The chapter is subdivided into sections detailing the evolution of thinking on structure - mechanism dominated by two factions: Lapworth-Thiele-Robinson and Michael – Flurscheim – Vorlander. The next section then summarizes salient components of these two schools of thought into a generalized electronic theory of organic reactions and structure, ascribing to Ingold much of the credit for providing a "coherent explanation of many chemical reactions outside of free radicals and certain rearrangements". In the next section, the author describes the important contributions of kinetic studies to discover, elucidate, or confirm mechanisms of reactions. This collaboration between what were traditionally considered tools of and concepts of physical chemistry and organic reaction analysis led to the recognition of the new field of organic chemistry. The spread and progress of physical organic chemistry was not extensive i the beginning nor was it smooth with the author alluding to the not-so-subtle rift between practitioners and theorists of the two fields as a possible impediment. The author then gives an account of further attempts at refining chemists' understanding of aromaticity in view of new principles of structure and mechanism. In the last section, attempts to describe and explain in precise language and conceptualization the formation of "ionic" intermediates ("with internal electrostatic arrangement") launched the chemists into an intense "non-classical ion debate" which contributed to further rehashing and, perhaps, refinement and extension of valence theory.
In conclusion, the author states,
"The Ingold revolution was to look for patterns in reactivity and to explain why reactions occurred in the way they did by relating reactivity to molecular structure. This mechanistic approach, like the periodic table in inorganic chemistry, simplified the task of the learner – but at the cost of demanding much more competence in mathematics than was had by Victorian chemists. Even so, by dint of specialization in quantum mechanics, chemists like Ingold and his followers were able to rely a good deal still on geometrical reasoning."
Some notes:
The Lapworth-Thiele-Robinson Tradition
Lapworth made one of the earlier attempts at classifying reactions on the basis of mechanisms in 1898. According to this classification, tautomeric reactions involve the movement of a univalent atom or group while desmotropic reactions involve two labile atoms of groups exchanging positions through a Williamson-Kekule intermediate. Lapworth’s insight on the key property of labile group was that its reactivity stems from its electropositive or electronegative character (ionic). This was supported by experiments in which salts of predicted ionic intermediate were isolated, “building up evidence that ions played a crucial role in reaction mechanisms”. He formulated a general theory of ‘alternative polarities’ in which atoms in a molecule are attributed a latent polarity that can be activated by a key atom. The latent polarity can be activated at a distance (“action at a distance in chemistry”) wherein the reactivity on one site can be induced by the activation of another site (“reactivity at one site influenced or induced action at another”). This theory of inductive effect was “fully enunciated” by Lapworth in 1920.
Johannes Thiele in 1899 developed the theory of residual valency after observing that addition of hydrogen atoms to unsaturated systems caused a shift in the positions of the double bonds, “as if not all the combining power of the carbon atoms at double bonds had been used”. The author compares Thiele’s and Robinson’s take on the explanation for this observation: “Whereas Thiele saw unsaturated atoms as possessing residual combining powers in addition to their normal bonding powers or valency value, Robinson preferred to think of the normal bond as splitting into two (or even fractional) partial valencies.”
The Michael – Flurscheim – Vorlander Tradition
Michael explained Markovnikov’s rule through polarization effects within a molecule that cause some carbon atoms to be more positive than others. H would normally attach to the more negative carbon atom which is typically the C in the C=C that has more H. (Lapworth explained this by invoking the polar inductive effect of the incoming halide atom.) Bernard Flurscheim believed that any carbon chain would have strong and weak links and “if two primary groups in a molecule had a strong affinity for one another, any addenda would be weakly bonded”. He explained benzene substitutions as the transmission within the cyclic chain of affinity demand resulting in changes in affinity. (His ideas were considered a hybrid of Thiele’s and Michael’s and Werner’s “notion of affinity over and above the normal valency value.) Daniel Vorlander’s explanation for substitution patterns in the benzene molecule pertains to different electrical charges arising between positive hydrogen atoms and negative nuclear atoms of the carbon chain. He then argued that the rate of substitution depended on the “relative electrochemical contrast between the incoming substituent and the C atom at the ortho, meta, and para positions.
In the author’s words, similarities are clear between all these theories of “residual affinity, alternative polarities, alterations of affinity intensity, and electrical contrasts”. “By 1920, therefore, organic chemists had a number of competing explanations for the mechanisms of reactions. Each was ad hoc and ultimately dependent upon there being a satisfactory explanation for the induction, or cause, of a polar, or stronger or weaker affinity.”
