Inductive and solvent effects

SOMe the enhancement in the meta-position is almost as large as in the para-position. The authors go on to show the applicability of op (g) values to certain solution processes, particularly those in non-aqueous solvents, but including the dissociation of thiophenols in 48% ethanol, the results of Bordwell and Andersen80 to which reference has been made earlier (Section III.A.1). A separation of field/inductive and resonance effects is also essayed for the gas-phase acidities of the phenols, and SOMe and S02Me feature in the discussion. There is reference to a oR° value of + 0.07 for SOMe as an unpublished result of Adcock, Bromilow and Taft (cf. 0.00 from Ehrenson and coworkers65 and — 0.07 from Katritzky, Topsom and colleagues128.)... 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

Streck and coworkers showed that in a range of solvents, the 13C carbonyl shifts in dialkyl ketones were affected similarly by branching at the a-position.127 In chloroform, the carbonyls of di-tert-butylketone and diisopropylketone were 11-12 ppm downfield of that of acetone, which they attributed to a mixture of inductive and steric effects. With tertiary systems, particularly in dipolar solvents, hindrance to solvent stabilisation of the polar, basic form of the carbonyl offsets the inductive stabilisation of the branched alkyl. 13C NMR data presented here support this. 

Structural and Solvent Effects in the Asymmetric Induction of Chalcone Epoxides... 

Han P, Bartels DM (1994) Encounters of H and D atoms with 02 in water relative diffusion and reaction rates. In Gauduel Y, Rossky P (eds) AIP conference proceedings 298. "Ultrafast reaction dynamics and solvent effects." AIP Press, New York, 72 pp Hasegawa K, Patterson LK (1978) Pulse radiolysis studies in model lipid systems formation and behavior of peroxy radicals in fatty acids. Photochem Photobiol 28 817-823 Herdener M, Heigold S, Saran M, Bauer G (2000) Target cell-derived superoxide anions cause efficiency and selectivity of intercellular induction of apoptosis. Free Rad Biol Med 29 1260-1271 Hildenbrand K, Schulte-Frohlinde D (1997) Time-resolved EPR studies on the reaction rates of peroxyl radicals of polyfacrylic acid) and of calf thymus DNA with glutathione. Re-examination of a rate constant for DNA. Int J Radiat Biol 71 377-385 Howard JA (1978) Self-reactions of alkylperoxy radicals in solution (1). In Pryor WA(ed) Organic free radicals. ACS Symp Ser 69 413-432... 

In constrast, kinetic regioselectivity does not usually correspond to the thermodynamic stability ratio between the two enolates. Indeed, when the ketone is ionised in protic solvents which make equilibration possible, the more substituted enolate is formed (e.g. [45] and [46] are in the ratios 10 90 and 40 60 for the lithium and sodium ion pairs, respectively, in dimethyl ether) (House, 1972). This means that the hyperconjugative effect, which is predominant in the enolate, is less important than inductive and steric effects in the transition state, a result which is in agreement with the carbanion character. The regioselectivity of preparative enolate formation in organic solvents has been reviewed by D Angelo (1976). 

The cumulative evidence from the studies cited here suggests that polarization effects generally are not transmitted through many carbon-carbon electrostatic effects can satisfactorily describe the experimental results. There are other analyses that seem more consistent with the dominance of an inductive effect for studies involving dissociation of positively charged ions or for investigations of the electrostatic theory in solvents of varying polarity. However, any attempt to partition the effect of a substituent into separate inductive and field effects... 

We begin our study of mechanisms in the context of acid-base chemistry in Chapter 3. Acid-base reactions are fundamental to organic reactions, and they lend themselves to introducing several important topics that students need early in the course (1) curved arrow notation for illustrating mechanisms, (2) the relationship between free-energy changes and equilibrium constants, and (3) the importance of inductive and resonance effects and of solvent effects. 

The more advanced, dsmamic coordination or functional approach considers the solute-solvent or ion-solvent interactions as a result of electron pair donor-acceptor interactions, and inductive and mesomeric effects. According to this theory, the equihbriiun between nonionized species and solvent-separated ion pairs is a function of the dsmamic electron pair acceptor and donor properties of the solvent, not simply their static polarity (73,74). Ionization may be affected by a nucleophilic attack of a donor (D) or by an electrophilic attack of an acceptor (A) directed to the substrate R—Y. The strong donor-acceptor interactions first lead to charge redistribution between and within the molecules (pile-up and spillover effects), and ultimately to the heterolytic scission of the covalent R—Y bond (eq. 8) ... 

The kinetics of the aminolysis (Et2NH) of ethyl formate at 45 °C in concentrated ethanol solutions have been described and a mechanism proposed. In kinetic studies on the aminolysis reactions of substituted aliphatic esters in a variety of apro-tic solvents, the reaction rate is strongly affected by inductive and steric effects of substituents in the acyl group, rising more than 10" -fold from cyanoacetate to trifluoroacetate. A study of the reactions between vinyl acetate (28) and diethylamine, piperidine, and morpholine in benzene and DMF has revealed that the corresponding enamines accompany the expected amides (29) secondary reactions, however, yielded a coloured oligomer. ... 

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