Curved-arrow notation

A full-headed curved arrow indicates the movement of two electrons from the tail of the arrow to the head. A half-headed curved arrow indicates the shift of one electron likewise. The two ways that a bond can break are heterolytic (two electrons) or homolytic (one electron). Homolytic processes are unusual and will be treated separately in Chapter 11. 

Arrows indicate a movement or flow of electrons that must come from a site of electron density, either a lone pair or a bond, and move to a site that can accept additional electron density. 

If an arrow comes from a bond, that bond is broken. If an arrow comes from a lone pair, the lone pair is removed and a new bond is formed at the head of the arrow. If the head of the arrow points between two atoms, it forms a new bond between them. If it points to an atom, it forms a new lone pair on that atom. 

A source of confusion for beginning students is that for intermolecular bond-forming reactions, some authors will point the arrow between the two atoms, whereas others will 

The bond or lone pair from which the first arrow in an electron flow originates is called the electron source. The head of the last arrow in an electron flow points to the electron sink. Arrows will always point away from negative charges and toward positive charges. Sometimes it is useful to use arrows to interconvert resonance structures, but those arrows do not really indicate electron flow. 

We keep track of electron pairs by noting changes in their location in molecules by means of curved arrows. The curved arrow depicts movement of an electron 

Organic Chemistry An Intermediate Text, Second Edition, by Robert V. Hoffman ISBN 0-471 -45024-3 Copyright 2004 John Wiley Sons, Inc. 

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IS first order in (CH3)3CC1 and first order in NaSCH2CH3 Give the symbol (El or E2) for the most reasonable mechanism and use curved arrow notation to represent the flow of electrons... 

FIGURE 9 5 (a) Curved arrow notation and (b) transition state for electrophilic addition of a hydrogen halide HXto an alkyne... 

Look closely at the acid-base reaction in Figure 2.5, and note how it is shown. Dimethyl ether, the Lewis base, donates an electron pair to a vacant valence orbital of the boron atom in BF3, a Lewis acid. The direction of electron-pair flow from the base to acid is shown using curved arrows, just as the direction of electron flow in going from one resonance structure to another was shown using curved arrows in Section 2.5. A cuived arrow always means that a pair of electrons moves from the atom at the tail of the arrow to the atom at the head of the arrow. We ll use this curved-arrow notation throughout the remainder of this text to indicate electron flow during reactions. 

Nevertheless, the full-blown mechanism that showed the role of the coenzyme was only written out in detail by Braunstein and M. M. Shemyakin in 1953 (Braunstein and Shemyakin, 1952, 1953). Their formulae (2), complete with the curved arrow notation of physical organic chemistry, clearly pointed out the role of the coenzyme as an electron sink in a ketimine mechanism. They showed how the coenzyme can function in transamination, racemization and, with some help from Hanke and his collaborators (Mandeles et al 1954), in decarboxylation. The mechanisms they advanced were exactly what we would postulate today, and constituted an early and successful application of theory to mechanistic enzymology. But it must be admitted that the theory appealed because it was reasonable the authors had little or no evidence, in terms of physical organic chemistry, to support their formulation, which is shown in part below. 

The reaction of PC14+ with Cl- is a Lewis acid-base reaction. Draw electron-dot structures for the reactants and products, and use the curved arrow notation (Section 15.16) to represent the donation of a lone pair of electrons from the Lewis base to the Lewis acid. 

If a bond is particularly weak and/or nonpolar, bond cleavage can occur by a nonpolar or homolytic process. One elecd on of die shared pah goes widi each of the two bonded atoms. Bond breaking dien is die movement of single elecdons rather than elecdon pairs and is indicated in curved-arrow notation as halfheaded arrows. Homolytic cleavage of a bond does not result in the formation of charge but does result in the formation of unpaired electron intermediates called free radicals. Free radicals normally have seven electrons in the valence... 

Curved-arrow notation is also a very useful device with which to generate resonance structures. In this application it is truly a bookkeeping system. Since individual canonical forms do not exist but are only thought of as resonance contributors to the description of a real molecule, the use of curved-arrow notation to convert one canonical form to another is without physical significance. Nevertheless it provides a useful tool to keep track of electrons and bonds in canonical structures. For example, the structures of carboxylate resonance contributors can be interconverted as follows ... 

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. 

Simple substitution reactions are shown in curved-arrow notation as... 

In aqueous solution, proton transfer to tile first formed intermediate is very rapid. However, again for illustrating die stepwise changes that must occur on the way from reactants to products using curved-arrow notation, these steps are shown independently. 

Although this process can be written to give a single canonical form A, it must be realized that the enolate is a delocalized species and resonance forms A and B can be generated as discussed previously using curved-arrow notation. This is not a mechanistic step since the delocalized product is a resonance hybrid of A and B—that is A is not converted to B, but rather the curved arrows merely indicate the changes in electron distribution that must be used to describe the canonical form B. 

There are other reactions, also easily describable by electron movement (curved arrows), in which n electrons are donated. In such reactions the n electrons are bonded electrons and hence die n-donor nucleophile is a weak electron donor. Consequently, a much stronger electron acceptor (stronger electrophile) is required for die electronic pull for electron donation to occur successfully. However, such descriptions are simply a matter of semantics because curved-arrow notation only shows changes in electrons, it does not indicate driving force. For a donor-acceptor interaction to occur productively, there has to be an energetic chiving force for the process, and die energy levels of die donor and acceptor must be matched so diat election movement from die donor to the acceptor can occur. 

Besides intermolecular reactions, curved-arrow notation is also useful in indicating bonding changes in intramolecular reactions and rearrangement. For example, Cope-type rearrangements are seen to involve changes in three pahs of bonded electrons. 

The anows can be written in either directional sense since these reactions are concerted rearrangements with all bond making-bond breaking taking place at die same time. This example emphasizes the fact that curved-arrow notation is merely an electron bookkeeping method. 

The curved-arrow notation clearly shows the electron flow needed to effect the rearrangement. What curved-arrow notation does not show is die timing of these events—that is, whether loss of a leaving group precedes or is concerted with 1,2-phenyl migration or if a bridged ion is an intermediate. Such considerations, if known, can be included in more detailed mechanistic sequences. 

Thus we see that, used properly, curved-arrow notation for electron movement is indispensable to the organic chemist as a way to depict chemical change in complex molecules. Furthermore, it can be extended to include a method for showing die mechanism if the ground rules are understood and followed carefully. 

Most undergraduate texts have a short section on curved-arrow notation or electron movement, and these discussions are tied in with the development of reaction mechanisms. 

For the following reactions, show die complete sductures of die reactants and products of the step shown, point out die bonds which have been made and/or broken, identify die electron donors and acceptors, and use curved-arrow notation to indicate electron flow. 

Using curved-arrow notation show how to derive three principal resonance structures for the following ... 

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