The Electronic Theory of Organic Reactions
Upon learning of Lewis’ shared electron pair, Robinson recognized that what he and Lapworth have been referring to as “saturated valency was a shared electron pair, a latent valency a free pair, and a virtual valency an incomplete octet”. In 1922, Robinson and Kermack published “An explanation of the property of induced polarity of atoms and an interpretation of the theory of partial valencies on an electronic basis” explaining that the basis of the ‘alternating effect’ is the “facile displacement of electrons in unsaturated systems”. In this paper, Robinson had recognized the “fact that unsaturated atoms share more electrons in common than saturated atoms” and “there will be a greater mobility of electrons”. Kermak and Robinson started the practice of using curved arrows (“the most important symbol in 20th century organic chemistry) to indicate the direction of electron mobility within bonded atoms.
Ingold presented experimental evidence (based on results of selective o m, p-substitution reactions) supporting Flurscheim’s prediction and contradicting Robinson’s and Lapworth’s prediction. These were later found to be anomalous. In 1925 Robinson presented a “coherent” and clarified version of his theory of electronic mechanisms: “ He now suggested that there were two different electronic mechanisms at work in aromatic and conjugative systems: changes in the ‘covalency functioning of electrons’ (the transfer of electron pairs), which he and Kermack had earlier represented by curved arrows, and another effect (which went some way towards a compromise with Flürscheim and Ingold) ‘due to electrostatic induction, the general effect requiring no changes in covalency’.” Robinson would later accuse Ingold of having “appropriated the electrostatic and covalency shift mechanisms”.
Organizing the Structure of Organic Chemistry
Ingold’s writings in the Annual Reports published by the Chemical Society between 1924 and 1928 helped “strengthen the vocabulary of the new electronic theory of organic chemistry”. Ingold’s final major generalization on tautomerism articulated in 1933: “the tautomeric effect represented the chemists’ best attempt to represent, or to capture on paper, the elusive structure of a compound”, renaming the tautomeric effect the ‘mesomeric’ effect (in between) recognizing the limitations of classical structural chemistry. In 1928, Ingold coined the terms ‘nucleophilic’ (electron pair donor) and ‘electrophilic’ (electron pair acceptor), leading to a generalized theory of aromatic substitution: “A nucleophilic (electron-releasing) group like alkyl reduced positive polarization in an aromatic molecule and encouraged electrophilic substituents into the ortho and para positions because of alternating polarity in the ring. Conversely, an electrophilic group, with electron-withdrawing tendencies, like CCl3, strongly retarded ortho – para substitution, but permitted meta.” Ingold’s principles provided a coherent explanation of many chemical reactions outside of free radicals and certain rearrangements.
Frank Whitmore presented a paper in 1932 proposing that all reactions proceeded through an electron deficient intermediate stabilized by 1) bonding to a nucleophile in the solvent, 2) losing a proton or other electrophilic group to form an unsaturated compound, and 3) a group or atom that is more electrophilic than another group, the less electrophilic group could cause an attached group to migrate with its electrons to form a new positive ion which is then stabilized by mechanisms 1 and 2. This new electronic theory of reaction mechanisms assert that “electron-withdrawing groups would slow down aromatic substitution”.
The Kinetics of Mechanism
Some key events in the development of kinetics:
The basic theory of reaction kinetics was developed by van’t Hoff in 1884 (Etudes de dynamique). In this publication, he classified reactions in terms of their molecularity (uni-, bi-, ter-, etc.) depending on how many concentrations are involved in the direct proportionality (linear, square or product of 2 concentrations, etc.). Ostwald distinguished between stoichiometric molecularity (number of reactants involved) and order of reaction (mathematical dependence on the power of the concentration or the product of concentrations). Ferdinand Wilhelmy in 1850 concluded based on results of experiments on sugar inversion that rate of inversion is directly proportional to concentration.
Berthelot and Leon Saint-Gilles in 1862 showed that the rate of reaction between acetic acid and ethyl alcohol was proportion to the concentrations of both reactants and the reaction reaches equilibrium after a certain amount of time.
Hughes studied the kinetics of the halogenations of organic acids and ketones and, using van’t Hoff’s differential method to determine rate order, found that the rate was independent of the halogen concentration. By 1935, based on kinetic studies that shed light on the possible mechanism, Hughes and Ingold have identified 4 different types of substitution and elimination reactions (SN2, SN1, E2, and E1, with each number indicating the number of molecules involved in the rate-limiting step), “codifying a great deal of organic chemistry”. In addition, they studied the effects of solvent polarity, steric hindrance, catalysts, salts, and stereochemistry on rate-mechanism. Their analysis was complemented by data from measurements of dipole moments, and dissociation constants; Hughes’ use of H isotope labeling; and thermodynamics to understand the activated state, all becoming standard tools for the study of organic reaction mechanisms. Not everyone was receptive and Ingold and Hughes received challenges to and criticisms of their kinetic method for determining mechanism.
