Effects electronic

The electronic effects of various substituents will clearly have an effect on a drug s ionization or polarity. This in turn may have an effect on how easily a drug can pass through cell membranes or how strongly it can bind to a receptor. It is therefore useful to have some measure of the electronic effect a substituent can have on a molecule. 

As far as substituents on an aromatic ring are concerned, the measure used is known as the Hammett substitution constant which is given the symbol a. 

The Hammett substitution constant (a) is a measure of the electron withdrawing or electron donating ability of a substituent and has been determined by measuring the dissociation of a series of substituted benzoic acids compared to the dissociation of benzoic acid itself. 

Benzoic acid is a weak acid and only partially ionizes in water (Fig. 9.7). 

An equilibrium is set up between the ionized and non-ionized forms, where the relative proportions of these species is known as the equilibrium or dissociation constant KH (the subscript H signifies that there are no substituents on the aromatic ring). 

As Ponec has pointed out, infrared measurements should be more sensitive than UPS in differentiating various types of surface sites, and hence particle size effects. When infrared measurements are made, it is found that the characteristic frequency of the adsorbed CO molecule at a constant surface coverage is dependent on the particle shape.2280 As the particle shape is changed from flat (film or single crystal) to curved (small particles) there is a shift to higher (for Cu) or to lower (for group VIII metals) frequency. Ponec has reviewed possible reasons for these changes in vibrational frequency.208 

Sheppard and T. T. Nguyen, Advances in IR and Raman Spectroscopy, Heyden, London, 1978, Vol. 5. 

Gallezot, J. Datka, J. Massardier, M. Primet and B. Imelik, Proc. 6th Int. Congress on Catalysis, London, 1977, The Chemical Society, Vol. 2, p. 696. 

Theoretical calculations, for example, for the adsorption of H2 on transition metals also show that much smaller clusters are required for chemisorption than for bulk properties.229 230 Also, for CO adsorption, extended Hiickel calculations on chains of Ni atoms showed only minor changes when the chain length exceeded 3 Ni atoms.231 This suggests a very small cluster as compared with the adsorption of halogen (see above). If the calculations are reliable this difference may reflect differences in the mode of bonding of CO and halogens.165 

Heats of reaction and bond dissociation energies allow the estimation of the feasibility of homolytic processes, as these are largely — but not solely — governed by thermochemical effects. The quantitative treatment of heterolytic processes, however, presents a far more difficult problem. Basic electrostatic considerations indicate that the dissociation of a covalent bond into positive and negative ions is inherently a highly endothermic process. It will be facilitated by any mechanism that allows dissipation or stabilization of the incipient charges. Chemists have come to differentiate these 

Our objective has been to develop methods that allow the calculation of various electronic parameters such as partial atomic charge, q, electronegativity, x, polarizability, a, for each atom of a molecule. In this way, the values assigned to an atom not only reflect the type of the atom, but also the particular molecular environment into which this atom is embedded (Fig. 18). The electronic parameters assigned to the atoms of a bond will then be used to arrive at a quantitative value for this bond which reflects its reactivity. A detailed description of a reaction will also have to include parameters characteristic of the reagent in order to account for its influences on bond breakage and formation. 

A common feature of the various methods that we have developed for the calculation of electronic effects in organic molecules is that they start from fundamental atomic data such as atomic ionization potentials and electron affinities, or atomic polarizability parameters. These atomic data are combined according to speciflc physical models, to calculate molecular descriptors which take account of the network of bonds. In other words, the constitution of a molecule (the topology) determines the way the procedures (algorithms) walk through the molecule. Again, as previously mentioned, the calculations are performed on the entire molecule. 

Concepts like electronegativity, inductive, resonance effects, etc. have been developed by the chemist to bring order into a wealth of experimental observations. They are of an empirical nature defying an unequivocal theoretical derivation. Any attempt to put these concepts onto a quantitative basis has to face up to this situation. Values calculated for these electronic effects must be compared with experimental data in 

Aryl phosphites were among the first ligands that were extensively studied. The results obtained with triphenylphosphine and triphenyl phosphite are strikingly similar at low ligand concentrations. At higher ligand metal ratios phosphites may retard the reaction. The electronic and steric properties of the phosphite have a large effect on the rate and selectivity of the reaction. 

