Resonance structure arrows

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 double-headed arrow is used to separate resonance structures. 

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. 

The compound above has two important resonance structures. Notice that we separate resonance structures with a straight, two-headed arrow, and we place brackets around the structures. The arrow and brackets indicate that they are resonance structures of one molecule. The molecule is not flipping back and forth between the different resonance structures. 

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 that we know how to identify good arrows and bad arrows, we need to get some practice drawing arrows. We know that the tail of an arrow must come either from a bond or a lone pair, and that the head of an arrow must go to form a bond or a lone pair. If we are given two resonance structures and are asked to show the arrow(s) that get us from one resonance structure to the other, it makes sense that we need to look for any bonds or lone pairs that are appearing or disappearing when going from one structure to another. For example, consider the following resonance structures ... 

It is pretty straightforward to see how to push only one arrow that gets us from one resonance structure to another. But what about when we need to push more than one arrow to get from one resonance structure to another Let s do an example like that. 

In this example, we can see that one of the lone pairs on oxygen is coming down to form a bond, and the C=C double bond is being pushed to form a lone pair on a carbon atom. When both arrows are pushed at the same time, we are not violating either of the two commandments. So, let s focus on how to draw the resonance structure. Since we know what arrows mean, it is easy to follow the arrows. We just get rid of one lone pair on oxygen, place a double bond between carbon and oxygen, get rid of the carbon-carbon double bond, and place a lone pair on carbon ... 

EXERCISE 2.20 Draw the resonance structure that you get when you push the arrows shown below. Be sure to include formal charges. 

PROBLEMS For each of the structures below, draw the resonance structure that you get when you push the arrows shown. Be sure to include formal charges. (Hint In some cases the lone pairs are drawn and in other cases they are not drawn. Be sure to take them into account even if they are not drawn—you need to train yourself to see lone pairs when they are not drawn.)... 

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. 

The arrow on the top structure violates the octet rule (giving carbon five bonds), and the arrow on the bottom structure does not violate the octet rule. The arrow on the bottom structure will therefore provide a valid resonance structure ... 

Once you learn to recognize this pattern (a lone pair next to a pi bond), you will be able to save time in calculating formal charges and determining if the octet rule is being violated. You will be able to push the arrows and draw the new resonance structure without thinking about it. 

Which of these options is the best Lewis structure Actually, no single Lewis structure by itself is an accurate representation of NO3. Any single structure of the anion shows nitrate with one NDO double bond and two N— O single bonds. In Section 9 1, we show that single and double bonds between the same types of atoms have different lengths and different energies. In contrast, experiments show that the three nitrate N—O bonds are identical. To show that the nitrate N—O bonds are all alike, we use a composite of the three equivalent Lewis structures. These are traditionally called resonance structures. Resonance stmctures are connected by double-headed arrows to emphasize that a complete depiction requires all of them. 

The two candidates are equivalent Lewis structures, so the best depiction of H2 PO4 shows two resonance structures connected by a double-headed arrow. 

Fig. 41. Proposed mechanisms for the reactions (a) Y + propene, (b) Y + cis-2-butene, (c) Y + 1-butene, (d) Y + isobutene. Note that the mechanism for Y + trans-2-butene is similar to that for Y + cis-2-butene and so is not shown. Double-sided arrows indicate resonance structures. See text for details.

Resonance structures or resonance contributors are connected by double-headed arrows (<- ) => the real molecule, radical, or ion is a hybrid of all of them. 

Figure 3.78 Generic arrow-pushing diagrams (left) and secondary resonance structures (right) for vicinal (upper) and geminal (lower) NBO donor-acceptor interactions.

Figure 11.11 shows there are some molecules which can legitimately be drawn in several different ways using Lewis structures, each conforming to the octet rule. These are resonance structures, and are equally valid, but the true structure is a hybrid of the two or more possible structures. This is indicated by the double-headed arrow, where the electrons are moved, but the atoms stay in position. However, in this example, the carbon oxygen bonds are of equal length - they do not rapidly interconvert from one version to another. The true... 

Sometimes when writing the Lewis structure of a species, we may draw more than one possible correct Lewis structure for a molecule. The nitrate ion, N03 , is a good example. The structures that we write for this polyatomic anion differ in which oxygen has a double bond to the nitrogen. None of these three truly represents the actual structure of the nitrate ion—it is an average of all three of these Lewis structures. We use resonance theory to describe this situation. Resonance occurs when more than one Lewis structure (without moving atoms) is possible for a molecule. The individual structures are called resonance structures (or forms) and are written with a two-headed arrow (<- ) between them. The three resonance forms of the nitrate ion are ... 

To communicate the bonding in SO2 more accurately, chemists draw two Lewis structures and insert a double-headed arrow between them. Each of these Lewis structures is called a resonance structure. Resonance structures are models that give the same relative position of atoms as in Lewis structures, but show different places for their bonding and lone pairs. 

To indicate resonance forms, we use a doubleheaded arrow between the contributing structures. This arrow is reserved for resonance structures and never used elsewhere. The difference between the two structures is that the electrons in the n bonds have been redistributed, and we can illustrate this by use of another type of arrow, a curly arrow. This arrow is used throughout chemistry to represent the movement of two electrons. In the benzene case, a cyclic movement of electrons accounts for the apparent relocation of double bonds, though there are two ways we might show this process both are equally satisfactory. 

The resonance terminology and the double-headed arrow may give the impression that the structures are rapidly interconverting. This is not true. We must appreciate right from the start that resonance structures are entirely hypothetical. They are our (sometimes clumsy) attempt to write down on paper what the bonding in the molecule might be like, and they may depict only the extreme possibilities. The molecule is presumably happily going about its business in a form that we cannot easily depict. Nevertheless, resonance structures are extremely useful and do help us to explain chemical behaviour. 

These simple examples illustrate the basic rules for mechanism and the use of curly arrows. The concepts are no different from those we have elaborated for drawing resonance structures (see Section 2.10) ... 

The resonance arrow, a single line with arrow heads at both ends (see Figure 2-1), separates different resonance structures. The actual structure is... 

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