The Spread of Physical Organic Chemistry
The term physical organic chemistry was first mentioned in a textbook authored by American Chemist Louis P. Hammett in 1940 (in his text Physical Organic Chemistry, Reaction Rates, Equilibria, and Mechanisms) In the preface to his text, Hammett “noted how it had been almost a point of honor with both physical and organic chemists to profess ignorance of the other’s fields [An undercurrent of this slight rivalry still existed when I went to graduate school where my research topic and method was considered as physical organic chemistry]. Monographs on the Physical Aspects of Organic Chemistry and Hammett’s text provided a physical chemistry approach to organic chemistry in British universities by the early 1940’s. “Although some American organic chemists were happily using physical techniques such as polarimetry and electrolysis in the 1920’s, there was a general scorning of thermodynamics, kinetics, and the new quantum mechanics within the organic chemistry community. No doubt there was a kernel of truth in the reputed physical chemists’ view of organic chemists as ‘grubby artisans engaged in an unsystematic search for new compounds’. This was to change in the late 1930’s as Ingold’s mechanistic viewpoint infiltrated textbooks and research programmes.” The author then went on to enumerate the relevant personalities in various universities and research centers that played significant roles in ensuring the sustainability and progress of physical organic chemistry in America that led to “eventual domination by Americans of the subject after 1945”.
Aromaticity
After Kekule deduced the structure of benzene, much research attention was given to discovering rules for predicting and theories for explaining substituent orientation in substitution reactions of benzene and its derivatives. “The advent of electronic theories, of course, allowed Robinson, Ingold, and others to explain the rule and its defects in terms of the inductive and electromeric effects and, more fundamentally, aromaticity itself as a reflection of mesomerism.” In 1925, Robinson “ascribed the uniquely ‘aromatic’ properties to benzene and its analogues to the six extra electrons that produced the ‘stable association which is responsible for the aromatic character’”. In 1931, Huckel applied quantum mechanical calculations to show that electron density in pi orbitals above and below the plane of the hexagon provided the “mesomeric, or resonance, energy that gave the molecule its extra stability by not containing its electrons to simple alternating single and double bonds”. Huckel also established what is now known as Huckel rule to predict aromatic properties in molecules that have 4n+2 (where n = 0, 1, 2, 3,…) pi electrons in a closed ring. Huckel’s rule using concepts of pi electrons, orbitals, and bonding and understanding and explanation of excited states in spectroscopy reinforced the advantages of molecular orbital theory over Pauling’s valence bond theory. In 1935, Kathleen Lonsdale confirmed the planar hexagonal structure of benzene using x-ray crystallography.
• By 1950, “aromaticity was understood as a property of any ‘cyclic compound with a large resonance energy where all its annular atoms take part in a single conjugated system’”.
The Non-Classical Ion Debate
• Michael J. S. Dewar published The Electronic Theory of Organic Chemistry in 1949.
• In 1969, Dewar published The Molecular Orbital Theory of Organic Chemistry in which he noted that any organic chemist who did not understand MO theory would be left “high and dry”.
• By the 1960’s, the inductive effect can be explained by MO theory as being due to the formation of polar sigma bond between carbon atoms in a chain and various substituent groups. “The formation of a positive carbonium ion, or a negative carbanion, brought about the further polarization of pi orbitals, which interacted in turn with the pi electron orbitals of substituents to produce what Ingold had called the electromeric effect.” Dewar preferred to call the intermediate a pi complex consisting of a transient covalent pi bonds wherein a single pair of electrons is shared between three nuclei.
• “Non-classical ion” referred to an intermediate formed containing an “internal electrostatic attachment”. Dewar felt that this and other terms (bridges, synartetic ions…) were confusing and that the phenomena can more precisely be described using pi terminology and symbolism. (In many cases, however, sigma bonding was involved and not pi).
• In the mid-1960’s NMR analysis was able to confirm the presence of non-classical carbocations in a concentrated solution prepared by George Olah using magic acid (intensely strong).
• The continuing debate on the structure and characteristic of these non-classical intermediates raged on but continued to contribute to the expansion and refinement of valence theory.
Conclusion
• The Ingold revolution was to look for patterns in reactivity and to explain why reactions occurred in the way they did by relating reactivity to molecular structure. This mechanistic approach, like the periodic table in inorganic chemistry, simplified the task of the learner – but at the cost of demanding much more competence in mathematics than was had by Victorian chemists. Even so, by dint of specialization in quantum mechanics, chemists like Ingold and his followers were able to rely a good deal still on geometrical reasoning.
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