Steric correction factor

When in a position, use steric correction factors, in Table 4. If there is more than one a substituent, consider the most substituted. 

Asterisks are meant to identify those substituents for which the steric correction factor s has to be added. ... 

Step 3. Correction factors are responsible for deviations from simple group additivity. In most cases correction factors reflect internal (electronic, steric and H-bonding) interactions between polar functional groups. Figure 14.2 describes them as two-way arrows between any two functional groups, thereby reflecting the bidirectional nature of interactions (interaction between the ith and jth fragments separated by the kth type of skeleton) as expressed in ... 

Comparison of these expressions indicates that the activation entropy is related to the steric factor for the reaction. One may interpret the steric factor in terms of the degree of order of molecular configurations required to bring about the reaction, and this viewpoint is generally regarded as more satisfactory from an intellectual viewpoint than is that which regards Ps as an a posteriori correction factor necessary to obtain agreement between theory and experiment. 

ClogP This method developed by Leo and Hansch presents some similarities with the method of Rekker, since the log P of a molecule is calculated by adding lipophUicity values attributed to multiatomic fragments and numerous corrections factors which take into account not only geometrical and topological effects but also electronic and steric effects (Equation 5.3) [39-41]. 

Additivity schemes with fixed bond energy (or enthalpy) parameters plus a host of corrective factors reflecting nonbonded steric interactions have a long history in the prediction of thermochemical properties, such as the classical enthalpy of formation of organic molecules. AUen-type methods, for example, nicely Ulustrate the usefulness of empirical bond additivity approaches [1,2]. 

As illustrated by the examples in Table 7.5, application of correction factors is necessary in those cases in which electronic and/or steric interactions of functional groups within a molecule influence the solvation of the compound. A positive correction factor is required if the interaction decreases the overall H-donor and/or... 

In Eq. (11.10) kp is the retention parameter of a parent compound, is the corresponding value for the derivative carrying n substituents and r, are retention increments due to individual substituents i. Having appropriate values for functional groups of interest one needs only to determine the retention of the parent structure and can next calculate the retention of a derivative. To get reliable predictions, the correction factors are introduced in Eq. (11.10) accounting for mutual interactions between substituents (electronic, steric, hydrogen bonding) [41,70], In cases of polyfunctional analytes the interactions between substituents make retention predictions of rather limited value. 

The correction factor P in (3A), called "probability" (or "steric") factor, is supposed to take into account that the reaction probability for a collision between two "activated" molecules A md B may be less than unity (for a large number of reactions P has a value between 10 and 10 ), however, for some reactions P > 1. 

The shortcomings of the collision theory have been realized very early through experimentation, such that the preexponentials predicted from experimental data did not agree with the predictions of the theory, fri most of the situations, the reaction rates were found to take place much more slowly thus, a correction factor to the collision frequency, called the probability factor or steric factor, was added to the prediction. Thus, the final form of the rate constant k was as follows ... 

The position of aniline in the above reactivity order deserves special comment. Aniline is less basic than pyridine by a relatively small factor, 0.65 pA units, but is appreciably more polarizable it then seems likely that the inverted order of reactivity is caused by the polarizability term in accordance with Edwards equation. If this is correct, in the reactivity order piperidine > aniline > pyridine, inversion with respect to basicity appears to result from an abnormally high reactivity of aniline rather than from a particularly low reactivity of pyridine. This view differs from that based on relative steric requirements of the reagents, but other factors besides basicity and polarizability may well contribute to the quantitative experimental picture. 

It has been suggested that the discrepancies between the value of k ikK observed and that predicted on the basis of simple statistics may reflect the greater sensitivity of combination to steric factors. Beckhaus and Rtichardt164 reported a correlation between log(A )/ ,<.,) (after statistical correction) and Taft steric parameters for a scries of alkyl radicals. 

These differences have been attributed to various factors caused by the introduction of new structural features. Thus isopentane has a tertiary carbon whose C—H bond does not have exactly the same amount of s character as the C—H bond in pentane, which for that matter contains secondary carbons not possessed by methane. It is known that D values, which can be measured, are not the same for primary, secondary, and tertiary C—H bonds (see Table 5.3). There is also the steric factor. Hence, it is certainly not correct to use the value of 99.5 kcal mol (416 kJ mol ) from methane as the E value for all C—H bonds. Several empirical equations have been devised that account for these factors the total energy can be computed if the proper set of parameters (one for each structural feature) is inserted. Of course these parameters are originally calculated from the known total energies of some molecules that contain the structural feature. 

E) alkenes. One explanation for this is that the reaction of the ylid with the carbonyl compound is a 2-1-2 cycloaddition, which in order to be concerted must adopt the [rt2s+n2al pathway. As we have seen earlier (p. 1079), this pathway leads to the formation of the more sterically crowded product, in this case the (Z) alkene. If this explanation is correct, it is not easy to explain the predominant formation of ( ) products from stable ylids, but (E) compounds are of course generally thermodynamically more stable than the (Z) isomers, and the stereochemistry seems to depend on many factors. 

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