAC 1997) (more) (E)-1,2-Dichloroethene and (Z)-1,2-Dichloroethene are constitutionally identical. They differ only in the spatial arrangement of the chlorine atoms (stereoisomers).
Molecular Entity	A molecular entity is any constitutionally or isotopically distinct atom, molecule, radical, complex, conformer etc., identifiable as a separately distinguishable entity. (IUPAC 1997) (more) see Chemical Species
Chemical Species	A chemical species is an ensemble of chemically identical molecular entities (on the time scale of the observation). (IUPAC 1997) (more) 1,2-Dichloroethane may be considered a chemical species, while (gauche)-1,2-Dichloroethane and (anti)-1,2- Dichloroethane may be considered molecular entities being distinct rotational conformers. Methylene (CH₂) may be considered a chemical species, while Triplet Methylene (³CH2) and Singlet Methylene (¹CH2) may be considered molecular entities being distinct electronic states.
Substituent	A substituent is an atom or group of bonded atoms that can be considered to have replaced a hydrogen atom (or two hydrogen atoms in the special case of bivalent groups) in a parent molecular entity (real or hypothetical). (IUPAC 1997) (more) OH is the substituent in CH₃CH₂OH (Ethanol) and (Phenol) Cl is the substituent in CH₃CH₂Cl (Chloroethane) and (Chlorobenzene) The full definition includes an exception for Group 6 atoms (O, S, etc.) where, for example, CH₃ is NOT considered a substituent in CH₃CH₂OCH₃ (Ethyl methyl ether) relative to the "parent" CH₃CH₂OH (Ethanol). CH₃, however, might be considered a "ligand" or a "group" in this example.
Ligand	Ligands are atoms or groups bound to a "central atom" in a polyatomic molecular entity. The term is generally used in connection with metallic "central atoms". (IUPAC 1994 Phys. Org. Chem.) H, Cl, and CH₃ are the ligands in AlH₃, AlCl₃, and Al(CH₃)₃, respectively. A "ligand" is similar, but more general than "substituent" since H atoms are also considered "ligands."
Group	A group is a defined linked collection of atoms or a single atom within a molecular entity. (IUPAC 1997) (more) Note that a "group" is somewhat more specific or detailed than "substituent" or "ligand". "Substituents" can be composed of a collection of groups. CH₃C(=O)CH₂- may be considered the substituent in (Benzyl methyl ketone) relative to the parent Benzene, and is composed of three groups CH₃-, -C(=O)-, and -CH₂-.
Conformation	A conformation is a distinct spatial arrangement of atoms that may convert to another conformation through rotation about a bond axis. (IUPAC 1997) (more) (gauche)-1,2-Dichloroethane and (anti)-1,2-Dichloroethane are different rotational conformations of the same chemical species. (rotamers)
Stereoisomers	Stereoisomers differ only in the spatial arrangements of their atoms. They are constitutionally identical. (same atoms and bond connectivity). (IUPAC 1997) ( more) (E)-1,2-Dichloroethene and (Z)-1,2-Dichloroethene are stereoisomers. They aretorsional isomers that differ in the position of the groups relative to a reference plane (). (gauche)-1,2-Dichloroethane and (anti)-1,2-Dichloroethane are stereoisomers. They aretorsional isomers that may interconvert by rotation about a single bond ().

Reaction Molecularity
Unimolecular Reaction	A unimolecular reaction is a reaction where there is a single reactant molecular entity that is involved in the microscopic chemical event constituting an elementary reaction. (IUPAC 1997) (more) (C₂H₆ → CH₃ + CH₃ is a unimolecular reaction, while C₂H₆ + CH₃ → C₂H₅ + CH₄ is a bimolecular reaction.
Bimolecular Reaction	A bimolecular reaction is where there are two reactant molecular entities that is involved in the microscopic chemical event constituting an elementary reaction. (IUPAC 1997) (more) C₂H₆ + CH₃ → C₂H₅ + CH₄ is a bimolecular reaction, while C₂H₆ → CH₃ + CH₃ is a unimolecular reaction.

