Basicity proton-transfer

Let us first discuss the four proton transfers in class II. We sec that in the last column all four values lie near 0.016 electron-volt, indicating that the value of Jel is nearly the same in all four cases. On the other hand, we notice from Table 9 that, for both glycine and alaniue, t.he value of Kn is a hundred times smaller than I i. We must ascribe this to the presence of a larger in the basic, proton transfer. According to (143) this implies in (130) a greater value of 0. The observed values of 0 for Kb are near 90°C, while for KA the values of 0 are smaller, namely 53.9° for glycine and 44.8° for alanine. 

We saw m Chapter 1 especially m Table 1 7 that alcohols resemble water m respect to their Brpnsted acidity (ability to donate a proton/rom oxygen) They also resemble water m their Brpnsted basicity (ability to accept a proton on oxygen) Just as proton transfer... 

The equilibrium shown m the equation lies to the right =10 for proton transfer from the conjugate acid of aniline to cyclohexylamine making cyclohexylamine 1 000 000 times more basic than aniline... 

If the substrate (M) is more basic than NHj, then proton transfer occurs, but if it is less basic, then addition of NH4 occurs. Sometimes the basicity of M is such that both reactions occur, and the mass spectrum contains ions corresponding to both [M + H]+ and [M + NH4]. Sometimes the reagent gas ions can form quasi-molecular ions in which a proton has been removed from, rather than added to, the molecule (M), as shown in Figure 1.5c. In these cases, the quasi-molecular ions have one mass unit less than the true molecular mass. 

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. 

It appears that the Aac2 mechanism gives a better agreement with experimental results than that of Goldschmidt. This is not in contradiction with the fact that the alcohol is more basic than the acid and that, consequently, the concentration of R OH is higher than that of RC(OH) since in any case a proton transfer from R OH to the acid is possible. 

For coupling with 2-naphthol-6,8-disulphonic-l-isotope effects (kK/kD) varied with the substituent in the benzenediazonium ion as follows 4-C1 (6.55) 3-C1 (5.48) 4-N02 (4.78), i.e. the reactivity of the ion was increased so that i correspondingly decreased. Base catalysis was observed127, 129, and there was a free energy relationship between this catalytic effect and the basicity of pyridine, 3- and 4-picoline. However, for 2-picoline and 2,6-lutidine, the catalysis was 3 times and 10 times less than expected from their basicities showing that, in this particular proton transfer, steric hindrance is important. 

On the basis of the examples given above, it is reasonable to suggest that the underlying principles for optimization of the overall reaction rate with respect to the choice of metal ion are similar. That is, there are basically three states along the reaction pathway which determine the most suitable choice of metal ion. These are (1) the reactant state with bound metal and substrate before the proton transfer step, (2) the intermediately created free OH nucleophile and, (3) the subsequent transition state associated with... 

To express the relative strengths of an acid and its conjugate base (a conjugate acid-base pair ), we consider the special case of the ammonia proton transfer equilibrium, reaction C, for which the basicity constant was given earlier (Kb = [NH4+l[OH ]/ NH3]). Now let s consider the proton transfer equilibrium of ammonia s conjugate acid, NH4+, in water ... 

Proton transfer equilibrium is established as soon as a weak base is dissolved in water, and so we can calculate the hydroxide ion concentration from the initial concentration of the base and the value of its basicity constant. Because the hydroxide ions are in equilibrium with the hydronium ions, we can use the pOH and pKw to calculate the pH. 

For each of the following weak acids, write the proton transfer equilibrium equation and the expression for the equilibrium constant Kv Identify the conjugate base, write the appropriate proton transfer equation, and write the expression for the basicity constant Kb. (a) HC102 (b) HCN ... 

In order to account for the inability of many enzymes to bind the protonated form of the basic inhibitors or permanently cationic ones better than uncharged analogs (for example, yS-o-galactosidase from E. coli, and P-v>-glucosidase from almonds), it was proposed that the enzyme could proton-ate the inhibitor at the active site by a cationic acid (for example, protonated histidine). If proton transfer cannot occur, the attractive forces due to the carboxylate would be canceled by the repulsion from the cationic acid. Experimental evidence for this proposal is, however, still lacking. In fi-D-gn-lactosidase from E. coli, a tyrosine is presumed to be responsible for the protonation of substrates. ... 

In molecules with both an acidic and a basic function, there is in principle the possibility of an intramolecular proton transfer leading to betaines (see above). Due to the values of the functional groups, pyridine and carboxylic acid, in [66] no intramolecular proton transfer takes place and no betaine is formed. 

To ensure that proton transfer takes place from the protonated catalyst 64-H and not from the acidic reagent itself, apolar solvents favoring contact rather than solvent separated ion pairs as well as a slow addition of the acidic substrate RX-H are required. In addition, it was sometimes found beneficial to lower the basicity of the catalyst, thus rendering the protonated species [catalyst-H" ] more acidic for the stereo-determining protonation of the enolate. This was accomplished by formally replacing NR2 by Me (see 64e, Fig. 36). 

Because of their basic properties, aikaioids were among the first naturai substances that eariy chemists extracted and purified. Morphine was isoiated from poppies in 1805 and was the first aikaioid to be characterized. When treated with aqueous strong acid, aikaioids accept protons to produce water-soiubie cations. The protonated aikaioids dissoive, ieaving the rest of the piant materiais behind. Adding strong base to the aqueous extract reverses the proton-transfer reaction, converts the aikaioid back to its neutrai base form, and causes pure aikaioid to precipitate from the soiution ... 

An aqueous solution of a soluble salt contains cations and anions. These ions often have acid-base properties. Anions that are conjugate bases of weak acids make a solution basic. For example, sodium fluoride dissolves in water to give Na, F, and H2 O as major species. The fluoride anion is the conjugate base of the weak acid HF. This anion establishes a proton transfer equilibrium with water ... 

We used DFT to optimize the geometries of various Hammett bases on cluster models of zeolite Brpnsted sites. For p-fluoronitrobenzene and p-nitrotoluene, two indicators with strengths of ca. -12 for their conjugate acids, we saw no protonation in the energy minimized structures. Similar calculations using the much more strongly basic aniline andogs of these molecules demonstrated proton transfer from the zeolite cluster to the base. We carried out F and experimental NMR studies of these same Hammett indicators adsorbed into zeolites HY and HZSM-5. 

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