Substrate standard measurement

As we have seen the nucleophile attacks the substrate m the rate determining step of the Sn2 mechanism it therefore follows that the rate of substitution may vary from nucleophile to nucleophile Just as some alkyl halides are more reactive than others some nucleophiles are more reactive than others Nucleophilic strength or nucleophilicity, is a measure of how fast a Lewis base displaces a leaving group from a suitable substrate By measuring the rate at which various Lewis bases react with methyl iodide m methanol a list of then nucleophihcities relative to methanol as the standard nucleophile has been compiled It is presented m Table 8 4... 

The hydrogen-isotope exchange reactions have already been discussed in previous sections. For various aromatic and heteroaromatic substrates, standard reaction rates in protodedeuteration and in nitration have been obtained. A plot of these shows a wide scatter. This means that there is no simple relation between these two measures for the susceptibility to electrophilic attack. No single reactivity index can be used as a measure to derive a unique order of the susceptibility of individual ring positions towards electrophilic attack (75TL1395). 

For the purpose of comparing the rates of nitration of different substrates, standard conditions have been defined as T = 25 °C, H0 = -6.6 (75% H2S04 at 25 °C). The procedure for the determination of standard rates from the experimentally measured rates has been evolved <75JCS(P2)1600>. The standard rates of individual substrates towards hydrogen exchange and nitration have been compared. This shows clearly that there is no unique scale of reactivity valid for electrophilic substitution reactions (75JCS(P2)1624). 

In the substrate standard method the absorption effect is determined using the transmittance ratio (T =1. /I ), The determination of the transmittance involves additional measurements, i.e., step scanning over the silver profile, to obtain I. and I , IA is measured for each filter sample and if can be obtained several ways as discussed below, gThese additional measurements can be performed very quickly compared to the more lengthy measurements of the internal standard because the silver peak is quite intense, A step scan of the silver diffraction profile plus background counting time can be accomplished in about two minutes with better than 1% precision,... 

Proponents of the internal standard procedure have questioned the validity of the substrate standard method to adequately correct for matrix absorption, Leroux and coworkers JLA) have presented data which support the method in addition, several measurements were performed in this laboratory to verify the validity of the method ( ), Mixtures of chrysotile in talc (1-7%) were prepared and various quantities "spiked" on silver filters. Table III illustrates the results obtained after correcting for matrix absorption as compared with the uncorrected data. Overall, there is excellent agreement between the corrected weight and the "spiked" weight. 

For each basis weight, thickness will determine the paper bulk or density. A short fiber pulp from hardwood, for example, will have a lower thickness for a specified basis weight. The thickness (caUper) of a paper substrate is measured as the perpendicular distance between two parallel surfaces under a pressure of 1 k cm. Paper having a uniform caliper will contribute to uniform plasma surface modiflca-tion. Variations in caliper can vary the paper s inherent strength, as well as treatment uniformity. The standard procedure for measuring thickness is represented in TAPPI T 411. 

Shear strength is measured via a simple single overlap shear specimen of standard dimensions (Fig. 9). In contrast to its simple appearance, the forces in a thin-adherend shear specimen can be quite complex due to the inherent offset loading of the specimen and subsequent bending in the substrates. The single overlap shear test is anything but a pure shear test, but the configuration is easy to manufacture, simple to test and is firmly entrenched in the industry as a primary examination technique for materials qualifications, inspection and process control. 

The available data are arranged by ring and ring-position in Tables II-VIII. The rate coefficients have been recalculated to the same units, where necessary the fact that different temperatures of reference were used in the publications should be noted. The temperatures used experimentally for a given substrate were chosen for a rate of reaction which was convenient to measure, and then for comparison, rate constants were calculated at a common temperature by means of the standard equations (cf. discussions by Ingold and by... 

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