Chemists 367 trend, observed

A study of the overall rate of addition of radicals to olefins shows us that polar forces, familiar to chemists working with ionic species in solution, are apparent in free radical reactions, i.e. in reactions involving uncharged species in the gas phase. The effects are smaller than in solution but are none the less clearly apparent. The results also show that polarity is not the whole story and that some of the trends observed in Tables 1 to 7 may be partly due to steric... 

There are a number of anomalies in the long form, whose exposition has caused the spilling of much ink. One of Mendeleev s greatest triumphs was his prediction of a new element ( eka-silicon ) between Si and Sn in his Table. Mendeleev had the audacity to predict some chemical properties of this new element, and his prophecies were substantially fulfilled a few years later by the isolation of germanium and a preliminary exploration of its chemistry. These predictions were made simply by interpolation between Si and Sn. Chemists, and chemistry students, have come to expect that the chemical properties within a Group follow monotonic trends properties can be predicted by interpolation and extrapolation. Experimental observations which do not fit such simple trends lead to the identification of anomalies . At one extreme, there may be a tendency to sweep such anomalies discreetly under the carpet, or even to question the validity of the data at the other extreme, strenuous efforts are made to account for anomalies by means of elaborate and sometimes fanciful theorising. 

The move toward catalytic reactions is reflected in the increase in the number of chapters in this book on the topic compared to the first edition. The trend has been observed by noted chemists in the previous decade. Professor Seebach, for example, in 1990 stated the primary center of attention for all synthetic methods will continue to shift toward catalytic and enantioselective variants indeed, it will not be long before such modifications will be available for every standard reaction. 6 Professor Trost in 1995 was a little more specific with catalysis by transition metal complexes has a major role to play in addressing the issue of atom economy—both from the point of view of improving existing processes, and, most importantly, from discovering new ones. 7 However, the concept can be extended to biological and organic catalysts and to those based on transition metals. 

Perhaps ironically, it was a herbicide—aminotriazole—which started the present regulatory trend and resulted most recently in rather specific government demands for environmental chemistry data to permit the registration of new herbicides and reregistration of old favorites (44). Modern society is being pushed inexorably toward a most serious dilemma the requirement for pest control vs the need for human and environmental safety. As we have seen in just the few examples of this Chapter, much— perhaps most—of our uncertainty arises from ignorance of the forces which act upon chemicals in the environment. Time is growing short for chemists to learn and apply the scientific fundamentals of the photochemical, microbial, and transportive phenomena which have been observed for centuries to influence us and our environment. 

It may come as a surprise to many readers, but the proportion, and even the total number, of women students at university reached a maximum in the late 1920s the trend applied equally to women chemists.10 In Chap. 13, we examine this phenomenon and provide observations on the subsequent decline. 

Trommsdorff s Journal der Pharmacie, apothecaries participated in the regular communication of the community of chemists.1191 would like to extend Hufbauer s argument to include not only communication and justification, but also apothecaries investigative practice in the laboratory. The possibility of publishing papers may have provided an incentive for more careful observation in the pharmaceutical laboratory and more systematic and extended chemical investigation. It may have reinforced a trend, which hinged on the correspondence between the material culture of the pharmaceutical and the chemical laboratories, to shift from the pharmaceutically useful to chemical analysis and the experimental history of substances.120 Thus chemical journals may have fed back into the actual practice of apothecaries and contributed to the emergence of the persona of the apothecary-chemist on many different levels. 

Many of the chemical properties of the elements can be understood IN TERMS OF THEIR ELECTRON CONFIGURATIONS. BECAUSE ELECTRONS FILL ATOMIC ORBITALS IN A FAIRLY REGULAR FASHION, IT IS NOT SURPRISING THAT ELEMENTS WITH SIMILAR ELECTRON CONFIGURATIONS, SUCH AS SODIUM AND POTASSIUM, BEHAVE SIMILARLY IN MANY RESPECTS AND THAT, IN GENERAL, THE PROPERTIES OF THE ELEMENTS EXHIBIT OBSERVABLE TRENDS. CHEMISTS IN THE NINETEENTH CENTURY RECOGNIZED PERIODIC TRENDS IN THE PHYSICAL AND CHEMICAL PROPERTIES OF ELEMENTS LONG BEFORE QUANTUM THEORY CAME ONTO THE... 

Of the many papers reporting spectra of Mn + in various host lattices, only a few where important trends or where superhyperfine splittings are observed and which are of relevance to an inorganic chemist will be mentioned and listed in Table XXXIV. 

The reaction is said to be regioselective. This trend was first observed in 1875 by Russian chemist Alexander M. Zaitsev (University of Kazan), and as aresult, the more substituted alkene is called the Zaitsev product. However, many exceptions have been observed in which the Zaitsev product is the minor product. For example, when both the substrate and the base are sterically hindered, the less substituted alkene is the major product ... 

Observations. These are the facts our ideas must explain. The most useful observations are quantitative because they can be analyzed to reveal trends. Pieces of quantitative information are data. When the same observation is made by many investigators in many situations with no clear exceptions, it is summarized, often in mathematical terms, as a natural law. The observation that mass remains constant during chemical change— made in the 18 century by the French chemist Antoine Lavoisier (1743-1794) and numerous experimenters since—is known as the law of mass conservation (Chapter 2). 

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