Hydrogen acids, relative acidities

Claisen condensations involve two distinct experimental operations The first stage concludes m step 4 of Figure 21 1 where the base removes a proton from C 2 of the p keto ester Because this hydrogen is relatively acidic the position of equilibrium for step 4 lies far to the right... 

Before we turn to other investigations of the stabilized ion it is interesting briefly to compare the rates of hydride shifts found in the strong acid systems with rates obtained under solvolytic conditions. Collins and Lietzke (1967) and Berson et al. (1967) have deduced the rates of 3,2- and 6,2-hydrogen migration relative to the rate of solvent capture for norbornyl ions in acetic acid, formic acid and other common solvents from elaborate C scrambling studies due to Roberts and Lee (1951) and Roberts et al., (1954) and tritium labelling studies of Lee and Lam (1966). From the ratio of these... 

Excellent results have been obtained in the asymmetric hydrogenation of itaconic acid (97% ee) and dimethyl ester (94% ee) using Rh/MonoPhos.31 The latter substrate could be hydrogenated in 99% ee using the piperidine ligand 24d.40 These hydrogenations are relatively fast and have been carried out on a 100-g scale with an S/C of 10,000. Preliminary results with some alkylidene and benzylidene succinates were also very promising. 

The hydrogen on the nitrogen is removed by the base. This hydrogen is relatively acidic due to the electron-withdrawing effect of the bromine and resonance stabilization of the conjugate base by the carbonyl group. 

Methylpyrimidine undergoes arylation not on the nuclei, but on the substituent (Eq. 70) [67]. This maybe attributed to the fact that the nuclei is less susceptible toward electrophiles and the methyl hydrogen is relatively acidic. 

Fig. 17. Correlation of relative basicity (APA) of pyridines with hydrogen-bonding relative energies (<5AH) of the pyridinium ion-HjO pair. The quantity shown on the vertical scale on the right gives the net atomic charge difference on the acidic H atom of the pyridinium ion. Open circles, versus APA full circles, SAfI versus APA (79JA1675).

One of the many important differences between phosphorus and nitrogen chemistry is the relative strengths of their bonds to hydrogen. The relatively weak P—H bond means that this functionality can be added across a wide variety of unsaturated molecules (alkenes, alkynes, carbonyls) and hence this represents an excellent method for preparing tertiary phosphines. The addition of P 11 compounds to C=0 and C=N has been described in detail by Gilheany and Mitchell.2 The reaction can be catalyzed by base (potassium hydroxide, butyllithium), acid (HC1, carboxylic acids, sulfonic acids, boron trifluoride), free radical (uv, organic peroxides, AIBN) or metal (simple metal salts, late transition-metal complexes). In some circumstances no catalyst is required at all for P 11 additions to proceed.60... 

Fig. 10.13. The position of the acceptor O atom in O - H. .. O hydrogen bonds relative to the O - H bond, projected onto the O - H. .. O plane. The circles correspond to observed positions. The curved line passing through the densely populated regions indicates the positions predicted by the Bond Valence Model [50]. The acceptor atoms shown near 100° are those found in the weak bonds in perchloric acid hydrates...

Figure 3.15 Transport number of hydrogen ions relative to sodium ions (PNaH) and current efficiency of cations to the composition of an amphoteric ion exchange membrane. (O, ) quarternized membranes (N-methyl pyridinium and sulfonic acid groups) (A, A) tertiary amino groups membrane (pyridinium hydrochloride and sulfonic acid groups) (, A) PNa 11 (O, A) current efficiency (%). Electrodialysis was carried out at a current density of 20 mA cm 2 using a mixed solution of 0.25 N sodium chloride and 0.25 N hydrochloric acid for 60 min at 25.0 °C.

When the strengths of hydrogen acids are compared in the same solvent and at the same temperature, one obtains a fairly good estimate of the relative strengths of the bonds between the protons and the... 

In the second step of the reaction, a base (apparently a glutamate residue) removes a proton from the a-carbon of the aldehyde. Recall that a-hydrogens are relatively acidic (Section 19.1). 

Let us consider the relative acidities of the following series of hydrogen acids, all of which are in the same period of the Periodic Table. 

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