The results of the early work by Pruett and Smith [4] together with two more recent data [63], are summarised in Table 8.5. We have added the %- and 0-values in this overview, which were not yet known when the catalytic work was reported. 

The trend towards higher linearities breaks down at two instances one involves a rather bulky ligand with a 0-value of 190°, the other one involves hexafluoroisopropyl phosphite that has a very high %-value. Both give rise to unstable rhodium carbonyl complexes either for steric reasons (vide infra) or for electronic reasons. The %-value of hexafluoroisopropyl phosphite is very high indeed and it is thought that this value of 51 means that electronically it is very similar to CO, i.e. a strong electron acceptor. Hence, the propensity of its 

The arrangement of atoms in a molecule is based on attractive and repulsive forces (see Fig. 2.2), as well as the directionality of the bonds, which is determined by the orientation of the bonding orbitals and their desire for maximum overlap. At first glance, the simple mechanical model (see Fig. 2.1) does not explicitly include specific electronic interactions. However, in developing a model that reproduces experimentally derived structural and thermodynamic data, it is inevitable that electronic factors are included implicitly to account for electronic effects responsible for some of the structural and thermodynamic variation present in the data used in the parameterization. Depending on the model used, the electronic effects may not be directly attributable to specific parameters. 

Electrostatic interactions can be used to account for the extra electron density transferred to the metal center from anionic rc-donating ligands such as carboxy-lates. Alternatively, the ensuing effects might be modeled by redefining the force constants related to metal centers coordinated to donors such as carboxylates, since, effectively, the electronic properties of the metal center have changed, allowing it to be defined as an electronically different species[57,65]. As in this ex- 

The Hammett type of correlation fails with aliphatic substrates just as it does with orf/2o-substituted aromatics. Taft provided the first successful correlations of aliphatic reactions by developing quantitatively an early suggestion of Ingold s that 

8rAG terms are additive functions of polar, resonance, and steric effects. 

In the acidic and alkaline hydrolysis rates of the same ester, the steric and resonance effects. re the same. 

The polar effects are much greater in alkaline hydrolysis than in acid hydrolysis. 

The difference - 8RAG cid for an ester hydrolysis should, if these 

The interaction of a light wave and electrons in atoms in a solid was first analysed by H. A. Lorentz using a classical model of a damped harmonic oscillator subject to a force determined by the local electric field in the medium, see Equation (2.28). Since an atom is small compared with the wavelength of the radiation, the electric field can be regarded as constant across the atom, when the equation of motion becomes  

E and P are produced by a light wave oscillating at frequency u but will differ 

Just as the dipole relaxation was described in terms of the real and imaginary components of the relative permittivity, the electronic response can be expressed in terms of the real and imaginary parts of the refractive index, i.e. n is written as n—ik, where k is the extinction coefficient. A propagating light wave is then described by  

The factor Ne2 in Equation (3.69) is the solvation or solvatochromic shift, which is a measure of the displacement of the resonance of an isolated atom, or molecule, when it is placed in a condensed medium. Thus (wg - Ne2/3eom) can be treated as a new resonant frequency Introducing the complex refractive index then leads to the results  

Provided that n - 1 and k are both small, approximate expressions for n and k are  

From photophysical measurements, the recommended best values for the o+ values of stannyl methyl groups are Ph3SnCH2 -0.73(2), Me3SnCH2 -0.81, and (Me3Sn)2CH -1.06.44 However, no systematic studies of chemical reactions or physical properties of organotin compounds appear to have been carried out with the aim of determining elec- 

The data in Table 3-1 indicate that a trialkylstannyl group should have only a weak electron attracting or electron releasing effect whatever the skeleton the group is attached to, or the phenomenon which is being studied. 

In 4-trimethylstannylbenzoic acid, with an observed pKa of 5.98 (estimated with the above data, 6.12) compared with benzoic acid, pKa 4.20, the stannyl substituent is electron releasing. On the other hand, the ESR spectra of the trimethylstannylcyclopen-tadienyl radical (see Section 20.2), and of the trimethylstannylbenzene radical anion (see Section 20.4) indicate that, under these conditions, the stannyl group is weakly electron attracting. 

