Nucleophilicity, LG, and PG Abilities

SCHEME 7.4 Two possible interactions displayed at the MC stage between the nucleophile and (a) the LG and (b) the o-NO group. Reprinted with permission from Um et ol. [71]. (2012) American Chemical Society. 

Using Mayr s equation, which relates nucleophilicity (N) and electrophilicity ( ) by log A = s( +Af) using kinetic coefficients [68, 85], it may be possible to obtain an electrophilicity order for XDNB derivatives. The empirical electrophilicity numbers obtained are =-14.1 (X=F), -17.6 (X=C1 and Br), and -18.3 (X=I) for the whole family of halides [11, 71]. The theoretical electrophilicity numbers obtained from Equation 7.3 are as follows = 1.31, 1.33, 1.30, and 1.28 for X = F, Cl, Br, and I, respectively. These results show that the electrophilicity index is not in line with the experimental figure even qualitatively. The explanation for this outcome may be traced to the absence of solvation effects that are incorporated in the solvent sensitivity parameter (s) in 

Ma3T s equation. Note that theoretical electrophilicities are obtained from the isolated ground state of molecules. In the next section, we consider this point by examining the electrophile-nucleophile interaction using a natural bond order population analysis. 

SCHEME 7.5 Reaction of chlorobenzenes and chloropyridine derivatives toward piperidine in MeCN at 25°C. is the rate coefficient for each reaction. Data taken from Crampton et al. [12] and [88]—Reproduced with permission from WILEY-VCH Verlag GmbH Co. KGaA, Weinheim. 

In the case of substrates 1 and 3, the comparison provides insights about the different reactivity with respect to a pyridine derivative [12, 88]. The reported reaction rates in MeCN reveal that when there is an NOj group in ortho position, the reaction is 20 times faster than the pyridine derivative. It is very important to stress at this point the role of the solvent as hydrogen bond donor-acceptor. The presence of a o-NO group in the substrate inhibits the formation of hydrogen bonds between the solvent and the substrate, due to the built-in solvation effect [19, 86, 87]. Nevertheless, in pyri-dines and other heterocycles, it may be possible to observe these interactions [89]. This result may 

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