(IUPAC Recommendations 1994)


Continue from terms starting with A.


Baldwin's rules; base; basicity; basicity function; bathochromic shift (effect); Bell-Evans-Polanyi principle; benzyne; bifunctional catalysis; bimolecular; binding site; biradical; Bodenstein approximation; bond; bond dissociation; bond-dissociation energy; bond energy (mean bond energy); bond migration; bond number; bond order; borderline mechanism; Bredt's rule; bridged carbocation; bridging ligand; Brønsted acid (Brönsted acid); Brønsted base (Brönsted base); Brønsted relation (Brönsted relation); Bunnett-Olsen equations

Baldwin's rules

A set of empirical rules for certain formations of 3- to 7-membered rings. The predicted pathways are those in which the length and nature of the linking chain enables the terminal atoms to achieve the proper geometries for reaction. The disfavoured cases are subject to severe distortions of bond angles and bond distances. BALDWIN (1976).


A chemical species or molecular entity having an available pair of electrons capable of forming a covalent bond with a hydron (proton) (see Brønsted base) or with the vacant orbital of some other species (see Lewis base). See also hard base, superbase.


For Brønsted bases it means the tendency of a compound to act as hydron (proton) acceptor. The basicity of a chemical species is normally expressed by the acidity of the conjugate acid (see conjugate acid-base pair). For Lewis bases it relates to the association constants of Lewis adducts and [pi]-adducts.

basicity function

See acidity function.

bathochromic shift (effect)

Shift of a spectral band to lower frequencies (longer wavelengths) owing to the influence of substitution or a change in environment. It is informally referred to as a red shift and is opposite to hypsochromic shift (blue shift). IUPAC PHOTOCHEMICAL GLOSSARY (1992).

Bell-Evans-Polanyi principle

The linear relation between energy of activation (Ea) and enthalpy of reaction ([Delta]Hr) sometimes observed within a series of closely related reactions.

Ea = A + B[Delta]Hr

DEWAR (1969), JENCKS (1985).


1,2-Didehydrobenzene (the aryne derived from benzene) and its derivatives formed by substitution. The terms m- and p-benzyne are occasionally used for 1,3- and 1,4-didehydrobenzene, respectively. IUPAC CLASS NAMES (1993).

bifunctional catalysis

Catalysis (usually for hydron transfer) by a bifunctional chemical species involving a mechanism in which both functional groups are implicated in the rate-controlling step, so that the corresponding catalytic coefficient is larger than that expected for catalysis by chemical species containing only one of these functional groups.

The term should not be used to describe the concerted action of two different catalysts ("concerted catalysis").


See molecularity.

binding site

A specific region (or atom) in a molecular entity that is capable of entering into a stabilizing interaction with another molecular entity. An example of such an interaction is that of an active site in an enzyme with its substrate. Typical forms of interaction are by hydrogen bonding, coordination, and ion pair formation.

Two binding sites in different molecular entities are said to be complementary if their interaction is stabilizing.


An even-electron molecular entity with two (possibly delocalized) radical centres which act nearly independently of each other, e.g.

Species in which the two radical centres interact significantly are often referred to as "biradicaloids". If the two radical centres are located on the same atom, the species are more properly referred to by their generic names: carbenes, nitrenes, etc.

The lowest-energy triplet state of a biradical lies below or at most only a little above its lowest singlet state (usually judged relative to kBT, the product of the Boltzmann constant kB and the absolute temperature T). The states of those biradicals whose radical centres interact particularly weakly are most easily understood in terms of a pair of local doublets.

Theoretical descriptions of low-energy states of biradicals display the presence of two unsaturated valences (biradicals contain one fewer bond than permitted by the rules of valence): the dominant valence bond structures have two dots, the low energy molecular orbital configurations have only two electrons in two approximately nonbonding molecular orbitals, two of the natural orbitals have occupancies close to one, etc.

The term is synonymous with "diradical". IUPAC PHOTOCHEMICAL GLOSSARY (1992). See also carbene, nitrene.

Bodenstein approximation

See steady state.


