Roberts, Caserio - Basic Principles of Organic Chemistry (2nd edition, 1977)

26-49 Account for the formation of the by-product, 15,in the reaction of 4-methyl- benzenol with trichloromethane in alkali:

26-50 The important polymer intermediate "bis-phenol A" [2,2-bis-(4-hydroxy- phenyl)propane] used, among other things, in epoxy resins, is made by an acidinduced condensation of 2-propanone and benzenol. Write a stepwise mechanism for this reaction that is consistent with the nature of the reactants and the products.

\

(Review Section 15-4E on electrophilic reactions of C=O compounds, Section

26-51* Devise syntheses from benzene of each of the photographic developers whose structure is shown in Section 26-2C. Some reactions you will need are discussed in Chapters 22 and 23.

26-52 Addition of hydrogen chloride to 1,4-benzenedione yields, among other products, 2,3,5,6-tetrachloro-1,4-benzenedione. Explain how this substance might be formed, with the knowledge that equilibria such as the following are established rapidly:

26-53 Nitrous acid can substitute the more reactive aromatic derivatives by attack of NOBon the ring and form Ar-N=O compounds. A product obtained from benzenol by this kind of reaction has the formula C,H,O,N. Exactly the same substance is formed from treatment of one mole of 1,4-benzenedione with one mole of azanol (hydroxylamine; Section 16-4C). On the basis of the reactions by which it is formed, write two likely structures for this substance and explain how you would decide which one was correct on the basis of chemical and spectroscopic tests.

26-54 Consider possible benzil-benzilic acid-type rearrangements occurring with 9,lO-phenanthrenedione and 9,lO-anthracenedione. Give your reasoning as to how easily these rearrangements might occur, relative to rearrangement of benzil itself (Section 26-4E).

Supplementary Exercises

26-55 The [2 -t 21 cycloadduct of tetrafluoroethene and 1,3-cyclopentadiene, when pyrolyzed at 700" to 750" and 5-mm pressure, produces (as the result of a sigmatropic rearrangement; Section 21-10) a mixture of two new substances, each having two ' double bonds. The pyrolysis mixture, when heated in aqueous ethanoic acid containing potassium ethanoate, forms tropolone in 70% yield. Write equations for the reactions involved, with particular attention to possible structures for the pyrolysis products.

26-56 How would you expect the properties of 3- and 4-hydroxy-2,4,6-cyclohepta- trienone to compare with those of tropolone? Explain.

26-59 Make an atomic-orbital model of benzenol, showing in detail the orbitals and electrons at the oxygen atom. From your model, would you expect one, or both, pairs of unshared electrons on oxygen to be delocalized over the ring? What would be the most favorable orientation of the hydrogen of the hydroxyl group for maximum d.elocalization of an unshared electron pair?

26-58 It has been reported that compound 16 with alkali rearranges to phenyl-1,2- cyclobutenedione, 3 (Section 26-2E). This reaction appears to be the first reported reverse benzil-benzilic acid rearrangement (Section 26-4E). Explain how and why this process occurs.

27

MORE ABOUT

SPECTROSCOPY MPORTANT, LESS-COMMON SPECTROSCOP METHODS

n Chapter 9, we gave an exposition of the most generally useful and practical spectroscopic methods currently employed in modern organic laboratories. However, in our discussions of nmr spectra, we passed rather quickly over the basis of understanding why some lines are broad and others sharp, why rate effects can cause chemical shifts to be averaged, and how to correlate spinspin splitting with the energies of nmr transitions. These topics will be discussed in this chapter along with a brief explanation of the remarkable effects on nmr spectra associated with some kinds of chemical reactions, namely, chemically ind~iceddynamic nuclear polarization (CIDNP).

In addition to the spectroscopic methods covered in Chapter 9, there are a number of other spectroscopic techniques that are less generally used, but can provide, and have provided, critical information with regard to specialized problems. Because some of these are relatively new and may become more widely used in the next few years, it is important that you be aware of them and their potentialities. However, because they may be peripheral to your present course of study, we have reserved consideration of them to this chapter.

