Resonance structures curved arrows

Although these curved arrows look exactly like the curved arrows used for drawing resonance structures, there is an important difference. When drawing resonance structures, curved arrows... 

Curved arrows are used to illustrate the mechanism by which an organic reaction occurs. Uniike their use in resonance structures, curved arrows in a reaction mechanism correspond to the actuai ma/ementdl electrons. 

Chemists do not like to use dotted lines when drawing structures because, unlike a solid line that represents two electrons, the dotted lines do not specify the number of electrons they represent. Therefore, chemists use structures with localized electrons (indicated by solid lines) to approximate the resonance hybrid that has delocalized electrons (indicated by dotted lines). These approximate stmctures are called resonance contributors. Curved arrows are used to show the movement of electrons in going from one resonance contributor to the next. 

Electron delocalization can be important in ions as well as in neutral molecules Using curved arrows show how an equally stable resonance structure can be generated for each of the following anions... 

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... 

Resonance forms differ only in the placement of their tt or nonbonding electrons. Neither the position nor the hybridization of any atom changes from one resonance form to another. In the acetate ion, for example, the carbon atom is sp2-hybridized and the oxygen atoms remain in exactly the same place in both resonance forms. Only the positions of the r electrons in the C=0 bond and the lone-pair electrons on oxygen differ from one form to another. This movement of electrons from one resonance structure to another can be indicated by using curved arrows. A curved arrow always indicates the movement of electrons, not the movement of atoms. An arrow shows that a pair of electrons moves from the atom or bond at the tail of the arrow to the atom or bond at the head of the arrow. 

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. 

The curved arrows show how one resonance structure relates to another. Notice that the formal negative charge is located on the ortho and para positions, exactly where reaction takes place most quickly. Other ortho- and para-directing groups include —NH2, —Cl, and —Br. All have an atom with a lone pair of electrons next to the ring, and all accelerate reaction. 

CURVED ARROWS THE TOOLS FOR DRAWING RESONANCE STRUCTURES ... 

In the beginning of the course, you might encounter problems like this here is a drawing now draw the other resonance structures. But later on in the course, it will be assumed and expected that you can draw all of the resonance structures of a compound. If you cannot actually do this, you will be in big trouble later on in the course. So how do you draw all of the resonance structures of a compound To do this, you need to leam the tools that help you curved arrows. 

Now we know what curved arrows are, but how do we know when to push them and where to push them First, we need to learn where we cannot push arrows. There are two important rules that you should never violate when pushing arrows. They are the two commandments of drawing resonance structures ... 

Now we have all the tools we need. We know why we need resonance structures and what they represent. We know what curved arrows represent. We know how to recognize bad arrows that violate the two commandments. We know how to draw arrows that get you from one structure to another, and we know how to draw formal charges. We are now ready for the final challenge using curved arrows to draw resonance structures. 

Mechanisms, like resonance structures, utilize curved arrows. (Resonance structures are ways of illustrating the Vcirious resonance forms that contribute to the resonance hybrid. If you need more review, refer to Organic Chemistry I For Dummies. ) Many of the same rules apply to both however, there are some important differences ... 

Here the curved arrows represent the direction of flow of electrons needed to convert from the one resonance structure to the other.) For similar reasons, some reactions require the use of special op+ constants for strongly electron-donating substitu-... 

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 ... 

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

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. 

Every curved arrow has a head and a tail. It is essential that the head and tail of every arrow be drawn in precisely the proper place. The tail shows where the electrons are coining from, and the head shows where the electrons are going (remember that the electrons aren t really going anywhere, but we treat them as if they were so we can make sure to draw all resonance structures) ... 

Some atoms, even in covalent compounds, carry a formal charge, defined as the number of valence electrons in the neutral atom minus the sum of the number of unshared electrons and half the number of shared electrons. Resonance occurs when we can write two or more structures for a molecule or ion with the same arrangement of atoms but different arrangements of the electrons. The correct structure of the molecule or ion is a resonance hybrid of the contributing structures, which are drawn with a double-headed arrow () between them. Organic chemists use a curved arrow (O) to show the movement of an electron pair. 

This is the acetate anion. The curved arrows are used to help keep track of how electrons are moved to get from the first resonance structure to the second. An unshared pair of electrons on the lower oxygen is moved in to become the pi electrons in the second structure. The pi electrons are moved to become an unshared pair on the upper oxygen. Resonance structures must always have the same total charge — in this case — I. These structures happen to be equivalent in other respects also, so they contribute equally to the resonance hybrid. With two important resonance structures, the acetate anion has a large resonance stabilization. It is significantly more stable than would be predicted on the basis of examination of only one of the structures. 

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