General Chemical Reaction Terminology
Chemical Reaction	A chemical reaction is a process that results in the interconversion of chemical species. Chemical reactions may be elementary reactions or stepwise reactions. (IUPAC 1997) (more)
Elementary Reaction	An elementary reaction is a reaction for which no Reaction Intermediates have been detected or need to be postulated in order to describe the chemical reaction on a molecular scale. An elementary reaction is assumed to occur in a single step and to pass through a single Transition State. (IUPAC 1997) (more)
Microscopic Chemical Event	A microscopic chemical event is a microscopic change involving molecular entities. (IUPAC 1997) (more)
Reaction Intermediate	A reaction intermediate is a molecular entity with a lifetime appreciably longer than a molecular vibration (corresponding to a local potential energy minimum of depth greater than RT) that is formed from the reactants and reacts further to give the products of a chemical reaction. (IUPAC 1997) (more) A Reaction Intermediate is often also termed "a stable intermediate". A Reaction Intermediate is often also defined as "A molecule that is created and destroyed during a particular reaction pathway."
Transition State	A transition state is a state of more positive molar Gibbs energy between the reactants and the products through which an assembly of atoms must pass on going from reactants to products in either direction. A Transition State is often also defined as "An higher energy and unstable configuration between reactants and products."
Reaction Mechanism (or Mechanism)	A reaction mechanism (or equivalently, mechanism) is a detailed description of the process leading from the reactants to the products of a reaction, including a characterization as complete as possible of the composition, structure, energy and other properties of reaction intermediates, products, and transition states.
Reaction Sequence	A reaction sequence is a "suggested mechanism" that is based on incomplete experimental data. For many reactions, all mechanistic information is not available. It is not appropriate to use the term "mechanism" to describe a statement of the "probable" sequence in a set of stepwise reactions. That should be referred to as a "reaction sequence", and not a "mechanism."
Composite Reaction	A composite reaction is a chemical reaction for which the expression for the rate of disappearance of a reactant involves rate constants of more than a single elementary reaction. Opposing Reactions, Parallel Reactions, and Stepwise Reactions are examples of Composite Reactions
Stepwise Reaction	A stepwise reaction is a chemical reaction with at least one reaction intermediate and involving at least two consecutive elementary reactions. An example of a stepwise reaction could be an abstraction followed by a radical elimination, such as C₂Cl₆ + H → C₂Cl₅ + HCl C₂Cl₅ → C₂Cl₄ + Cl
Opposing Reaction	An opposing reaction is a chemical reaction for which the rate of change in a reactant or product involves constants of two "opposed" chemical reactions. The simplest example is where the net rate of disappearance of a reactant is governed simply by the difference between the forward and reverse rates of an overall reaction. For the reaction C₂H₆ + CH₃ → C₂H₅ + CH₄, the rate of disappearance of C₂H₆ is given by d[C₂H₆]/dt = -k_f[C₂H₆][CH₃] + k_r[C₂H₅][CH₄] In this example, although the individual forward and rates may be fast, the observed net rate may be slow, when the overall reaction is near equilibrium.
Parallel Reaction	A parallel reaction is a chemical reaction for which the expression for the rate of disappearance of a reactant is governed by the rate constants relating to several simultaneous reactions to form different respective products from a single set of reactions. The simplest example is where the rate of disappearance is the sum of similar, but different channels (weighted by reaction path degeneracy). For the reaction C₂HCl₃ + H → C₂HCl₂ + HCl, the rate of disappearance of C₂H Cl₃ (trichloroethene) is the sum of two reaction channels forming trichloroethenyl radicals. CHCl=CCl₂ + H → CHCl=CCl* + HCl CHCl=CCl₂ + H → *CH=CCl₂ + HCl

