Substitution reactions illustrating operation

In what follows we will be concerned with the rates of ionic reactions under nonequilibrium conditions. We shall use the term nucleophile repeatedly and we want you to understand that a nucleophile is any neutral or charged reagent that supplies a pair of electrons, either bonding or nonbonding, to form a new covalent bond. In substitution reactions the nucleophile usually is an anion, Y 0 or a neutral molecule, Y or HY . The operation of each of these is illustrated in the following equations for reactions of the general compound RX and some specific examples ... 

The above systematics of substitution reaction mechanisms are automatically deducibie from the complete chemical set of entities for any atom (constructed by us above). To demonstrate this, it is sufficient to note that the universal operator for all the ligand-electron networks described in the preceding section precisely corresponds to all the classical mechanisms for mono- and bimolecular substitution at a saturated atom. To facilitate understanding of this operator, and the canonical designations of the corresponding mechanisms, we illustrate it in Fig. 4.23 for a typical stable hydrocarbon (the methane molecule). Naturally, the conclusions arrived at can be readily extended to include any other E—X bonds and D reagents. 

The lignin-degrading peroxidases operate by generating hydrogen peroxide, which dissociates into hydroxyl radicals that react with lignin by free radical substitution reactions to break down the network structure of the highly crosslinked, three-dimensional polymer. This reaction sequence is illustrated below for a representative section of the complex lignin macromolecule ... 

When the reagent functions exclusively as a nucleophile (and not as a base), only substitution reactions will occur (not elimination), as illustrated in Figure 8.26. The substrate will determine which mechanism operates. Si,j2 will predominate for primary substrates, and S l will predominate for tertiary substrates. For secondary substrates, both Sivf2 and S l pathways are viable, although Si,j2 is generally favored. The rate of an Sn2 process can be further enhanced by using a polar aprotic solvent, as described in Section 7.8. 

In the example (Expt 6.79) the reaction of the diazonium salt from o-chloroaniline with benzene to yield 2-chlorobiphenyl is illustrative. It should be noted, however, that when the liquid aromatic compound in which substitution is to occur is of the type ArZ, the directive influences which are used to explain electrophilic substitution processes are not operative. Thus irrespective of the nature of the substituent Z, ortho-para substitution predominates this result supports the assumption that the substitution process is radical in type. Although the classical reaction occurs in a two-phase system, the use of the more stable diazonium fluoroborates together with the phase-transfer catalyst 18-crown-6 can sometimes be more convenient. The literature method for the preparation of 4-chlorobiphenyl in this way is given as a cognate preparation in Expt 6.19 ... 

A main objective of this work is to develop the relationship between the many reaction pathways leading to ligand substitution at square-planar molecules. Concentrating on more recent results to illustrate the processes under discussion, we examine in detail the evidence for operation of the less common and sometimes controversial routes such as dissociative ligand exchange (6). It cannot be stressed too much, however, that the field is still dominated by associative reactions, so to maintain a balance, as well as to provide the now necessary comparative evidence, we also cover the essential features of nucleophilic ligand replacements. 

In Eq. (a) K is the associative constant for Cr with [cis-Ru(NHj) Cl2]. This form is compatible with either a scheme III mechanism or that illustrated in Fig. lA, but because [CrCl] is the product, the outer-sphere path is not operative. Another way to establish this is to use the calculated value of the ratio of reactivity of Cr to V for outer-sphere reagents 0.02. Because the reaction of with [cis-Ru(NHj) Cl2] takes place by an outer-sphere mechanism (it is too fast to allow substitution on either metal center) with k = 9.8 X 10 M s , a value of the outer-sphere reactivity of Cr with cis-Ru(NHj) Clj] can be calculated to be ca. 2 X lO M s . Another correlation suggests lO M s . If the mechanism of Fig. lA is correct, the required outer-sphere reactivity of Cr with this Ru(III) complex must be 7 X 10 M s clearly this system requires scheme III kinetics, with the pattern shown in Fig. IB. Some other reactions between metal ions also exhibit this, or a slight modification of this, pattern of reactivity ... 

Reactions 25.14 and 25.15 illustrate the trans-eSect in operation cis- and traHx-[PtCl2(NH3)2] are prepared cally by different substitution routes. ... 

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