organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...

, the Baker–Nathan effect is observed with

reaction rate The reaction rate or rate of reaction is the speed at which a chemical reaction takes place, defined as proportional to the increase in the concentration of a product per unit time and to the decrease in the concentration of a reactant per u ...

s for certain

chemical reaction A chemical reaction is a process that leads to the chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...

s with certain substrates where the order in reactivity cannot be explained solely by an inductive effect of

substituents In organic chemistry, a substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety (chemistry), moiety in the resultant (new) molecule. The suffix ''-yl'' is used when naming organic compounds that conta ...

. This effect was described in 1935 by John W. Baker and W. S. Nathan. They examined the

chemical kinetics Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is different from chemical thermodynamics, which deals with the direction in which a ...

for the reaction of

pyridine Pyridine is a basic (chemistry), basic heterocyclic compound, heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom . It is a highly flammable, weak ...

with benzyl bromide to form a pyridinium salt, and a series of benzyl bromides having different

alkyl group In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl group is derived from a cy ...

s as

substituent In organic chemistry, a substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. The suffix ''-yl'' is used when naming organic compounds that contain a single bond r ...

s at the para position. : The reaction is facilitated by electron-releasing substituents (the inductive effect) and in general the observed order (with decreasing reactivity) is

tert-butyl In organic chemistry, butyl is a four-carbon alkyl radical or substituent group with general chemical formula , derived from either of the two isomers (''n''-butane and isobutane) of butane. The isomer ''n''-butane can connect in two ways, giv ...

> isopropyl > ethyl >

methyl In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula (whereas normal methane has the formula ). In formulas, the group is often abbreviated as ...

. The observed order in this particular reaction however was methyl > ethyl > isopropyl > tert-butyl. In 1935 Baker and Nathan explained the observed difference in terms of a

conjugation Conjugation or conjugate may refer to: Linguistics *Grammatical conjugation, the modification of a verb from its basic form *Emotive conjugation or Russell's conjugation, the use of loaded language Mathematics *Complex conjugation, the change o ...

effect and in later years after the advent of hyperconjugation (1939) as its predecessor. A fundamental problem with the effect is that differences in the observed order are relatively small and therefore difficult to measure accurately. Other researchers have found similar results or very different results. An alternative explanation for the effect is differential solvation as orders invert on going from the solution phase to the gas phase. Today, the conjugation of neighbouring pi orbitals and polarised sigma bonds is known as hyperconjugation. Numerous anomalous physical measurements, including bond lengths and dipole moments, have been examined through this concept. The original formulation of the Baker–Nathan effect is no longer employed due to more logical reasons for rate accelerations in solutions and its historical context is discussed by Saltzman. ''John William Baker and the origin of the Baker-Nathan effect'' Martin D. Saltzman Bull. Hist. Chem.; 2012; 37(2); 82–90.

References

{{DEFAULTSORT:Baker-Nathan effect Physical organic chemistry