Steric effects in acidity

Hydrolysis reactions involving tetrahedral intermediates are subject to steric and electronic effects. Electron-withdrawing substituents faciUtate, but electron-donating and bulky substituents retard basic hydrolysis. Steric effects in acid-cataly2ed hydrolysis are similar to those in base-cataly2ed hydrolysis, but electronic effects are much less important in acid-cataly2ed reactions. Higher temperatures also accelerate the reaction. 

The hypothesis put forth concerning steric effects in acid hydrolysis would have as its corollary the proposal that the role of the Ci component in cellulase enzyme system complexes is to disrupt the engagement of the C6 oxygen in the bifurcated intramolecular hydrogen bond and thus permit rotation of the C6 group into positions more favorable to hydrolytic attack. 

The standard substituent is by convention taken as methyl whereas for a, it is hydrogen. Ingold [54] indicated that steric effects in acid and base hydrolysis of a given ester should be identical this is supported by modern ideas on ester hydrolysis where both mechanisms involve similar transition states in their rate-limiting steps (Eqns. 66 and 67). 

Taft began the LFER attack on steric effects as part of his separation of electronic and steric effects in aliphatic compounds, which is discussed in Section 7.3. For our present purposes we abstract from that treatment the portion relevant to aromatic substrates. Hammett p values for alkaline ester hydrolysis are in the range +2.2 to +2.8, whereas for acid ester hydrolysis p is close to zero (see Table 7-2). Taft, therefore, concluded that electronic effects of substituents are much greater in the alkaline than in the acid series and. in fact, that they are negligible in the acid series. This left the steric effect alone controlling relative reactivity in the acid series. A steric substituent constant was defined [by analogy with the definition of cr in Eq. (7-22)] by Eq. (7-43), where k is the rate constant for acid-catalyzed hydrolysis of an orr/to-substituted benzoate ester and k is the corresponding rate constant for the on/to-methyl ester note that CH3, not H, is the reference substituent. ... 

Wasserman, H.H. in Newman Steric Effects in Organic Chemistry, Wiley NY, 1956, p. 352. See also Buchanan, G.L. Kean, N.B. Taylor, R. Tetrahedron, 1975, 31, 1583. StericaUy hindered P-keto acids decarboxylate more slowly Meier, H. Wengenroth, H. Lauer, W. Krause, V. Tetrahedron Lett., 1989, 30, 5253. 

Similarly to peroxycarboxylic acids, DMDO is subject to cis or syn stereoselectivity by hydroxy and other hydrogen-bonding functional groups.93 However a study of several substituted cyclohexenes in CH3CN —H20 suggested a dominance by steric effects. In particular, the hydroxy groups in cyclohex-2-enol and... 

The literature on basic- and acid-catalyzed alkylation of phenol and of its derivatives is wide [1,2], since this class of reactions finds industrial application for the synthesis of several intermediates 2-methylphenol as a monomer for the synthesis of epoxy cresol novolac resin 2,5-dimethylphenol as an intermediate for the synthesis of antiseptics, dyes and antioxidants 2,6-dimethylphenol used for the manufacture of polyphenylenoxide resins, and 2,3,6-trimethylphenol as a starting material for the synthesis of vitamin E. The nature of the products obtained in phenol methylation is affected by the surface characteristics of the catalyst, since catalysts having acid features address the electrophilic substitution in the ortho and para positions with respect to the hydroxy group (steric effects in confined environments may however affect the ortho/para-C-alkylation ratio), while with basic catalysts the ortho positions become the... 

Another hypothesis was provided by Mikio Shimitso (1982) on the basis of studies of steric effects in molecular models. It had been noted years previously that the fourth nucleotide at the 3 end of the tRNA molecules (referred to as the discrimination base) might have a recognition function. In the case of certain amino acids (i.e., their tRNA-amino acid complexes) this base pair, in combination with the anticodon of the tRNA molecule, can select the amino acid corresponding to the tRNA species in question this is done on the basis of the stereochemical properties of the molecule. Since the anticodon of a tRNA molecule and the fourth nucleotide of the acceptor stem are far apart in space, two tRNA molecules must complex in a head-to-tail manner. The pocket thus formed can then fit specifically to the corresponding amino acid. 

Experimental Rate Constant Data Illustrating the Role of Steric Effects in Strengthening the Intramolecular Hydrogen Bond in the Monoanion of 2,2-Disubstituted Malonic Acids in Aqueous Solution (25 C, 0.1 M NaClO )... 

Since the solvent properties of dimethyl sulfoxide are widely different from those of hydrocarbons and halogenated hydrocarbons, it may be difficult to compare the kinetic and thermodynamic data for the C02H group (Table 16) directly with others. However, heating the carboxylic acid (68, X = OH) in toluene affords the sp isomer almost exclusively. Probably, the observed results with the carboxylic acid derive from difficulty in the formation of a hydrogen bond owing to a steric effect, in addition to the nonplanar conformation of the carboxyl group relative to the naphthalene. 

Development of a predictive model for the reactivity of annelated benzenes is facilitated by the partitioning of substituent effects into their steric and electronic parameters. In order for this to be accomplished, reference reactions are needed which depend uniquely on one type of parameter. Typically conformational equilibria serve as good references of steric effects, whereas acid-base equilibria serve as good references of electronic effects. 

Ground-state and excited-state reactions of chiral Meldrum s acid derivatives 39 with the enone function have been reviewed with an emphasis on the facial selectivity in the C=C bond (Figure 2) <1996H(42)861>. Top-face preference, even when it is sterically more hindered than bottom-face attack, is supported by hyperconjugation no —r c=c 39a, whereas bottom-face preference is dominated by steric effects in the sofa conformation of the molecule 39. The trajectory of the attacking reagent plays a balancing role. 

A c2 pathway, 241, 384 A,4 1 pathway, 241, 380 acid-catalysed, 240, 378 acyl-oxygen fission, 88, 240, 242 alkyl-oxygen fission, 240 isotope labels in, 88, 241 steric effects in, 242 Esters... 

---