Nucleophilic substitution reactions nucleophile strength

The labels 7a and 7d are used to denote mechanisms which approach, but fail to reach, the extremes of A and D character respectively. In the case 7d, the E-Y bond is perceptibly weakened prior to any significant bonding of Z to E, but EX is never present as a free entity. The label 7a is appropriate where there is evidence of incipient E-Z bond formation while the E-Y bond remains more or less intact. It is often impossible to decide whether a mechanism should be labelled A, 7or 7a, on the strength of the experimental evidence alone. If the intermediate Y—EX —Z is a plausible species, having a right to exist , we may tend to favour A as opposed to 7. If, on the other hand, the intermediate offends our notions about bonding and stability, we are inclined to postulate an 7 mechanism. Thus the well-known nucleophilic substitution reaction at a saturated carbon atom ... 

Use of glycerol as a monomer causes the cross-linking that gives Glyptal its strength. 31.27 Repeated nucleophilic acyl substitution reactions result in the formation of Melmac. 

AUyl compounds are highly reactive towards nucleophiles and examples of dissociative (5 1) and associative (5 2) reactions, and nucleophilic substitution with rearrangement are well documented . Differentiation between these mechanisms when azide ion is the nucleophile is cften extremely difficult due to the possibility of rearrangement of the allylic azide resulting from substitution, and for this reason the relative nucleophilic strength of azide ion in allylic 5n reactions has not been delineated. 

The order of nucleophilic strength of entering ligands [i.e. the order of the rate constants k in equation (21-27)] for substitution reactions on Pt" is... 

The reactions involving 64, 66, and 68 used three different types of nucleophiles, cyanide, an azide, and an alkoxide. The ability to evaluate the relative strength and effectiveness of various nucleophiles is obviously as important for Sn2 reactions as it was for nucleophilic acyl substitution reactions. A general list of nucleophiles in order of their ability to form bonds to sp carbon in a Sn2 reaction is ... 

This order is opposite to the known order for the tran -effect of ligands in substitution reactions of square platinum complexes. Therefore, the result obtained does not agree with the mechanism of usual electrophihc FF substitution in hydrocarbons, where the hydrocarbon molecule acts as a nucleophile, since a normal order of trans-effect would have been expected for a mechanism of this type. The order in the hgand effect is apparently determined by the strength of the covalent Pt-C bond formed and, possibly, of the Pt-H bond as well, both of which are weaker for softer ligands. Thus, the vibrational frequency of the Pt-C... 

The measured half-lives range from 19 sec to 7000 yr, suggesting that structure variation can have significant effects on hydrolysis rates. The reactivity of these chemicals can be rationalized in terms of the limiting mechanisms presented for nucleophilic substitution. It is apparent from the data in Table 2.2 that the fluori-nated aliphatics are much more stable than the chlorinated aliphatics, which in turn are more stable than the brominated aliphatics. This trend in reactivity reflects the strength of the carbon-halogen bond, which follows the order F>Cl>Br, that is broken in the nucleophilic substitution reaction. 

As with nucleophilic substitution reactions, rates of dehydrohalogenation reactions will be dependent on the strength of the C-X bond being broken in the elimination process. Accordingly, it is expected that the ease of elimination of X will follow the series Br>Cl>F. The relative reactivities of Br and Cl toward elimination is evident from the hydrolysis product studies of 1,2-dibromo-3-chloropropane (DBCP Burlinson et al., 1982). DBCP has been used widely in this country as a soil fumigant for nematode control and has been detected in groundwaters (Mason et al., 1981) and subsoils (Nelson, et al., 1981). Hydrolysis kinetic studies demonstrated that the hydrolysis of DBCP is first order both in DBCP and hydroxide ion concentration above pH 7. Below pH 7, hydrolysis occurs via neutral hydrolysis however, the base-catalyzed reaction will contribute to the overall rate of hydrolysis as low as pH 5. Product studies performed at pH 9 indicate that transformation of DBCP occurs initially by E2 elimination of HBr and HCl (Figure 2.4). 

To assess the importance of nucleophilic substitution reactions of naturally occurring nucleophiles it is necessary to have some measure of their reactivity, relative to OH" and HjO. A number of properties of nucleophiles, all of which are some measure of the nucleophile s ability to donate electrons to an electrophile, have been used to correlate nucleophilic reactivity. These closely related properties include basicity, oxidation potential, polarizability, ionization potential, electronegativity, energy of the highest filled molecular orbital, covalent bond strength, and size (Jencks, 1987). 

In platinum(II) substitution reactions, however, the order of nucleophilic strength is I" > Br > Cl and is independent of solvent. This is clearly shown by the results in Table 6.3.1 which compares values of ky for the reaction ... 

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