Base strength nucleophilicity

There is also evidence for stable 3,4-adducts from the X-ray analysis of 2-amino-4-ethoxy-3,4-dihydropteridinium bromide, the nucleophilic addition product of 2-aminopteridine hydrobromide and ethanol (69JCS(B)489). The pH values obtained by potentiometric titration of (16) with acid and back-titration with alkali produces a hysteresis loop, indicating an equilibrium between various molecular species such as the anhydrous neutral form and the predominantly hydrated cation. Table 1 illustrates more aspects of this anomaly. 2-Aminop-teridine, paradoxically, is a stronger base than any of its methyl derivatives each dimethyl derivative is a weaker base than either of its parent monomethyl derivatives. Thus the base strengths decrease in the order in which they are expected to increase, with only the 2-amino-4,6,7-trimethylpteridine out of order, being more basic than the 4,7-dimethyl derivative. 

Diffusion-limited rate control at high basicity may set in. This is more eommonly seen in a true Br nsted plot. If the rate-determining step is a proton transfer, and if this is diffusion controlled, then variation in base strength will not affect the rate of reaction. Thus, 3 may be zero at high basicity, whereas at low basicity a dependence on pK may be seen. ° Yang and Jencks ° show an example in the nucleophilic attack of aniline on methyl formate catalyzed by oxygen bases. 

The tendency for N-nitrosamides to undergo hydrolysis by a nucleophilic catalysed pathway has been confirmed by studies of N-alkylnitroso acetamides (22) Results summarised in Table I for N -n-butyl-JJ -nitroso acetamide show that its decomposition is also subject to steric constraints (2,6-lutidinestrong nucleophiles (eg. imidazole, thiols) irrespective of their base strength (pK ). Further, the second order dependence on [Imidazole] is more clearly defined for the decomposit-... 

A base is a nucleophile Electronic effects which shift electron density to the atom with the lone-pair increases base-strength. 

The more hindered (37c) is to be preferred the PB is less susceptible to Michael addition and (37c) as well as (37cH) are less nucleophilic than those of the lower esters (see Sect. 14.8.5 for an example). In the absence of side reactions these PBs are, upon workup, converted into the dihydro derivatives that can be reoxidized back to the PBs by bromine or by anodic oxidation [68, 87, 88]. The base strength of (38) can be modified either by substitution [89] or by complexation with alkali metal counterions [86, 89]. 

A molecule with a lone pair of electrons to donate can behave as a nucleophile. The strength of the nucleophile (the nucleophilicity) is often related to basicity. A strong nucleophile is usually a strong base and vice versa. But nucleophilicity and basicity aren t the same. Basicity refers to the ability of a molecule to accept (bond with) an H. The base strength is shown by its equilibrium constant. On the other hand, nucleophilicity refers to the ability of a lone pair of electrons to attack a carbon on an electrophile. 

The lone pair of electrons on the N atom of amines accounts for their base strength and nucleophilicity. They abstract protons from water, react with Lewis acids, and attack electrophilic sites such as carbonyl carbon. 

Steric effects Base strength is relatively unaffected by steric effect, because a base removes a relatively unhindered proton. Thus, the strength of a base depends only on how well the base shares its electrons with a proton. On the other hand, nucleophilicity is affected by the steric effects. A bulky nucleophile has difficulty in getting near the backside of the sp carbon. Therefore, large groups tend to hinder this process. 

What then is the difference between an acid and an electrophile, or between a base and nucleophile No great difference until we try to use the terms in a quantitative sense. For example, if we refer to acid strength, or acidity, this means the position of equilibrium in an acid-base reaction. The equilibrium constant Ka for the dissociation of an acid FIA, or the pKa, is a quantitative measure of acid strength. The larger the value of Ka or the smaller the pKa, the stronger the acid. 

We saw in Chapter 8 (Section 8.3) that the difference between bases and nucleophiles is function. We even saw the difference between the terms nucleophilic-ity (which is a kinetic concept) and basicity (which is a thermodynamic concept). But there is another important difference between the strengths of bases and nucleophiles that you must know. Students rarely see this difference, and it causes them much unnecessary anguish when doing problems that involve substitution and elimination reactions at the same time (we will do problems like this in Chapter 12). Let s avoid the anguish by clearing up the difference now. 

Triflate, tosylate and mesylate are the anions of strong acids. The weak conjugate bases are poor nucleophiles. Nucleophilicity increases in parallel with the base strength. Thus, amines, alcohols and alkoxides are very good nucleophiles. Base strength is a rough measure of how reactive the nonbonding electron pair is thus, it is not necessary for a nucleophile to be anionic. 

However, as a nucleophile s base strength and steric hindrance increase, its basicity tends to be accentuated. If there are abstractable protons at the p-position of the electrophile, an elimination pathway can compete with the nucleophilic substitution. 

The major trend in nucleophilicity is to parallel base strength. However, sometimes differences between basicity and nucleophilicity of a species occur because the two are somewhat different. Basicity measures attack on hydrogen and it is... 

When the surface of fully dehydrated MgO is contacted with ethylene oxide at RT (260), the first surface product formed is the a adduct between the ethylene oxide molecules and the most coordinatively unsaturated Mg2+ ions of the surface, presumably localized on edges, steps, and corners (hereafter Mg ) (Fig. 9 IR bands labeled A) (Scheme 6). This first step is not unexpected since ethylene oxide, with its medium base strength, acts as a surface probe for the most reactive Lewis acid sites (three- and fourfold coordinated Mg2+ ions). This precursor species transforms at RT into a truly chemisorbed species, leading to the formation of a structure plausibly suggested to be the cyclic structure represented below (bands B in Fig. 9) because of the nucleophilic attack of the basic coordinatively unsaturated oxygen ion (O ) in adjacent position on the CH2 group (Scheme 7). 

The strength of the base or nucleophile determines the order of the reaction. 

These complex anions are very weakly basic, i.e. of very low nucleophilicity, and have been used deliberately as counter-ions in the isolation of solid compounds where the cation is highly electrophilic and subject to disproportionation or other reaction with species of reasonable base strength. Relative basicities of anions can be of great importance in isolating such compounds. An example which will be discussed in detail in Sec. 11.3.4.6 involved attempts to synthesise solid compounds of IJ and BrT. These were unsuccessful when the counter-ion was fluorosulfate, because of disproportionation of the halogen cations, whereas the cations were stable in the presence of less basic fluoro-antimonates. 

These reactivities can be explained by the oxidative dimerization potential derived for equation (139) and the base strength of the nucleophile. 

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