Curly arrows substitution

Mistakes with valencies As electrons are moved around via curly arrows, it is imperative to remember how many electrons are associated with a particular atom, and not to exceed the number of bonds permitted. The usual clanger is five-valent carbon, typically the result of making a new bond to a fully substituted carbon (four bonds, eight electrons) without breaking one of the old bonds. This is the case in the example shown. 

The OH group is said to be orfho, para-directing towards electrophiles. No substitution occurs in either meffl position. We can understand this by looking at the curly arrow mechanisms or by looking at the molecular orbitals. In Chapter 21 (p. 526) we looked at the K system of an enolate and saw how the electron density is located mainly on the end atoms (the oxygen and the carbon). In phenol it is the ortho and para positions that are electron-rich (and, of course, the oxygen itself). We could show this using curly arrows. 

Part Z The Mechanism of Substitution and Part 3 Elimination and Addition Pathways and Products are concerned with organic reaction mechanisms. Curly arrows are introduced and the key features of the two common mechanisms of nucleophilic substitution are reviewed. Including kinetic features, stereochemical outcome and reaction coordinate diagrams. This leads to a discussion of the features of El and E2 elimination reactions. The book finishes with a discussion of the factors that affect the competition between substitution and elimination reactions. Much of the teaching of substitution mechanisms Is carried out via interactive CD-ROM activities. 

Early depictions relevant to the development of the electronic theory of organic chemistry A) representation of the reaction between 1,3-butadiene and Br2, in the manner of Robinson, depicting partial bonds and polarities B) the "birth of the Robinson-Kemack curly arrows representing partial valences in 1,3-butadiene C) application by Robinson of curly arrows toward the explanation of meta substitution in nitroso benzene D) early representations by Thiele and others of the equivalence of all bonds in benzene E) Robinson s 1925 representation of benzene... 

The reason was the first ever use of curly or curved arrows, in this case to represent tautomerism in A,A-dimethylamino derivatives. While the arrows were not then meant to indicate movement of electrons (as was later universal in the electronic theory of organic reactions), it is most probable that the symbols were adopted by Robert Robinson who, with Watson, worked at British Dyes during World War I123. Arthur Lapworth and Alfred Werner had already used arrows in mechanistic studies, the former perhaps influenced by the inventor of the TNA process, Bernard J. FlUrscheim, who explained benzene substitution patterns in terms of affinity demand , indicated by arrowed bonds124. 

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