Unimolecular Reaction Types
Homolysis (or Bond Fission or Bond Dissociation)	An homolysis is the cleavage (or fission or dissociation) of a bond so that each of the molecular fragments between which the bond is broken retains one of the bonding electrons. A unimolecular reaction involving homolysis of a bond (not forming a cyclic structure) in a molecular entity containing an even number of (paired) electrons results in the formation of two radicals.
Beta Scission	A beta scission reaction is a chemical reaction where the main feature is the scission of a bond beta (connected to an adjacent atom) to the atom bearing a radical. A unimolecular reaction involving beta scission of a bond in a molecular entity results in the formation of a radical as one product with concomitant formation of an unsaturation in the other product.
Elimination	An elimination is a reaction where the main feature is elimination of two ligands (atoms or groups). In a 1,2-Elimination, the ligands are lost from adjacent centers with concomitant formation of an unsaturation in the molecule. In a 1,n-Elimination (n>2), the ligands are lost from non-adjacent centers which may result in formation of a new ring. In a 1,1-Elimination, the resulting product is a carbene or "carbene analogue."
Molecular Rearrangement	A molecular rearrangement is traditionally any reaction that involves a change of connectivity.
Isomerization	An isomerization is chemical reaction, the principal product of which is isomeric with the principal reactant. An intramolecular isomerization that involves the breaking or making of bonds is a special case of a molecular rearrangement.
Stereoisomerization	Stereoisomerization is isomerization resulting in a different spatial arrangement of atoms without any differences in connectivity or bond multiplicity between the isomers.
Torsional Stereoisomerization	Torsional Stereoisomerization s isomerization resulting in a different spatial arrangement of atoms arising from actual or conceptual torsion about a bond axis (including a double bond).
Rotational Isomerization	Rotational isomerization dis isomerization resulting in a different spatial arrangement of atoms arising from restricted rotation about one single bond.
Migration	A migration is the intramolecular transfer of an atom or group during the course of a molecular rearrangement.
Bond Migration	A bond migration is the movement of a bond to a new position within the same molecular entity during the course of a molecular rearrangement.
Substitution	A substitution reaction is a reaction, elementary or stepwise, in which one atom or group in a molecular entity is replaced by another atom or group.
Displacement (or Nucleophilic Substitution)	A displacement reaction is a reaction in which one atom or group in a molecular entity is replaced by another atom or group contained in an "attacking" nucleophile, which is a molecular entity that brings a pair of electrons (such as in H₂O).

Bimolecular Reaction Types
Abstraction	An abstraction reaction is a chemical reaction where the main feature is the bimolecular removal of an atom from a molecular entity. (C₂H₆ + CH₃ → C₂H₅ + CH₄ is an abstraction reaction.
Metathesis	A metathesis is a bimolecular process formally involving the exchange of a bond (or bonds) between similar interacting chemical species so that the bonding affiliations in the products are identical (or closely similar) to those in the reactants. (This is the definition in physical organic chemistry, requiring only one bond to be exchanged). A metathesis reaction or "double decomposition" reaction is a chemical reaction between two compounds in which parts of each are interchanged to form two new compounds (AB + CD = AD + CB). (This is the definition in inorganic chemistry, requiring two parts to be interchanged).
Disproportionation	A disproportionation is any chemical reaction of the type A + A → A' + A" where A, A' and A" are different chemical species. The splitting of a chemical compound into two new compounds, one more oxidized and one more reduced than the original compound.
Radical Disproportionation	A radical disproportionation is a special case of disproportionation and is loosely described in organic chemistry as a chemical reaction between two radicals where the main feature is the transfer of one hydrogen atom from one chemical species to another with concomitant formation of an unsaturation in the first chemical species.
Hydrogen Exchange Disproportionation	A hydrogen exchange disproportionation is hydrogenation of a molecule by adding hydrogen to an unsaturated bond (making it less unsaturated) by transfer of hydrogen from another hydrocarbon.

References
IUPAC 1997	"IUPAC Compendium of Chemical Terminology" (Gold Book) by A. D. McNaught and A. Wilkinson (Blackwell Science: 1997)
IUPAC 1994 Phys. Org. Chem.	"Glossary of Terms used in Physical Organic Chemistry" (IUPAC Recommendations 1994) by P. Müller. Pure Appl. Chem. 66, 1077-1184 (1994).