Compounds with partly filled d-orbitals have two peculiarities  

The electronic ground state that a particular metal center adopts is a function of the chromophore. In many cases the ground state can be derived from chemical knowledge (e.g., octahedral cobalt(III) ( Aig) or tetrahedral Ni(II) ( Tj) complexes). However, based on the molecular mechanics formalism alone, this problem cannot be solved in a general way. Let us consider coordination compounds that are dose to the spin-crossover limit (for example hexacoordinate iron(II) ( A]g/ T2g)). In these cases it is not possible to assign the atom type of the metal center without further information (experimental or theoretical). Therefore, molecular mechanics alone is not always able to predict the structural properties. 

Even with an assumed or experimentally determined knowledge of the electronic ground state (e.g., by ligand field spectroscopy), the modeling of specific electronic effects due to partly filled d-orbitals such as Jahn-Teller effects, trans influences and 

The description of the bonding in ferrocene suggests that there is a higher electron density on the ir-cyclopentadienyl ring compared with that in 

Moreover, acetylation or mercuration of ir-C4H4Co-w-C5H5 results in preferential substitution of the C4-ring. 

In agreement with the above order of reactivities, Friedel-Crafts acylation of C6H6Cr(CO)3 does not proceed very easily. The reduced electron density on the arene ring compared with that in the free ligand, however, enhances its susceptibility to nucleophilic substitution  

As shown on p 211, nucleophilic attack on C7H7M(CO)3 cations proceeds by addition to yield cycloheptatriene complexes. 

In substituted ferrocenes, the substituent affects the reactivity of the substituted ring, the relative reactivity of the two rings, and hence the reactivity of the molecule as a whole. Thus the ease of oxidation of ferrocene is reduced by electron withdrawing, -I, substituents and, in acid media, the relative ease of oxidation is alkylferrocenes ferrocene acyl-ferrocene. A striking example is ferrocenyl-carboxaldehyde which resists 

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This means that the methods developed for the calculation of physicochemical effects can also be used to deepen our understanding of biochemical rcaaions. Clearly, electronic effects within the substrate molecule arc not the only ones determining its reactivity, The binding of the substrate to the enzyme is also influenced... 

Excluding the phenomenon of hyperconjugation, the only other means by which electronic effects can be transmitted within saturated molecules, or exerted by inductive substituents in aromatic molecules, is by direct electrostatic interaction, the direct field effect. In early discussions of substitution this was usually neglected for qualitative purposes since it would operate in the same direction (though it would be expected to diminish in the order ortho > meta > para) as the cr-inductive effect and assessment of the relative importance of each is difficult however, the field effect was recognised as having quantitative significance. ... 

In unsaturated molecules electronic effects can be transmitted by mesomerism as well as by inductive effects. As with the latter, the mesomeric properties of a group are described by reference to hydrogen. Groups which release electrons to the unsaturated residue of the molecule are said to exert a +Af effect, whereas groups which attract electrons are said to exert a —Af effect. In aromatic structures the important feature of an M-substituent is that it influences the 0- and p-positions selectively. 

In a M.o. treatment of the electronic effect of the methyl group it was found necessary to take into account both inductive and hypercon-jugative effects. This treatment is commented on in 9.3 below. 

Differences of electronic effect between 2-amuio and 2-acetamido substituents are also illustrated by ... 

Sandstrom et al. (65) evaluated the Kj value for 4,5-dimethyl-A-4-thiazoline-2-thione (46) in water (Scheme 19) K-j= 10. A-4-Thiazoline-2-thiones are less basic in the first excited state (61) than in the ground state, so application of Forster s cycle suggests that the thione form is even more favored in the first excited state. Huckel molecular orbital (HMO) calculations suggest that electronic effects due to substitution in... 

Deviation includes, in fact, the summation of steric and electronic effects, and basicity is somewhat a useful predictor for properties of complex dyes (solvent sensitivity, isomeric forms of trinuclear dyes) and gives also semiquantitative data for color structure relation (atomic)... 