There is a chemical bond between two atoms or groups of atoms in case that the forces acting between them are such as to lead to the formation of an aggregate with sufficient stability to make it convenient for the chemist to consider it as an independent "molecular species". In the context of this Glossary, the term refers usually to the covalent bond. PAULING (1967). See also agostic, coordination, hydrogen bond, multi-centre bond.

bond dissociation

See heterolysis, homolysis.

(In ordinary usage the term refers to homolysis only.)

bond-dissociation energy, D (SI unit: kJ mol-1, or J (per molecule))

The enthalpy (per mole) required to break a given bond of some specific molecular entity by homolysis, e.g. for
CH4 [arrow] H3C. + H.
symbolized as D(CH3-H) (cf. heterolytic bond-dissociation energy). See also bond energy.

bond energy (mean bond energy)

The average value of the gas-phase bond-dissociation energies (usually at a temperature of 298 K) for all bonds of the same type within the same chemical species. The mean bond energy for methane, for example, is one-fourth the enthalpy of reaction for

CH4(g) [arrow] C(g) + 4H(g)

Tabulated bond energies are generally values of bond energies averaged over a number of selected typical chemical species containing that type of bond.

bond migration

See migration.

bond number

The number of electron-pair bonds between two nuclei in any given Lewis formula. For example in ethene the bond number between the carbon atoms is two, and between the carbon and hydrogen atoms is one.

bond order

A theoretical index of the degree of bonding between two atoms relative to that of a normal single bond, i.e. the bond provided by one localized electron pair. In the valence-bond theory it is a weighted average of the bond numbers between the respective atoms in the contributing structures. In molecular-orbital theory it is calculated from the weights of the atomic orbitals in each of the occupied molecular orbitals. For example, in valence-bond theory (neglecting other than Kekulé structures) the bond order between adjacent carbon atoms in benzene is 1.5; in Hückel molecular orbital theory it is 1.67. Other variants of molecular orbital theory provide other values for bond orders.

borderline mechanism

A mechanism intermediate between two extremes, for example a nucleophilic substitution intermediate between SN1 and SN2, or intermediate between electron transfer and SN2.

Bredt's rule

A double bond cannot be placed with one terminus at the bridgehead of a bridged ring system unless the rings are large enough to accommodate the double bond without excessive strain. For example, while bicyclo[2.2.1]hept-1-ene is only capable of existence as a transient, its higher homologues having a double bond at the bridgehead position have been isolated: e.g.

BREDT (1924); see also FAWCETT (1950); WISEMAN (1967); KEESE and KREBS (1972). For an alternative formulation see WISEMAN and CHONG (1969).

bridged carbocation

A carbocation (real or hypothetical) in which there are two (or more) carbon atoms that could in alternative Lewis formulae be designated as carbenium centres but which is instead represented by a structure in which a group (a hydrogen atom or a hydrocarbon residue, possibly with substituents in non-involved positions) bridges these potential carbenium centres. One may distinguish "electron-sufficient bridged carbocations" and "electron-deficient bridged carbocations". Examples of the former are phenyl-bridged ions (for which the trivial name "phenonium ion" has been used), such as (A). These ions are straightforwardly classified as carbenium ions. The latter type of ion necessarily involves three-centre bonding. Structures (C) and (D) contain five-coordinate carbon atoms. The "hydrogen-bridged carbocation" (B) contains a two-co-ordinate hydrogen atom. Hypercoordination, which includes two-coordination for hydrogen and five- but also higher coordination for carbon is generally observed in bridged carbocations. OLAH, SURYA PRAKASH, WILLIAMSON, FIELD, and WADE (1987). See also carbonium ion, multi-centre bond, neighbouring group participation

bridging ligand

A ligand attached to two or more, usually metallic, central atoms. IUPAC INORGANIC NOMENCLATURE (1990).

Brønsted acid (Brönsted acid)

A molecular entity capable of donating a hydron (proton) to a base, (i.e. a "hydron donor") or the corresponding chemical species. For example: H2O, H3O+, CH3CO2H, H2SO4, HSO4-, HCl, CH3OH, NH3. See also conjugate acid-base pair.

Brønsted base (Brönsted base)

A molecular entity capable of accepting a hydron (proton) from an acid (i.e. a "hydron acceptor") or the corresponding chemical species. For example: OH-, H2O, CH3CO2-, HSO4-, SO42-, Cl-. See also conjugate acid-base pair.