2 7 - M O W CAN WE UNDERSTAND LINE-WIDTH DIFFERENCES IN NMR SPECTROSCOPY? THE UNCERTAINTY PRINCIPLE

If you look at the nmr spectra of many different kinds of organic compounds, you will notice that some resonances are sharp and others are broad. In a few spectra, all of the peaks may be broad as the result of poor spectrometer performance, but this is not true for the spectra of Figures 9-29 (p. 3 12) and 24-2 (p. 1173) where, within a given spectrum, some resonances will be seen to be sharp and others broad. We can understand these differences by consideration of the lifetimes of the magnetic states between which the nmr transitions 0ccur.l The lifetimes of the states can be related to the width of the lines by the Heisenberg uncertainty principle.

You may have heard of the uncertainty principle, but if you have not studied chemical physics you may have little idea of its possible importance to organic chemistry. The usual statement of the principle is that there are limits to how precisely we can specify the momentum and the position of a particle at the same time. An alternative statement has more relevance to spectroscopy and chemistry, namely, that the precision with which we can define the eizergy of a state depends on the lifetime of the state. The shorter the lifetime, the less the certainty with which we can define the en erg^.^

Let us consider an example. Suppose a magnetic nucleus in a ground state with a long lifetime and rather precisely defined energy goes to an excited state with a short lifetime, At.3 The uncertainty principle tells us that the energy of the excited state cannot be defined precisely. It will have an inherent uncertainty in its energy so that an imprecise v, having an uncertainty in frequency Av, will take the nucleus from the ground state to the excited state. The imprecision of the energy AAE, or the imprecision A v in the transition frequency, v, depends on At, and is given approximately by the relationship

in which h is Planck's constant. What this means is that the absorption line corresponding to the transition will have an uncertainty in line width that is inversely proportional to At (see Figure 27-1).

IIt may be helpful to you before proceeding to review the introductions to Section 9-10 and 9-10A in which the general characteristics of the nuclear magnetic states are described.

'A brief exposition of the basis of the uncertainty principle is given by R. P. Feynman, Lect~lt-esin Physics, Addison-Wesley, Reading, Mass., 1963, Vol. 1, pp. 6-1 0.

:'The uncertainty principle will be applied in this section to nmr spectroscopy but, as we will see later, it is applicable to all other forms of spectroscopy.

27 More About Spectroscopy. Important, Less-Common Spectroscopic Methods

Figure 27-1 Schematic representation of the range of absorption frequencies involved in a transition from a long-lived ground state to an excited state of short (right) and longer (left) lifetime. The line width Av can be taken to be the width of the line in frequency units at half maximum height.

It is most convenient to think of line widths in frequency units because most of our spectra are plotted this way. If the scale is wavelength or energy, it can be converted to frequency by the procedures given previously (Section 9-3). Division of Equation 27-1 by h leads to the relationship Av - 1/(2n x At). In nmr spectroscopy, we may wish to consider spin-spin splittings or chemical shifts involving lines no farther than 1 H z apart. However, two lines 1 H z apart will not be clearly distinguishable unless Av of each is less than about 1 Hz, which corresponds to a At, the lifetime of the excited state, of 1/(2n) = 0.16 sec. If AV is 2 2 Hz, lines that are 1 H z apart will be so poorly resolved as to appear as one line (cf. Figure 27-2). A A v of 2 H z corresponds to a At of 1/(2 x 2n) = 0.08 sec. Clearly, line separations observed in nmr spectroscopy and, in fact, in all forms of spectroscopy, depend on the lifetimes of the states between which transitions take place. The lifetime of 0.16 sec required for Av to be 1 H z is a long time for a molecule! During 0.16 sec, a molecule such as ethanol in the liquid phase may undergo 1011collisions with other molecules, 101° rotations about the C-C bond, and 1012vibrations of each of the various bonds, and may even undergo a number of chemical changes. The properties of magnetic states that have lifetimes of this order clearly must be an average over all of these happenings.

It is possible to shorten the lifetime of an excited nuclear magnetic state (or increase its relaxation rate) in a number of ways. For a liquid, the simplest way is to dissolve in it paramagnetic metal ions, such as Cu(II), Fe(III), Mn(II), and the like, or other substances (0,, NO, and so on) that have unpaired electrons. Another way is to reduce the rate of motion of magnetic nuclei in different molecules with respect to one another, which is easily done by increasing the viscosity. Without going into details of the mechanisms by which substances with unpaired electrons or increased viscosity shorten the lifetime of excited nuclear magnetic states, it is important to know that dramatic line broadening thereby can be produced. Thus the proton resonance line of water is enormously broadened by adding paramagnetic Mn(I1) ions or by freezing (water molecules in ice move much more slowly relative to one another than in liquid water).