The quatemization of the nitrogen atom of the thiazole ring (the Menschutkin s reaction) by alkyl halide or methyl tosylate can be used to measure the reactivity of this atom and thus to evaluate steric and electronic effects of ring substituents. 

Although Noyce and Fike have recently found for the solvolysis of 2-thiazolyl-ethyl chlorides analogous modality of substituent electronic effect transmission from position 2 toward position 5 and from position 5 toward position 2(60). a more general conclusion indicates that the... 

An effect that results when two or more atoms or groups rnteract so as to alter the electron drstnbutron rn a system rs called an electronic effect The greater stability of more highly substituted alkenes is an example of an electronic effect... 

The first example of a stereo electronic effect in this text concerned anti elimination in E2 reactions of alkyl halides (Section 5 16)... 

We have seen this situation before m the reaction of alcohols with hydrogen halides (8ection 4 11) m the acid catalyzed dehydration of alcohols (8ection 5 12) and m the conversion of alkyl halides to alkenes by the El mechanism (8ection 5 17) As m these other reactions an electronic effect specifically the stabilization of the carbocation intermediate by alkyl substituents is the decisive factor The more stable the carbo cation the faster it is formed... 

Table 17 3 compares the equilibrium constants for hydration of some simple aldehydes and ketones The position of equilibrium depends on what groups are attached to C=0 and how they affect its steric and electronic environment Both effects con tribute but the electronic effect controls A hydr more than the steric effect... 

Consider first the electronic effect of alkyl groups versus hydrogen atoms attached to C=0 Recall from Section 17 2 that alkyl substituents stabilize C=0 making a ketone carbonyl more stable than an aldehyde carbonyl As with all equilibria factors... 

A striking example of an electronic effect on carbonyl group stability and its rela tion to the equilibrium constant for hydration is seen m the case of hexafluoroacetone In contrast to the almost negligible hydration of acetone hexafluoroacetone is completely hydrated... 

Steric and electronic effects influence the rate of nucleophilic addition to a proton ated carbonyl group m much the same way as they do for the case of a neutral one and protonated aldehydes react faster than protonated ketones... 

Fatty acid synthetase (Section 26 3) Complex of enzymes that catalyzes the biosynthesis of fatty acids from acetate Field effect (Section 19 6) An electronic effect in a molecule that IS transmitted from a substituent to a reaction site via the medium (e g solvent)... 

Induced dipole/mduced dipole attraction (Section 2 17) Force of attraction resulting from a mutual and complemen tary polanzation of one molecule by another Also referred to as London forces or dispersion forces Inductive effect (Section 1 15) An electronic effect transmit ted by successive polanzation of the cr bonds within a mol ecule or an ion... 

Stereochemistry (Chapter 7) Chemistry in three dimensions the relationship of physical and chemical properties to the spatial arrangement of the atoms in a molecule Stereoelectron ic effect (Section 5 16) An electronic effect that depends on the spatial arrangement between the or bitals of the electron donor and acceptor Stereoisomers (Section 3 11) Isomers with the same constitu tion but that differ in respect to the arrangement of their atoms in space Stereoisomers may be either enantiomers or diastereomers... 

Schematic diagram showing how placing a thin layer of highly dispersed carbon onto the surface of a metal filament leads to an induced dipolar field having positive and negative image charges. The positive side is always on the metal, which is much less electronegative than carbon. This positive charge makes it much more difficult to remove electrons from the metal surface. The higher the value of a work function, the more difficult it is to remove an electron. Effectively, the layer of carbon increases the work function of the filament metal. Very finely divided silicon dioxide can be used in place of carbon.

Disubstituted Boranes. Even slight differences in stetic or electronic effects of substituents may have an effect on the hydroboration reaction course. These effects are well demonstrated in disubstituted boranes, and consequentiy a range of synthetically useful reagents has been developed. 

The BDE theory does not explain all observed experimental results. Addition reactions are not adequately handled at all, mosdy owing to steric and electronic effects in the transition state. Thus it is important to consider both the reactivities of the radical and the intended coreactant or environment in any attempt to predict the course of a radical reaction (18). AppHcation of frontier molecular orbital theory may be more appropriate to explain certain reactions (19). 

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