Electronic induction

A silicon atom might be expected to release electrons inductively, but because of empty 7-orbitals shows the overall character ( + 7 —717). Nitration of trimethylsilylbenzene with nitric acid in acetic anhydride at —10 to o °C gives 25-5,39-8,30-2 and 6-8 %, respectively, of 0-, m-, and /)-nitro-trimethylsilylbenzene and nitrobenzene, with a rate of reaction relative to that of benzene of about 1-5. The figures give no indication of an important conjugative effect. 

A favorite strategem is to compare the reactivities of ortho- and para-substituted aromatic substrates, such as 13 and 14 the electronic (inductive and resonance) effects of the substituent should be about the same in the two cases, but only the o/-f/io-substituted compound can undergo intramolecular catalysis. 

Because of the frequent mutual interference of electronic, inductive, and steric effects, and because of the influence of ring strain, the carbonyl stretching frequency is naturally not an absolute criterion for the methylation course. The heterocyclic systems in question are too diverse for this to hold. Careful inspection of Table I discloses certain deviations from the relationships mentioned. These deviations will now be discussed. 

Note again that halogens, hydroxyl, alkoxyl, and amino groups withdraw electrons inductively and donate electrons by resonance. Halogens have a... 

The most frequently encountered hydrolysis reaction in drug instability is that of the ester, but curtain esters can be stable for many years when properly formulated. Substituents can have a dramatic effect on reaction rates. For example, the tert-butyl ester of acetic acid is about 120 times more stable than the methyl ester, which, in turn, is approximately 60 times more stable than the vinyl analog [16]. Structure-reactivity relationships are dealt with in the discipline of physical organic chemistry. Substituent groups may exert electronic (inductive and resonance), steric, and/or hydrogen-bonding effects that can drastically affect the stability of compounds. A detailed treatment of substituent effects can be found in a review by Hansch et al. [17] and in the classical reference text by Hammett [18]. 

From this point of view it is of interest to examine the consequences of full ther-malization of the classical Drude oscillators on the properties of the system. This is particularly important given the fact that any classical fluctuations of the Drude oscillators are a priori unphysical according to the Bom-Oppenheimer approximation upon which electronic induction models are based. It has been shown [12] that under the influence of thermalized (hot) fluctuating Drude oscillators the corrected effective energy of the system, truncated to two-body interactions is... 

Sn2 attack on the CH2 in (10) is found to proceed at very much the same rate as on that in MeCH2Cl, suggesting that any adverse steric crowding in the T.S. by the bulky C6H5 group is compensated by a small electronic (inductive ) effect promoting reaction. 

Extensive collections of pK values are available in the literature, e.g., [98-101]. It is also possible to predict pK values for a broad range of organic acids and bases using linear free energy relationships based on a systematic treatment of electronic (inductive, electrostatic, etc.) effects of substituents which modify the charge on the acidic and basic center. Quantitative treatment of these effects involves the use of the Hammett Equation which has been a real landmark in mechanistic organic chemistry. A Hammett parameter (a), defined as follows ... 

Several methods have been employed to extract the rate constant of the addition of nucleophiles to the aryl radicals from the kinetics of Sr I reactions. Relative reactivities of two nucleophiles towards the same aryl radical have been obtained from the ratio of the two substitution products after preparative-scale reaction of the substrate with a mixture of the two nucleophiles under photochemical or solvated-electron induction (Galli and Bunnett, 1981). 

The relationship described by equation 4 indicates that most of the variation in carcinogenicity within the series of acyclic nitrosamines can be associated with water-hexane partition coefficients and electronic inductive effects of substituents on the a-carbons. 

Alkyl groups are able to decrease the concentration of positive charge on the carbocation by donating electrons inductively, thus increasing the stability of the carbocation. The greater the number of alkyl groups bonded to the positively charged carbon, the more stable is the carbocation. Therefore, a 3° carbocation is more stable than a 2° carbocation, and a 2° carbocation is more stable than a 1° carbocation, which in turn is more stable than a methyl cation. 

We note that the induced absorption bands mentioned are associated with single or double electronic transitions. Such induced bands are not limited to oxygen similar bands of a few other systems (benzene, for example) were known for some time. However, most of the common diatomic molecules have electronic states that are many eV above the ground state. As a consequence, for those molecules, electronic induced transitions commonly occur in the ultraviolet region of the spectrum where interference from allowed electronic spectra may be strong. Electronic induction is not nearly as common as rovibro-translational induced absorption. 

This dependency underscores the role of the bases in catalase activity (Figure 21) tuning the metal center (see also Scheme 6), assisting in H202 dehydronation (and hence its binding to Mn) and facilitation, via electronic inductive effects, of 0—0 splitting in Mn(H—O—O—H) intermediates. 

The ionization of monoprotic acids and bases and hence their solubility and absorption is dependent on their pK, the pH at which the drug is 50% ionized. First developed by Hammett and published in 1940, the relation between the dissociation constants of benzoic acid derivatives and the longer range electronic (inductive, mesomeric and field) effects is linear and additive. 

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