Shorthand mechanisms

You will meet many more mechanisms like this, in which an unspecified base removes a proton from an intermediate. As long as you can satisfy yourself that there is a base available to perform the task, it is quite acceptable to write either of these shorthand mechanisms. 

In the first reaction there are two nucleophilic substitutions and you must decide which nucleophile attacks first. The amine is a better nucleophile than the alcohol and the cyclization occurs because it is an equilibrium with two equal leaving groups (both alcohols) but one (EtOH) goes away when it leaves while the other is attached and cannot escape. The second reaction is more straightforward. The product is used to control the stereochemistry of new molecules as you will see in Chapter 45. For the first time we are using shorthand mechanisms. Note the double-headed arrow on the carbonyl group and the omission of proton transfer steps. If you drew the full mechanism, you did a better job. If you removed the amide (NH) proton before reaction with the acid chloride in the second step you also did a better job. 

The conventional representation is a powerful and familiar shorthand for discussing reaction mechanisms, and will be used here for that reason with the proviso that its shortcomings must constantly be borne in mind [see, for example, Albery (1993)]. 

A shorthand addition-elimination mechanism sometimes encountered is also shown. This employs a double-headed curly arrow to indicate the flow of electrons to and from the carbonyl oxygen we prefer and shall use the longer two-step mechanism to emphasize the addition intermediate. 

The necessary control of metabolism and of growth is accomplished largely through mechanisms that regulate the locations, the amounts, and the catalytic activities of enzymes. The purpose of this chapter is to summarize these control mechanisms and to introduce terminology and shorthand notations that will be used throughout this book. Many of the control elements considered are summarized in Fig. 11-1. 

The creation of tables of chemical affinities was an attempt to encapsulate all possible reactions between the constituents of chemical compounds. The goal was not only to provide a summary and key to known reactions but also to predict reactions that had not yet been observed. Tables of affinities thus had both a descriptive and a predictive role they could be used as a shorthand for a description and classification of observed reactions, and they could function as instruments of discovery. It was also possible, although not necessary, to use affinity tables as a clue to the mechanism of chemical reactions. It was along... 

Finally, it should be stressed that organic electron transfers only rarely occur as isolated steps because of the high chemical reactivity of odd-electron species. Normally, they are part of multi-step mechanisms together with other types of elementary reaction, such as bond forming and breaking. In organic electrochemistry a useful shorthand nomenclature for electrode mechanisms denotes electrochemical (= electron transfer) steps by E and chemical ones by C, and it is appropriate to use the same notation for homogeneous electron-transfer mechanisms too. Thus, an example of a very common mechanism would be the ECEC sequence illustrated below by the Ce(IV) oxidation of an alkylaromatic compound (14-17) (Baciocchi et al., 1976,... 

Before we go any further, we should introduce to you a little bit of chemical shorthand that makes writing many mechanisms easier. 

Now, this is a useful shorthand, but it is not a substitute for understanding the true mechanism. Certainly, you must never ever write... 

Do not be concerned about the details of the mechanisms note that we have used the H+I shorthand introduced in Chapter 13. and have abbreviated the mechanism where water is eliminated and the oxime formed—the full mechanism of imine (and oxime) formation can be found in Chapter 14, p. 349. in this chapter, we are much more concerned just with the structure of the products. 

In the Dirac notation, quantum mechanical integrals are represented by a shorthand notation using brackets, for instance ... 

This shorthand version of the mechanism assumes that one odd hydrogen radical (OH) is consumed per SO2 oxidized. This approach allows modellers to simulate SO2 oxidation in the troposphere. 

The sequential mechanisms can be subdivided in those which have a compulsory order and those which have a random order of S binding. These mechanisms are often described with the popular shorthand notation given by Cleland (1963), in which uni, bi, ter, etc, denote the number of S and P species. The /Cm values of the various reactants are concentration values at which half of the maxi-... 

The characterization of the interrelations between chemical bonding and molecular shape requires a detailed analysis of the electronic density of molecules. Chemical bonding is a quantum mechanical phenomenon, and the shorthand notations of formal single, double, triple, and aromatic bonds used by chemists are a useful but rather severe oversimplification of reality. Similarly, the classical concepts of body and surface , the usual tools for the shape characterization of macroscopic objects, can be applied to molecules only indirectly. The quantum mechanical uncertainty of both electronic and nuclear positions within a molecule implies that valid descriptions of both chemical bonding and molecular shape must be based on the fuzzy, delocalize properties of electronic density distributions. These electron distributions are dominated by the nuclear arrangements and hence quantum mechanical uncertainly affects electrons on two levels by the lesser positional uncertainty of the more massive nuclei, and by the more prominent positional uncertainty of the electrons themselves. These two factors play important roles in chemistry and affect both chemical bonding and molecular shape. 

The amino acid cyanoalanine is found in leguminous plants Lathyrus spp.) but not in proteins. It Is made In the plant from cysteine and cyanide by a two-step process catalysed by pyridoxal phosphate. Suggest a mechanism. We suggest you use the shorthand form of pyridoxal phosphate shown here. 

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