Organic reactions, mechanism notation

The joint memoirs of Prevost and Kirrmann self-consciously presented a general theory of organic chemistry that constituted an application of physical methods and principles to the problem of organic reaction mechanisms. However, the language system devised by Prevost and Kirrmann was not adopted by chemists in general, and that part of their notation which was new was not used outside France. Indeed, there was very little interest in their work inside France. 

This is an extension of the skeletal notation. It is very good at indicating the structure of ring compounds and the cyclic transition states of aliphatic compounds, and so is particularly useful when writing organic reaction mechanisms that involve such species. 

Use of curved-arrow notation to depict die mechanisms of organic reactions requires that appropriate mechanistic principles be superimposed on the correct use of curved arrows to denote movement of electrons. The mechanism of a reaction is the stepwise process by which reactants are converted to products, and generally each step involves bond making and/or bond breaking that can readily be depicted by curved-arrow notation. 

The notation for junctive processes is applicable to mechanisms of organic reactions as well. In the two alternative mechanisms for the free-radical chlorination of cyclopropane, the reaction pathway proceeds towards the transition state through either (201+202- [203] ) or (l,2)j - (201+202- [206] ), whereas the transition state transforms into products through either (l,l)d- ([203] -204+205) or (l,2)d- ([206] -204+205). 

CHAPTER 3 ACIDS AND BASES An Introduction to Organic Reactions and Their Mechanisms CURVED-ARROW NOTATION... 

We begin our study of mechanisms in the context of acid-base chemistry in Chapter 3. Acid-base reactions are fundamental to organic reactions, and they lend themselves to introducing several important topics that students need early in the course (1) curved arrow notation for illustrating mechanisms, (2) the relationship between free-energy changes and equilibrium constants, and (3) the importance of inductive and resonance effects and of solvent effects. 

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,... 

This sequence is one of the most ubiquituous in organic or organometallic chemistry and has already been discussed in preceding parts of this section. The chemical step may involve reactions (or a succession of chemical reactions) of high-order molecularities, not only first-order reactions as suggested by the notation. Classic examples are given by activated olefin electrohydrodimerization mechanisms ... 

Acids and bases Chapter 2 on acids and bases serves two purposes. It gives students experience with curved arrow notation using some familiar proton transfer reactions. It also illustrates how some fundamental concepts in organic structure affect a reaction, in this case an acid-base reaction. Since many mechanisms involve one or more acid-base reactions, 1 emphasize proton transfer reactions early and come back to this topic often throughout the text. 

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