Brønsted relation (Brönsted relation)

The term applies to either of the equations

kHA/p = G(KHAq/p)[alpha]

kA/q = G(KHAq/p)-[beta]

(or their logarithmic forms) where [alpha], [beta] and G are constants for a given reaction series ([alpha] and [beta] are called "Brønsted exponents"), kHA and kA are catalytic coefficients (or rate coefficients) of reactions whose rates depend on the concentrations of HA and/or of A-. KHA is the acid dissociation constant of the acid HA, p is the number of equivalent acidic protons in the acid HA, and q is the number of equivalent basic sites in its conjugate base A-. The chosen values of p and q should always be specified. (The charge designations of HA and A- are only illustrative.)

The Brønsted relation is often termed the "Brønsted catalysis law" (or the "Catalysis Law"). Although justifiable on historical grounds, this name is not recommended, since Brønsted relations are known to apply to many uncatalysed and pseudo-catalysed reactions (such as simple proton (hydron) transfer reactions). The term "pseudo-Brønsted relation" is sometimes used for reactions which involve nucleophilic catalysis instead of acid-base catalysis. Various types of Brønsted parameters have been proposed such as [beta]lg, [beta]nuc, [beta]eq for leaving group, nucleophile and equilibrium constants, respectively. See also linear free-energy relation.

Bunnett-Olsen equations

The equations for the relation between lg([SH+]/[S]) + Ho and Ho + lg[H+] for base S in aqueous mineral acid solution, where Ho is Hammett's acidity function and Ho + lg[H+] represents the activity function lg([gamma]S[gamma]H+)/[gamma]SH+ for the nitroaniline reference bases to build Ho.

lg([SH+]/[S]) - lg[H+] = ([Phi] - 1)(Ho + lg[H+]) + pKSH+

lg([SH+]/[S]) + Ho = [Phi](Ho + lg[H+]) + pKSH+

BUNNETT and OLSEN (1966); HAMMETT (1970). See also Cox-Yates equation.


BALDWIN, J. E. (1976), J. Chem. Soc., Chem. Commun., 734-736.

BREDT, J. (1924), Justus Liebigs Ann. Chem., 437, 1-13.

BUNNETT, J. F., and OLSEN, F. P. (1966), Can. J. Chem., 44, 1899-1916, 1917-1931.

DEWAR, M. J. S. (1969), "The Molecular Orbital Theory for Organic Chemistry", McGraw-Hill, New York.

FAWCETT, F. S. (1950), Chem. Rev., 47, 219-274.

HAMMETT, L. P. (1940, 1970), "Physical Organic Chemistry", 1st and 2nd editions, McGraw Hill, New York.

*IUPAC CLASS NAMES (1993). IUPAC: Organic Chemistry Division. Glossary of Class Names of Organic Compounds and Reactive Intermediates Based on Structure. IDCNS and public review; now published in Pure Appl. Chem., 67, 1307-1375 (1995).

*IUPAC INORGANIC NOMENCLATURE (1990). IUPAC: Nomenclature of Inorganic Chemistry, Recommendations 1990, (LEIGH, G. J., Ed.), Blackwell, Oxford.

*IUPAC PHOTOCHEMICAL GLOSSARY (1992). IUPAC: Organic Chemistry Division: Commission on Photochemistry. Glossary of Terms Used in Photochemistry. Draft 1, provisional.

JENCKS, W. P. (1985), Chem. Rev., 85, 511-527.

KEESE, R., and KREBS, E.-P. (1972), Angew. Chem., Int. Ed. Engl., 11, 518-520.

OLAH, G. A., SURYA PRAKASH, G. K., WILLIAMS, R. E., FIELD, L. D., and WADE, K. (1987), "Hypervalent Chemistry", Wiley, New York.

PAULING, L. (1967), "The Chemical Bond", Cornell University Press, Ithaca, New York.

WISEMAN, J. R. (1967), J. Am. Chem. Soc., 89, 5966-5968.

WISEMAN, J. R., and CHONG, J. A. (1969), J. Am. Chem. Soc., 91, 7775-7777.

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