R & R (Resonance and Rationalization)

Introduction

As was mentioned earlier, the concept of resonance was developed to rationalize the failure of valence bond theory to accurately represent the electron distribution in certain molecules. Evidence that points out this limitation comes from a variety of experiments. In tis discussion we will restrict our attention to spectroscopic evidence, namely chemical shift data in 1H-NMR spectroscopy. Before we look at that data, let's take a second look at Figure 4 from our introductory discussion of resonance to make certain we understand the implications of the structures and the arrows depicting the movement of electrons.

Figure 4 (revisited)

The Resonance Formalism

You should be aware of the following implications:

1. In resonance contributors 1 and 2 the electron denisty is highest on the oxygen atoms that bear the formal negative charge.

2. The arrow labeled 1 implies that one of the three sp3 hybridized, non-bonding orbitals on the negatively charged oxygen is beginning to overlap with the unhybridized p orbital on the adjacent carbon atom. The conversion of a non-bonding pair of electrons that is localized on the oxygen atom to a bonded pair that is shared between the oxygen atom and the adjacent carbon results in a decrease in electron density around the oxygen atom and an increase around the carbon.

3. The octet rule applies. In resonance contributor 1 there are 8 electrons around the carbon. Transferring electron density from the oxygen to the carbon, as implied by arrow 1, results in the structure shown in Figure 1A. This is the same structure that was shown in red in the animation in Figure 3 of the first page of this topic.

Figure 1A

Breakin' the Rules

4. The increase in orbital overlap implied by arrow 1 is accompanied by a comparable decrease in orbital overlap as shown by the arrow labeled 2. This prevents a violation of the octet rule.

5. The net result of the orbital interactions depicted by arrows 1 and 2 is a transfer of electron density from one oxygen atom to the other as indicated in resonance contributor 2 in Figure 4.

Exercise 1 How many valence electrons are there in the structure shown in Figure 1A?

Exercise 2 How many electrons are there around the carbon atom in Figure 1A.

Using Resonance to Rationalize Experimental Data

Now let's look at a comparable situation where this type of transfer of electron density manifests itself spectroscopically. Consider the resonance interaction depicted in Figure 2A.

Figure 2A

The Give and Take of Resonance

Exercise 3 According to Figure 2A, does the resonance interaction indicated by arrows 1 and 2 increase or decrease the electron density around the methylene carbon? increase decrease

Exercise 4 Based on what you have learned about the correlation between chemical shifts and electron density, would you predict the resonance interaction shown in Figure 2A to shift the signal for the methylene protons to higher or lower d values relative to a reference system where no such interaction is possible? higher lower

Now let's see how the prediction you made in Exercise 4 agrees with the expeimental facts. To do so we have to consider NMR data for four different compounds, butane (CH3CH2CH2CH3), methyl ethyl ether ( CH3CH2OCH3), and 1-butene ( CH2=CHCH2CH3), and methyl vinyl ether ( CH2=CHOCH3), the compound shown in Figure 2A. The formulas above are color-coded to distinguish between that part of each molecule giving rise to the signal we will be considering (shown in blue) and that part of the molecule, shown in red, to which we attribute the change in the chemical shift of that signal. The chemical shift data is summarized in Figure 3A.

Figure 3A

Everything's Shifting

There are several points to be aware of before we consider the data.

1. There are no resonance interactions possible in butane or methyl ethyl ether. Therefore we can attribute chemical shift differences soley to differences in the inductive effects, i.e. electronegativities, of the substituents.

2. In comparing butane and methyl ethyl ether, we are looking at the effect of changing a C (in the CH2 group) to an O, i.e. we are looking at the effect of increasing the electronegativity of the substituent attached to the CH2 group on the chemical shift of the methyl group attached to that CH2 group.

3. We are going to assume that the inductive effects of the CH2CH3 and the OCH3 groups are the same in butene and methyl vinyl ether as they are in butane and methyl ethyl ether, respectively.

Now let's see how we rationalize the experimental data. First, we'll focus on butane and methyl ethyl ether. When we replace a CH2 group with an O, the chemical shift of the methyl group increases from 0.33 to 1.14 ppm, i.e. Dd = (1.14-0.33) = 0.81 ppm. We attibute this change to a decrease in electron density around the methyl hydrogens which, in turn, we attribute to the greater electronegativity of the oxygen atom compared to the carbon atom.

Now let's compare 1-butene and methyl vinyl ether. Here the effect of replacing a CH2 group with an O is just the opposite; the chemical shifts of the methylene protons decrease; Dd = (3.88-4.87) = - 0.99 ppm and (4.03-4.94) = -0.91 ppm. Clearly the electron density around the methylene hydrogens in methyl vinyl ether is greater than it is in 1-butene. This increase is rationalized in terms of the resonance interactions depicted in Figure 2A.

