Qualitative Guidelines

The problem is to determine the most reasonable mechanism(s) which will predict a rate law that is consistent with the observations. Very often this is done by analogy to previous studies on related systems, but there are some general guidelines that can be useful for writing a mechanism that will produce the desired form of the rate law. 

The mechanism is composed of elementary reactions whose rate laws are implied from the stoichiometry of each reaction. The elementary mechanistic steps are usually unimolecular or bimolecular reactions termolecular reactions are very rare because of the improbability of bringing three species together. 

The form of the experimental rate law provides some guidelines for the construction of a mechanism. The following generalizations assume that the reaction is monophasic, but they may apply to individual steps in a multiphasic reaction. It also should be remembered that the experimental rate law may be incomplete because of experimental constraints. Then, the predicted rate law may contain terms not observed experimentally, but it should be possible to show that the extra terms are minor contributors under the conditions of the experiment. 

For the simplest cases, in which rate = / g p[A][B] or rate = ifce,p[A], the kinetics only requires a one step mechanism involving the species in the rate law. In the second case, the solvent also may be involved because its concentration will be constant and may be included in 

If the rate law has a half-order term, such as [A] , then the mechanism probably involves a step in which A is split into two reactive species before the rate-determining step. 

---

A urethane may have bulk properties of some of the stiffest plastics, on one extreme, or it may be as flexible and conformable as a PSA, on the other extreme. This section will give qualitative guidelines to achieving the desired adhesion characteristics of properly prepared substrates. 

Although the present results are only theoretical and not yet verified by experiments, it is felt that this approach, although very much simplified with respect to practice, can be used at least as a qualitative guideline in choosing new experiments for product development. These results are therefore useful in a qualitative sense to show sensitivites, trends and directions. Experimental verification may improve the applicability to (semi-)quantitative predictions, or indicate shortcomings in the present approach, which may be accordingly adjusted. 

Some qualitative guidelines can be given to make an a priori estimate of the relative weight of dipolar, contact, and Curie relaxation contributions. Consider first the fast motion limit where Rim = Rim and none of the frequency-dependent terms is dispersed. The equations take the simple form already noted ... 

The Importance of the Tg of the polymer in determining permeability is evident from the relationship in Figure 2. In most cases, the Tg is available from literature compilations (.11). Methods of estimating Tg from substituent group contributions have been described (12,13). Some qualitative guidelines for predicting the change in Tg with polymer structure are ... 

Note As with many rules in chemistry, the above qualitative guidelines must be applied cautiously. 

Many liquids have been proposed as stationary phases in the development of gas-liquid chromatography. Currently less than a dozen are in common use. The proper choice of stationary phase is often crucial to the success of a separation. Qualitative guidelines exist for making this choice, but in the end, the best stationary phase can only be determined in the laboratory. 

The initial layer formation step has hardly been investigated. Some qualitative guidelines can be abstracted from theoretical considerations and experimental observations. The first intensive and systematic study of the kinetics and mechanism of membrane formation was performed by Leenaars et al. [2-4] on boehmite and Y-AI2O3 membranes with pore diameters in the range of 3-5 nm. 

Modem quantum mechanics can provide accurate excited-state PESs,14,16,17 but such calculations are demanding and in order to obtain useful answers one must ask the right questions. The chemist s intuition or knowledge from related reactions is instmmental in suggesting conceivable photoproducts of the initial reactant. Qualitative guidelines to... 

The treatment of multiple reactions is more complex, because it involves not only the objective of optimal productivity, but also the desired selectivity. The optimal temperature path in a reactor can be found by computer simulation. As a qualitative guideline, it should be kept in mind that high temperature favours reactions with higher activation energy, while low temperatures is recommended for reactions with low activation energy. Let s consider the following consecutive-series first-order reactions ... 

Be able to perform a conceptual synthesis of plantwide control structures (pairings) based on degrees-of-freedom analysis and qualitative guidelines. 

We use the qualitative guidelines described above to predict whether entropy increases or decreases. 

A particularly important consideration in designing contactors for gas-liquid reactions is the validity of the ideal flow assumption [plug flow (PF) or mixed flow (MF)] normally used in the design. Numerous studies have been reported on the role of nonideality (i.e., backmixing) in gas-liquid contactors, and based on these, some qualitative guidelines... 

To date, much of our fluoroalkylphosphine work has focused on the coordination chemistry of the dfepe chelate. Substitution of sterically demanding dfepe for small monodentate carbonyl ligands should maintain the electron-poor nature of the metal center and provide unprecedented access to stable low coordinate electrophilic metal systems with unusual stractural and electronic properties. As a useful qualitative guideline, (dfepe)MLn complexes are conceptually viewed as chelating electronic mimics to (CO)2MLj complexes with steric properties equal to or exceeding that of (dppe)MLn donor phosphine analogues. 

---