Steric effects acid-catalyzed

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. ... 

Fundamental to the interpretations of cr as a measure of electronic effects is the validity of the assumption that the steric effect is identical in acid- and base-catalyzed... 

If the assumptions underlying the Taft treatment of the separation of electronic and steric effects are valid, then the relative rates of acid-catalyzed reactions of esters should be a measure of the steric effect. Taft " accordingly defined a steric constant Es by Eq. (7-52). 

A very interesting steric effect is shown by the data in Table 7-12 on the rate of acid-catalyzed esterification of aliphatic carboxylic acids. The dissociation constants of these acids are all of the order 1(T, the small variations presumably being caused by minor differences in polar effects. The variations in esterification rates for these acids are quite large, however, so that polar effects are not responsible. Steric effects are, therefore, implicated indeed, this argument and these data were used to obtain the Es steric constants. Newman has drawn attention to the conformational role of the acyl group in limiting access to the carboxyl carbon. He represents maximum steric hindrance to attack as arising from a coiled conformation, shown for M-butyric acid in 5. 

Both have the same steric and conjugative effects, but the polar effect is largely absent in the acid-catalyzed reactions. Subtraction of the logarithms of the two relative rates therefore leaves just the polar effect. This is written... 

Taft, following Ingold," assumed that for the hydrolysis of carboxylic esters, steric, and resonance effects will be the same whether the hydrolysis is catalyzed by acid or base (see the discussion of ester-hydrolysis mechanisms. Reaction 10-10). Rate differences would therefore be caused only by the field effects of R and R in RCOOR. This is presumably a good system to use for this purpose because the transition state for acid-catalyzed hydrolysis (7) has a greater positive charge (and is hence destabilized by —I and stabilized by +1 substituents) than the starting ester. 

Taft was also able to isolate steric effects. For the acid-catalyzed hydrolysis of esters in aqueous acetone, long (kJh was shown to be insensitive to polar effects. In cases where resonance interaction was absent, this value was proportional only to steric effects (and any others that are not field or resonance). The equation is... 

A Hammett plot for para-substituted benzaldehydes showed that electron-rich aldehydes gave higher ees (r = -0.4). As in Shibuya s related results (Section 5.3.3.1 above), this indicates that aldehyde coordination is important in enantiodifferentia-tion, but the lower rvalue (compared to Shibuya s r = -1.30) suggests a weaker electronic influence, probably due to the relative Lewis acidities of A1 and La. For ortho-substituted aldehydes, lower ees were observed, presumably due to steric effects. Although Al-Cl and Al-triflate complexes 29-30a-b did not catalyze the reaction, they... 

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... 

The acid-catalyzed ester hydrolysis provides a good target for MM treatments. DeTar first used hydrocarbon models in which an ester was approximated by an isoalkane (74) and the intermediate (75) by a neoalkane (76). He assumed that if the rate of reaction truly is not influenced by polar effects but is governed only by steric effects of R, as has been generally postulated, the rate must be proportional to the energy difference (AAH ) between 74 and 76. The AAH f is mainly determined by the van der Waals strain in these branched alkanes. Nonsteric group increment terms were carefully adjusted, and statistical mechanical corrections for conformer populations... 

Hirai et al.129 studied the hydrogenation of olefins catalyzed by poly(acrylic acid)-Rh(II) complexes in homogeneous solutions. The catalytic activity of the polymer-Rh complex was about 103 times that of the acetato-Rh complex. When olefins having another functional group, such as diallylether, allylaldehyde, and cyclohexene-1 -one, were used as the substrates, the olefinic bond was preferentially hydrogenized by the polymer-Rh complex. The polymer ligand was presumed to exercise a steric effect. 

Taft based his steric effect constants on the assumption that rates of esterification of carboxylic acids with alcohols and of acid catalyzed hydrolysis of carboxylate... 

The scope and limitations of the Lewis acid-catalyzed additions of alkyl chlorides to carbon-carbon double bonds were studied.51 Since Lewis acid systems are well-known initiators in carbocationic polymerizations of alkenes, the question arises as to what factors govern the two transformations. The prediction was that alkylation products are expected if the starting halides dissociate more rapidly than the addition products.55 In other words, addition is expected if the initial carbocation is better stabilized than the one formed from the dissociation of the addition product. This has been verified for the alkylation of a range of alkyl-and aryl-substituted alkenes and dienes with alkyl and aralkyl halides. Steric effects, however, must also be taken into account in certain cases, such as in the reactions of trityl chloride.51... 

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