Nucleophiles basicity

These results show that cyclohexenyl iodonium salt readily gives cyclohexenyl cation under poorly nucleophilic/basic conditions. When a stronger nucleophile like bromide is added to the solution of 3, 1-bromocyclohexene 13 is formed as a main product (eq 5). It is noteworthy here that the rate of the reaction is strongly retarded by the added bromide salt as a nucleophilic reagent (Figure 1), although bromide does react with the substrate to give substitution product 13.6... 

The pA a-values quoted for nucleophilic, basic or acidic groups are generally those measured in the course of the work cited, but values have been estimated in a few instances or taken from tables. A small error in pKt affects the calculated EM relatively little in most cases. 

This nucleophilic/basic attack consumes OH (acidification) and releases silicic acid and network-forming and networkmodifying elements. During this alteration step, =Si-0 groups that are formed are eventually reprotonated by surrounding water molecules. The gel layer, which thickens during hydrolysis and extensive... 

The [Co(NH3)sOH]2+-promoted hydrolysis (and that promoted by other exchange-labile anc non-labile metal hydroxide species) of 2,4-dinitrophenyI acetate and 4-nitrophenyl acetate hat been studied in some detail.19 Kinetic studies (25 °C, / = 1.0 M) show that these reactions follov shallow Bronsted slopes (/ = 0.33 and 0.40) for the two substrates which extend over a range o 1010 in nucleophile basicity. A correlation is reported which allows the prediction of reactioi... 

The general base catalysis mechanism, (b) in Scheme 11.4, has a More O Ferrall-Jencks diagram (Fig. 11.4B) which shows that decreasing the nucleophilicity/basicity of the nucleophile (decreasing the energy, , of just the top-left corner) would cause movement of the transition structure coordinates towards the top-left. Resolving this into its components indicates an increase in /9. The value of /3 is related to that of a by a = ft — 1, so that the value of a should become less negative, which is not consistent with the observed results. 

In contrast, oxidations always involve the most nucleophilic (basic, negative-charge density) center (substrate or substrate-base combination) that produces... 

Furthermore, the decisive electrophilicity/nucleophilicity (basicity) balance of the involved 2-benzimidazolyl-center and the pyridine nitrogen, respectively, cannot be predicted. The basicity of the latter can only be used as a surrogate para-... 

The same reasons that made hydroxide ion basic (chiefly that it is unstable as an anion and therefore reactive ) make it a good nucleophile. Basicity is just nucleophilicity towards a proton and nucleophilicity towards carbon must be related. You saw in Chapter 12 that nucleophilicity towards the carbonyl group is directly related to basicity. The same is not quite so true for nucleophilic attack... 

Any nucleophile basic enough to remove the ortho proton can carry out this reaction. Known examples include oxyanions, amide anions (R2N"), and carbanions. The rather basic alkoxide t-butoxide will do the reaction on bromobenzene if the potassium salt is used in the dipolar aprotic solvent DMSO to maximize reactivity. 

Nature makes use of this property by having imidazole groups attached to proteins in the form of the amino add histidine and using them as nucleophilic, basic and acidic catalytic groups in enzyme reactions (this will be discussed in Chapters 49 and 50). We use this property in the same way when we add a silyl group to an alcohol. Imidazole is a popular catalyst for these reactions. 

In contrast to acidity characterization with basic probes, the use of acidic molecules to probe surface basicity is far less satisfactory. In fact, all acidic (or electrophilic) molecules (Table 3.12) also contain accessible nucleophilic (basic) atoms. It seems impossible to find a molecule that actually only interacts specifically with basic sites. On the other hand, metal oxides that display significant surface basicity... 

Monomer Structures, Ring Strains and Nucleophilicities (Basicities)... 

Since the electrons of the carbonyl double bond hold together atoms of quite different electronegativity, we would not expect the electrons to be equally shared in particular, the mobile tt cloud should be pulled strongly toward the more electronegative atom, oxygen. Whatever the mechanism involved, addition of an un-symmetrical reagent is oriented so that the nucleophilic (basic) portion attaches itself to carbon, and the electrophilic (acidic) portion attaches itself to oxygen. 

In amidines transamination is possible,especially useful for such purposes are imidoylimid-azolides, e.g. (341 equation 172). In some cases it is usefiil to prepare amidines by ring opening of suitable heterocycles, which can be achieved by treatment with amines or other nucleophilic (basic) compounds. Examples are the synthesis of amidines firom 1,3,5-oxadiazinium salts (342), 3-amino-l,2-benzisothiazoles (343), 2-ethoxycarbonyl-3,l-benzoxazin-4(4//) one (344), 1,2,5-oxa-diazolo[3,4]pyrimidine 1-oxides (pyrimidofuroxans 345) and l,2,4,6-thiatriazenium-5-olate 1,1-dioxides (345) as shown in Scheme 58. 

The reactions of furansulfonyl chlorides with anilines (entry 11)123 in MeOH yield a linear Bronsted plot with fix =0.51, which indicates an Sw2 mechanism rather than a stepwise process. This value is quite similar to those found for the reactions of anilines with benzenesulfonyl chloride (0.63)117, with 2-thiophenesulfonyl chloride (0.53)124 and with 3-thiophenesulfonyl chloride (0.54)125. Thus their anilinolysis mechanism is also expected to be S/y2. The reaction rate therefore depends not only on the nucleophile basicity but also on the substrate reactivity. Comparison of the reaction rates leads to the following reactivity order for the Ar moiety benzene > 3-thiophene > 3-furan > 2-furan > 2-thiophene. This reactivity sequence follows the order of the resonance interaction between the... 

---