Attacking reagent

The value of k was fixed at 0-5 and the n electron energy when the orbital representing the attacking reagent was positioned near to a particular position in the aromatic nucleus was computed, using values of h var3nng from — 3 to +3. 

In many addition reactions the attacking reagent unlike H2 is a polar molecule Hydro gen halides are among the simplest examples of polar substances that add to alkenes... 

Anti addition (Section 6 3) Addition reaction m which the two portions of the attacking reagent X—Y add to opposite faces of the double bond... 

A further complication arises with Ingold s suggestion" that both the inductive and resonance effects are composed of initial state equilibrium displacements that reveal themselves in equilibrium properties like dipole moments and equilibrium constants and of time-dependent displacements produced during reaction by the approach of an attacking reagent, observed rate effects being resultants of both types of electronic effects. Hammett, however, claims that it is not necessary or possible to make this distinction. 

IV, C, 1, d). Second, for both classes of aromatic compounds such values show a surprisingly small dependence on the nature of the attacking reagent, probably indicating the predominant role of the reorganization of the substrate toward a new state represented by structure 63 or 65. FinaUy, it may not be fortuitous that a correspondence is found between structural effects on substitution rates and on ionization constants (Section IV,C, l,a). Bond-making would in fact be the essential analogy between these phenomena [Eqs. (16) and (17)], and... 

Substitutions. If heterolytic, these can be classified as nucleophilic or electrophilic depending on which reactant is designated as the substrate and which as the attacking reagent (very often Y must first be formed by a previous bond cleavage). 

In nucleophilic substitution the attacking reagent (the nucleophile) brings an electron pair to the substrate, using this pair to form the new bond, and the leaving group (the nucleofuge) comes away with an electron pair ... 

Several distinct mechanisms are possible for aliphatic nucleophilic substitution reactions, depending on the substrate, nucleophile, leaving group, and reaction conditions. In all of them, however, the attacking reagent carries the electron pair with it, so that the similarities are greater than the differences. Mechanisms that occur at a saturated carbon atom are considered first. By far the most common are the SnI and Sn2 mechanisms. 

From what has been said thus far, it is evident that the electrophile in Friedel-Crafts alkylation is a carbocation, at least in most cases. This is in accord with the knowledge that carbocations rearrange in the direction primary — secondary —> tertiary (see Chapter 18). In each case, the cation is formed from the attacking reagent and the catalyst. For the three most important types of reagent these reactions are... 

The iyn-preference of 105b and 105c is similar to those observed in the reduction of the related ketones, 34 and in the epoxidation and dihydroxylation of the related olefins 71 [104]. Although the trajectories of the attacking reagents are considered to be different in these reactions [83-87, 170, 171], all three types of reactions favor iyn-addition, which excludes a predominant role of divergent trajectories in these dibenzobicyclic systems. 

Eq. (4.4 a) is satisfied in the position 1 of azulene. Eq. (4.4b) is valid in position 6 of fulvene, position 6 of azulene, position 3 of fluoranthene, and position 5 of acenaphthylene. Even in a few exceptional cases where the previous relations do not hold, a consideration of the coulombic effect of attacking reagents leads to a conclusion favorable to the hypothesis of frontier density growth. An example of such cases is position 3 of... 

It is found in practice that the value of the steric parameter, Eg, for a particular group, R, differs to some extent from one reaction to another. This is not altogether surprising as both the local environment of R and the size of the attacking reagent will vary from one reaction to another. It means, however, that on incorporating Es into the Hammett type equation, [12], it is necessary to introduce a yet further parameter, 8, as a measure of a particular reaction s... 

The description of chemical reactivity implies, among other aspects, the study of the way in which a molecule responds to the attack of different types of reagents. In order to establish this response, one usually adopts the electronic structure of the molecule in its isolated state as the reference point and considers the effects of an attacking reagent on this state. This procedure leads to the description of what we may call the inherent chemical reactivity of a molecule. 

In Scheme 2.29b the nucleophile is indole (see Chapter 7), acting as an enamine. It seems likely that the size of the attacking reagent in this case is influential in directing addition solely to C-4. The reaction in Scheme 2.29c exemplifies nucleophilic addition followed by a concerted retro-cyclization process. 

Ground-state and excited-state reactions of chiral Meldrum s acid derivatives 39 with the enone function have been reviewed with an emphasis on the facial selectivity in the C=C bond (Figure 2) <1996H(42)861>. Top-face preference, even when it is sterically more hindered than bottom-face attack, is supported by hyperconjugation no —r c=c 39a, whereas bottom-face preference is dominated by steric effects in the sofa conformation of the molecule 39. The trajectory of the attacking reagent plays a balancing role. 

Reactions whose rate is not greatly affected by coordination. Here the reactive site of the ligand is usually quite distant from the coordination center, though a very active attacking reagent can lead to much the same result if it is sufficiently unselective. 

General Order of Rate Constants. The rate constants of electrophilic reactions of aromatic ligands and their metal complexes fall in the order fo, > kML > kffL. The difference between these rate constants becomes greater as the activity of the attacking reagent decreases. When L is a phenolate, HL is the phenol when L is an amine, HL is the corresponding ammonium derivative. The possible synthetic applications of this sequence can be appreciated from the fact that 8-hydroxyquinoline is usually sulfonated with 15 to 30% oleum, while its copper (II) complex can be readily sulfonated in 70% sulfuric add (5). 

If Za is the charge on the attacking reagent, ZB that on the ligand and ZB2 is the charge on the metal ion prior to coordination, then... 

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