Arrow formalism

Whenever we can write two or more structures for a molecule with different arrangements of the electrons but identical arrangements of the atoms, we call these structures resonance structures. Resonance is very different from isomerism, for which the atoms themselves are arranged differently. When resonance is possible, the substance is said to have a structure that is a resonance hybrid of the various contributing structures. We use a double-headed arrow ( — ) between contributing structures to distinguish resonance from an equilibrium between different compounds, for which we use.  

Each carbon-oxygen bond in the carbonate ion is neither single nor double, but something in between—perhaps a one-and-one-third bond (any particular carbon-oxygen bond is single in two contributing structures and double in one). Sometimes we represent a resonance hybrid with one formula by writing a solid line for each full bond and a dotted line for each partial bond (in the carbonate ion, the dots represent one-third of a bond). 

PROBLEM 1.27 Draw the three equivalent contributing resonance structures for the nitrate ion, NOj . What is the formal charge on the nitrogen atom and on each oxygen atom in the individual structures What is the charge on the oxygens and on the nitrogen in the resonance hybrid structure Show with curved arrows how the structures can be interconverted. 

Arrows in chemical drawings have specific meanings. For example, in Section 1.12 we used curved arrows to move electrons to show the relatedness of the three resonance structures of the carbonate ion. Just as it is important to learn the structural representations and names of molecules, it is important to learn the language of arrow formalism in organic chemistry. 

Curved arrows are used to show how electrons are moved in resonance structures and in reactions. Therefore, curved arrows always start at the initial position of electrons and end at their final position. In the example given below, the arrow that points from the C=0 bond to the oxygen atom in the structure on the left indicates that the two electrons in one of the covalent bonds between carbon and oxygen are moved onto the oxygen atom  

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Fig. 1.11. Radical initiators and their mode of action (in the "arrow formalism" for showing reaction mechanisms used in organic chemistry, arrows with half-heads show where single electrons are shifted, whereas arrows with full heads show where electron pairs are shifted).

The curved-arrow formalism is universally used for keeping track of the flow of electrons in reactions. We have also used this device (in Section 1-9, for example) to keep track of electrons in resonance structures as we imagined their flow in going from one resonance structure to another. Remember that electrons do not flow in resonance structures they are simply delocalized. Still, the curved-arrow formalism helps our minds flow from one resonance structure to another. We will find ourselves constantly using these (red) curved arrows to keep track of electrons, both as reactants change to products and as we imagine additional resonance structures of a hybrid. 

PROBLEM 18.4 Represent the reaction of chlorine with each of the enol forms of 2-butanone (see Problem 18.3) according to the curved arrow formalism just described. 

Using correct arrow formalism, write the contributors to the resonance hybrid structure of the acetate ion, CH3C02. Indicate any formal charges. 

Formal Charge, Resonance, and Ciuwed-Arrow Formalism... 

Explain the different products of the following two reactions by considering the mechanism by which each reaction proceeds. As part of your explanation, use the curved arrow formalism to draw a mechanism for each reaction. 

In a reaction of HCl and ammonia, identify the electron-donating atom of the hase and draw the reaction using the curved arrow formalism, giving the products of the reaction. 

In the reaction of butanal with a nucleophile (Y), note the arrow formalism for transfer of two electrons from the electron-rich Y (the nucleophile) to the electron-deficient carbonyl carbon. In other words, when the nucleophile collides with the 6+ carbon of the carbonyl, a new C-Y bond forms and the weaker x-bond in the C=0 unit breaks. The electrons in that bond are transferred to the more electronegative oxygen atom to form an alkoxide, 29 (see the blue arrow in the illustration). This overall process is called nucleophilic... 

Problem 1L7. Using the curved arrow formalism, write a suitable pathway from a-santonin to santonic acid and then to santonide (Woodward, 1950). Answered. (Note the details of the mechanism are suggestions )... 

Problem 11.11. Examine the chemistry of Scheme 11.78. Describe (with suitable curved arrow formalism) what is happening in each step. 

Learn how to use the curved arrow formalism to push pairs of electrons in writing resonance forms and in sketching electron flow in chemical reactions. 

PROBLEM 1.16 Use the arrow formalism to convert each of the following Lewis stmctures into another resonance form. Notice that part (e) of this question asks you to do something new—to move electrons one at a time in writing Lewis forms. 

WORKED PROBLEM 1.17 Use the arrow formalism to Avrite resonance forms that contrihute to the structures of the following molecules ... 

Both of these ways of producing " CH3 and CH3 involve the concept of breaking a carbon-hydrogen bond in unsymmetrical fashion, a process known as heterolytic bond cleavage (p. 37 and Fig. 2.13). Remember the curved arrow formalism—the red arrows of Figure 2.13 move the pair of electrons in the carbon—hydrogen bond to the hydrogen or to the carbon. 

In Figure 2.60, the curved arrow formalism shows the electron flow as the new bond is formed in this illustrated nucleophile-electrophile (Lewis base-Lewis acid) reaction. 

FIGURE 2.60 The curved arrow formalism for the formation of methyl chloride from the methyl cation (Lewis acid) and a chloride ion (Lewis base). 

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