And nucleofuge

Two new reactivity indices related to electrophilicity and nucleophilicity, electro-fugality and nucleofugality, have recently been introduced by Ayers et al. [59-61]. Electrofugality AEe is defined as... 

The proportion of C-alkylation increases in the order OTs < Br < I, a sequence which is often associated with the balance of hardness between nucleophile and nucleofuge (Smith and Hanson, 1971). The work of Kurts et al. (1974) indicates that the overall reaction rate of the crown ether-assisted alkylation increases in the order Na+ < K+ < Rb+ < Cs+, which, according to these authors, reflects the increasing distance between cation and anion in the ion pairs. The high reactivity of the tetraphenylarsenate also fits in with this picture. The decrease of the kc/k0 ratio is only small in good cation-solvating solvents such as dimethyl sulfoxide (DMSO). Alkylation of the sodium derivative of [103] with ethyl iodide in DMSO gave kc/kQ = 15.7 addition of... 

Hammett s p = 1.07 in MeCN. The Ad -E route is the dominant reaction pathway, as revealed by the effects of the changes in the substituent, solvent, nucleophile and nucleofuge no competitive 5n1 reaction was observed. ... 

For thermoneutral identity reactions, there is no thermochemical driving force. In the case of non-identity nucleophilic substitution reactions - when the nucleophile and nucleofuge are different - reaction exothermicity may be taken quantitatively into account. This can be quite elegantly considered by applying the simple Marcus equation [104-109]. For cationic reactions, where interactions with the neutral nucleophile and nucleofuge are quite weak,... 

Hayami, J., Hihara, N., Kaji, A. Sn2 reactions in dipolar aprotic solvents. IX. An estimation of nucleophilicities and nucleofugicities of anionic nucleophiles studied in the reversible Finkelstein reactions of benzyl derivatives in acetonitrile - dissociative character of the reaction as studied by the nucleofugicity approach. Chem. Lett. 1979, 413-414. 

The more stable the negative charge of the nucleophilic atom, the less nucleophilic it is. For example, due to differing resonance stabilization, an alkoxide is more nucleophilic than a phenoxide, which in turn is more nucleophilic than a carboxylate. This trend reflects the relative basicities, as well. Below we will describe LFERs for nucleophilicity and nucleofu-gality that correlate the pfCgS of the conjugate acids of the nucleophiles and nucleofuges to sensitivity parameters Pnuc and Piq). 

One fact about the use of quaternary ions as phase transfer catalysts should be noted. In general, the large, lipophilic quaternary ions are soft in the HSAB sense [25]. As a consequence, the quat tends to pair with the softest anion available in solution. If both iodide ions and hydroxide ions were present, for example, the quat would pair with iodide. If reaction with hydroxide was desired, the catalyst would be poisoned by the presence of iodide. The source of an ion such as iodide could be from the catalyst originally added or it could be the leaving group in the substitution reaction. The choice of reaction conditions should therefore include a consideration of cation, anion, nucleophile and nucleofuge. 

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