Exercise 5 Consider the methylene proton in 1-butene that resonates at 4.94 ppm. What would you expect the chemical shift of the corresponding proton in methyl vinyl ether to be if our assumption about inductive effects being the same in butene and methyl vinyl ether as they are in butane and methyl ethyl ether were true? ppm
We are not going to abandon our assumption that the inductive effects of the CH2CH3 and the OCH3 groups are the same in butene and methyl vinyl ether as they are in butane and methyl ethyl ether. Rather, we take that statement to be true and interpret the decrease in chemical shift to mean that the resonance effect of OCH3 group more than counterbalances its inductive effect. We can even estimate the magnitude of the chemical shift that is due to the resonance effect of the oxygen atom.
Exercise 6 The chemical shift decrease of -0.91 ppm represents the net result of the inductive and resonance effects of the methoxy group. What is the value of the chemical shift decrease that is due soley to the resonance effect? ppm
Now let's consider the chemical shift data for ethylbenzene and anisole that is summarized in Figure 4A.

Figure 4A

More Give and Take

Exercise 7 Does this data indicate that the electron density is higher in ethylbenzene or anisole? ethylbenzene anisole

Exercise 8 What is the chemical shift of the hydrogen atom in anisole that has the highest electron density around it? ppm

Exercise 9 Draw the structure of the resonance contributor which indicates that the electron density is high at the ortho position(s) of anisole. (In a monosubstituted aromatic ring, the carbon to which the substituent is attached is labeled C-1, positions C-2 and C-6 are called the ortho positions, while C-3 and C-5 are the meta positions, and C-4 is called the para poisition.)

Exercise 10 Draw the structure of the resonance contributor which indicates that the electron density is high at the para position of anisole.

Exercise 11 The chemical shift of the meta hydrogen in anisole is slightly higher than that of the meta position in ethylbenzene. Is this difference an inductive effect or a resonance effect? inductive resonance

Because the overlap of one of the orbitals containing a lone pair of electrons on the oxygen atom with the p orbitals of the pi system of the aromatic ring results in an increase in electron density within that pi system, the methoxy group is classified as an electron donating group. The type of NMR data we have been considering provides one line of experimental evidence that chemists have used to classify various groups as either electron donating or as electron withdrawing groups. We'll end this discussion by considering chemical shift data that is typical of a molecule that contains an electron withdrawing group. Figure 5A presents the chemical shifts of ethylbenzene and acetophenone. The substituent on the aromatic ring in acetophenone is called an acetyl group.

Figure 5A

Take and Give

Notice that the chemical shifts of all of the hydrogen atoms in acetophenone are higher than those of the comparable hydrogens in ethylbenzene. This indicates that the electron density around all of the hydrogens in acetophenone is lower than it is in ethylbenzene. Hence the acetyl gorup is classified as an electron withdrawing group. More important than the fact that the acetyl group reduces the electron density in the pi system of the aromatic ring is the fact that is does so selectively. The electron withdrawing effect of the acetyl group is felt most strongly at the ortho position, then at the para position, and least at the meta position. Figure 6A shows how resonance theory rationalizes the reduction of electron density at the ortho position.

Figure 6A

Move 'em Out

It is important to understand the implications of Figure 6A. Resonance theory is not saying that C-2 of acetophenone is positively charged, nor that the oxygen atom is negatively charged. What resonance theory does say is that, to the extent that there is some overlap of the p orbitals of the C-O pi bond and p orbitals of the pi system of the aromatic ring, as implied by arrows 1 and 2, the electron density at C-2 of the aromatic ring is reduced while the electron density around the oxygen atom is increased.In other words, the resonance interaction increases the "positive character" of C-2 and the "negative character" of the oxygen atom.

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Exercise 12 Draw the structure of the resonance contributor which indicates that the electron density is reduced at the para position of acetophenone.

Exercise 13 Draw the structure of the resonance contributor which indicates that the electron density is reduced at the other ortho position of acetophenone.

The electronic effect of a substituent attached to an aromatic ring is the composite of the inductive effect and the resonance effect of that substituent. The inductive effect of a substituent affects all of the atoms in the aromatic ring, although the magnitude of the effect decreases rapidly with distance. The resonance effect of a substituent affects the carbon atoms in positions ortho and para to the substiuent selectively. In anisole the resonance effect of the methoxy group increases the electron density selectively at the ortho and para positions. In acetophenone it reduces the elecron density selectively at those positions. The resonance effect of a substituent does not affect the carbon atoms meta to the substituent.
Exercise 14 Is the inductive effect of an acetyl group electron withdrawing or electron donating? withdrawing donating

Exercise 15 The chemical shifts of the aromatic protons in phenyl acetate ,, are 7.12, 7.22, and 7.40 ppm. Is the inductive effect of an acetoxy group electron withdrawing or electron donating? dontaing withdrawing

Exercise 16 The signal at 7.12 ppm in the 1H-NMR spectrum of phenylacetate may be assigned to the ortho hydrogens meta hydrogens para hydrogens

Exercise 17 Draw the structure of the resonance contributor that best rationalizes the increase in electron density at the para position of phenylacetate.

Exercise 18 In benzonitrile,, the ortho protons resonate at 7.51 ppm, while the signal for the meta protons occurs at 7.44 ppm. Would you classify the nitrile group as electron withdrawing or electron donating? withdrawing donating

Exercise 19 Draw the structure of the resonance contributor that best rationalizes the decrease in electron density at the para position of benzonitrile.

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