Curved Arrows and Chemical Reactions

In Section 1.8 we introduced curved arrows as a tool for systematically converting one resonance contributor to another. Their more common use is to track electron flow in chemical reactions. The remainder of this chapter introduces acid-base chemistry and illustrates how curved-arrow notation enhances our understanding of chemical reactions by focusing on electron movement. 

There are two kinds of curved arrows. A double-barbed arrow (/ ) shows the movement of a pair of electrons, either a bonded pair or a lone pair. A single-barbed, or fishhook, arrow (/ ) shows the movement of one electron. For now, we ll concern ourselves only with reactions that involve electron pairs and focus on double-barbed arrows. 

We ll start with some simple examples,— reactions involving only one electron pair. Suppose the molecule A—B dissociates to cation A+ and anion B . A chemical equation for this ionization could be written as  

The reaction is the same but the second equation provides more information by including the bond that is broken during ionization and showing the flow of electrons. The curved arrow begins where the electrons are originally—in the bond—and points to atom B as their destination where they become an unshared pair of the anion B . 

Dissociations of this type are common in organic chemistry and will be encountered frequently as we proceed tluough the text. In many cases, the species A has its positive charge on carbon and is referred to as a carbocation. Dissociation of an alkyl bromide for example, involves breaking a C—Br bond with the two electrons in that bond becoming an unshared pair of bromide ion. 

To illustrate, the ionization of carbonic acid corresponds to that just described for A—B. A neutral molecule (H2CO3) dissociates to a cation (H ) and an anion (HC03 ). 

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Section 1.12 Curved Arrows and Chemical Reactions introduces the student to the notational system employed in all of the mechanistic discussions in the text. 

In Section 1 9 we introduced curved arrows as a tool to systematically generate resonance structures by moving electrons The mam use of curved arrows however is to show the bonding changes that take place in chemical reactions The acid-base reactions to be discussed in Sections 1 12-1 17 furnish numer ous examples of this and deserve some preliminary comment... 

Chemists use curved arrows to show the electronic changes that occur during a chemical reaction. Fot example, the arrows describing the Sn2 reaction below show formation of a CC bond and loss of a Cl bond. 

Each of these variables will be considered in this book. We start with concentrations, because they determine the form of the rate law when other variables are held constant. The concentration dependences reveal possibilities for the reaction scheme the sequence of elementary reactions showing the progression of steps and intermediates. Some authors, particularly biochemists, term this a kinetic mechanism, as distinct from the chemical mechanism. The latter describes the stereochemistry, electron flow (commonly represented by curved arrows on the Lewis structure), etc. 

Four different regimes of the I-V curve for moderately doped silicon electrodes in an HF electrolyte are shown in Fig. 3.2. These regimes will now be discussed in terms of the charge state of the electrode, the dependence on illumination conditions, the charge transfer, the mass transport, and accompanying chemical reactions. Transient effects are indicated in Fig. 3.2 by a symbol with an arrow. 

The organic chemist made an important step in the understanding of chemical reactivity when he realized the importance of electronic stabilization caused by the delocalization of electron pairs (bonded and non-bonded) in organic molecules. Indeed, this concept led to the development of the resonance theory for conjugated molecules and has provided a rational for the understanding of chemical reactivity (1, 2, 3). The use of "curved arrows" developed 50 years ago is still a very convenient way to express either the electronic delocalization in resonance structures or the electronic "displacement" occurring in a particular reaction mechanism. This is shown by the following examples. 

The flow of electrons from a Lewis base to a Lewis acid is often indicated with a curved arrow. The arrow starts on a pair of nonbonding electrons on the Lewis base and points toward the Lewis acid with which it reacts. Because adding a pair of electrons to one point on a molecule often displaces electrons in the molecule, combinations of curved arrows are often used to describe even simple chemical reactions. Consider the following example, in which a pair of electrons on an NH2 ion are donated to the H+ ion formed when the electrons in one of the CH bonds in acetylene are given to the carbon atom instead of being shared by the C and H atoms in this bond. 

Since we are describing a dynamic process of electron movement from one molecule to another in this last reaction, it is natural to use some sort of arrow to represent the process. Organic chemists use a curved arrow (called a curly arrow ) to show what is going on. It is a simple and eloquent symbol for chemical reactions. 

Chemical reactions entail the breaking and forming of covalent bonds. The flow of electrons in the course of a reaction can be depicted by curved arrows, a method of representation called "arrow pushing." Each arrow represents an electron... 

Curved arrowrs with full heads are used to describe the breaking of bonds and the formation of new ones in chemical reactions. Half-headed arrows are introduced to cover reaction situations where pairs of electrons forming bonds are supposed to move in different directions and join up in new pairings. 

Chemical reaction mechanisms, which trace the formation and breakage of covalent bonds, are communicated with dots and curved arrows, a convention known informally as electron pushing. ... 

The description of the step-by-step process by which reactants (e.g., alkene + HBr) are changed into products (e.g., alkyl halide) is called the mechanism of the reaction. To help us understand a mechanism, curved arrows are drawn to show how the electrons move as new covalent bonds are formed and existing covalent bonds are broken. In other words, the curved arrows show which bonds are formed and which are broken. Because the curved arrows show how the electrons flow, they are drawn from an electron-rich center (at the tail of the arrow) to an electron-deficient center (at the point of the arrow). An arrowhead with two barbs / represents the simultaneous movement of two electrons (an electron pair). An arrowhead with one barb represents the movement of one electron. These are called curved arrows to distinguish them from the straight arrows used to link reactants witli products in chemical reactions. 

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