Aliphatic compounds Taft equation

The Hemmett equation is not applicable well to the reactions of aliphatic compounds partly due to some steric interference between the substituents and reaction site. For aliphatic compounds, Taft has proposed the following equation... 

The reactivity of aliphatic compounds, where steric hindrance near the substituent site is not important, is aceommodated by the Taft modification of the Hammett equation ... 

Taft found that the rate or equilibrium constants for a variety of reactions of aliphatic compounds conform to Equation 2.18 or Equation 2.19, respectively. 

Hammett s success in treating the electronic effect of substituents on the equilibria rates of organic reactions led Taft to apply the same principles to steric, inductive, and resonance effects. The Hammett o constants appear to be made up primarily of two electronic vectors field-inductive effect and resonance effect. For substituents on saturated systems, such as aliphatic compounds, the resonance effect is rarely a factor, so the o form the benzoic acid systems is not applicable. Taft extended Hammett s idea to aliphatics by introducing a steric parameter ( .). He assumed that for the hydrolysis of esters, steric and resonance effects will be the same whether the hydrolysis is catalyzed by acid or base. Rate differences would be caused only by the field-inductive effects of R and R in esters of the general formula (XCOOR), where X is the substituent being evaluated and R is held constant. Field effects of substituents X could be determined by measuring the rates of acid and base catalysis of a series XCOOR. From these rate constants, a value a could be determined by Equation (5.9) ... 

Since solvatochromic parameters are derived from direct measurements of the energy resulting from intermolecular interaction, they can be used to predict solubility, which is determined by solute-solute, solvent-solvent, and solute-solvent interaction energies. For nonself-associated liquid aliphatic compounds with a weak or nonhydrogen-bond donor (Taft etal., 1985 Kamlet etal., 1986), the solubility in water at 29S was related to molar volunWjf, hydrogen-bond basicity j and polarity/polarizability (jf) by a linear solvation energy relationship (LSER) as in Equation 3.55 ... 

The Hammett equation describes reasonably well the inductive and mesomeric influences on the rate constant when there are substituent changes in meta- and para-substituted aromatic compounds. Ort/io-substituted aromatics and substituted aliphatic compounds can be represented by the Taft equation [81, 83]... 

The factor 2.48 puts a on the same scale as Hammett s er, and the k0 values are rate constants for acid and base hydrolysis of acetic acid esters (i.e., R is a methyl group in the reference compound). Usually R is an ethyl or methyl group, but in many cases the rate constants do not depend on the nature of R. Equation 8 is based on the fact that acid hydrolysis rates of substituted benzoic acid esters are only slightly affected by the nature of the substituent, but acid hydrolysis rates of aliphatic esters are strongly affected by substituents. These effects were taken to be caused by steric factors thus log(/c//c0)acid defines s. It is reasonable to assume that steric factors affect base-catalyzed rates in the same way. Substituent effects on base hydrolysis of aliphatic compounds are composed of both polar and steric effects, and subtraction of the latter yields a measure of the former. The parameter a is important because it allows one to evaluate substituent effects on aliphatic reaction rates by a formula analogous to the Hammett equation, or by a bivariate relationship, the Taft-Pavelich equation (Pavelich and Taft, 1957) ... 

Values for many more substituents are available along with Hammett equations for many additional acids along with Taft equations and substituent constants for aliphatic compounds." " So this approach is quite versatile and it is possible to predict pAa values for molecules with rather complex structures. This procedure provides a system for evaluating how substituents influence the distribution of charge in a molecule. 

Taft equation. Taft (1956) has extended the Hammett-type correlation to aliphatic systems. Because steric effects of substituents in aliphatic systems cannot be ignored as they were for m- and / -substituted benzene compounds, Taft recognized the need to develop separate terms for the polar and steric effects for substituent constants. Based on the observation that the acid-catalyzed hydrolysis of meta- and para-substituted benzoic acid esters are only slightly affected by the electronic nature of the substituent group (p values are near 0), Taft concluded that the acid-catalyzed hydrolysis of aliphatic esters would also be insensitive to polar effects of substituent groups. Any effect on rate due to substituent groups could therefore be attributed to steric effects. Taft defined a steric substituent constant, E, by ... 

The second application of a values for QS ARs in environmental sciences is for the estimation of values. In contrast to the substituent constants, the latter represent properties of the entire molecules and are used to quantify the degree of dissociation at a given (e.g. ambient) pH. In this way, like log values may appear in two ways as an endpoint by itself in hazard-assessment schemes and as a chemical descriptor to account for environmental conditions affecting chemical properties. For a large number of chemicals, values are available in the literature and may also be retrieved from databases. If such are not available and experimental determinations are not feasible, estimates may be obtained from a compound class-specific Hammett equation for aromatic substances or a Taft equation for aliphatic acids ... 

Based on a large experimental material, L. Hammett in 1937 proposed the known prate constant of the reaction of an aromatic compound to the dissociation constant of the corresponding benzoic acid. A year later, M. Evans and M. Polanyi derived the empirical correlation A = oAH for the reaction of sodium atoms with alkyl halides. In 1954 N. N. Semenov considered and showed the applicability of this correlation to many reaction of radical abstraction. Simultaneously (1952-1953) R. Taft advanced a postulate about the additive influence of structural factors. A diverse experimental material on various reactions of aromatic and aliphatic compounds and application of the Hammett, Taft, and Polanyi-Semenov equations to them was obtained in fifties-seventies. 

The dependence has the form log( x/ o)" p o. Later this equation was modified. Taft introduced the principle of additivity of the influence of different ctors on the reactivity of a compound each factor, being it inductive, conjugation or steric, affects AG independently, so that for each reaction series log( / o) or logfX/ATp) can be presented as the sum 2p,aliphatic compounds the Taft equation has the form... 

For example, Figure 2.3 shows plots of the a constants of X vs. log p/T of aliphatic carboxylic acids (XCOaH) and vs. log k for the dehydration of acetaldehyde hydrate by XC02H. Deviations from Equations 2.18 and 2.19 occurwhen the rate of reaction or position of equilibrium becomes dependent on steric factors. For example, Taft studied the enthalpies of dissociation, A Hd, of the addition compounds formed between boron trimethyl and amines (X1X2X3N) and found that when the amine is ammonia or a straight-chain primary amine the dissociation conforms to Equation 2.20, in which 2 ° is the sum of the a values for the... 

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