Figure 27-3 Proton nmr spectra of 2,2,3,3-tetrachlorobutane in 2-pro- panone solution at different temperatures. The curves on the left are experimental curves and those on the right are theoretical spectra calculated in accord with the uncertainty principle for different values of At. The large peak at -44" corresponds to 1, the smaller one to the enantiomers 2 and 3. The change of At with the temperature indicates that the energy barrier to rotation is about 14 kcal mole-'.

Now consider a mixture of the conformations 1, 2, and 3 in which the ltfetimes of the conformations before they convert one into the other are At.

Assuming that the lifetimes of the +l/2 and -l/2 magnetic states are long compared to At, then uncertainty In the transition energies will depend on the lifetimes of the chemical states (conformations) with different chemical shifts for the protons. The chemical-shift difference between 1 and 2 or 3 at -44", as shown by Figure 27-3, is about 5 Hz. From Equation 27-1, we can see that 5 Hz also will be the degree of the uncertainty in the frequency when At - 1/(2nAv) = 1/(2n X 5 HZ)= 0.03 sec. Thus if 1 has a lifetime much longer than 0.03 sec, say 1 sec, before going to 2 or 3, it will give a sharp resonance of its own and, of course, 2 and 3 will also. However, if 1, 2, and 3 have lifetimes much shorter than 0.03 sec, say 0.001 sec, then we expect one average resonance for 1, 2, and 3.

Either condition can be realized for 2,2,3,3-tetrachlorobutaneby taking the proton nmr spectrum at different temperatures (Figure 27-3). At -44", at which At is 1.0 sec, we see the separate peaks for 1 and for 2 and 3. At -20°, at which At is 0.045 sec, the uncertainty is such that the lines have coalesced and we no longer can see the separate peaks. When the spectrum is taken at room temperature, at which At is about 0.00005 sec, a single very sharp line is observed. We get a sharp line at this temperature because, for practical purposes, there is no uncertainty about the average chemical shift of 1, 2, and 3. The line width now is determined again by the lifetimes of the +l/2 and -l/2 magnetic states, not by the lifetimes of the conformations.

Exercise 27-1 The lifetime for rotation about the C-C bond in ethanol is -10-lo sec at room temperature. Approximately what (large) chemical-shift difference, in Hz, would a given hydrogen (marked with *) have to have between 4 and either of the conformations 5 and 6 to permit the observation of separate chemical shifts for the CH, hydrogens in these conformations? Show your reasoning.

Exercise 27-2 The 19F nmr spectrum of 1,2-difluorotetrachloroethane shows two peaks with unequal areas separated by about 0.90 ppm at -120" but a single sharp resonance at room temperature. Explain this change in the spectrum.

Exercise 27-3 The nmr spectrum of the tert-butyl protons of 3,3-dibromo-2,2-di- methylbutane is shown as a function of temperature in Figure 27-4. Explain the two peaks observed at -64". Calculate the approximate mean lifetime of the process that causes the lines to coalesce at -33".

Figure 27-4 Proton spectra of 3,3-dibromo-2,2-dimethylbutane in

CF,CI, as solvent at various temperatures

Exercise 27-4 Referring to Figure 9-8 (p. 271), we see that the microwave spectrum of 1-iodopropane shows separate rotational peaks for the trans and gauche forms. Peaks about 0.35 GHz apart are clearly resolved. What lower limit can we then put on At for the lifetime of interconversion of the trans and gauche forms of 1-iodopropane? Show your reasoning.

Exercise 27-5 Figure 9-29 (p. 312) shows some rather remarkable changes in the spectrum of ethanol as a function of concentration in CCI, solution.

a.Explain the origin of the approximately 5 Hz, 1:2:1 triplet observed for the HO proton at 10% concentration.

b.The washing-out of the triplet splitting of the HO resonance in 100% ethanol is a consequence of intermolecular HO proton exchange (C2H50H:" C2H50H7-'C2H50H

+C2H,OHZk).Any given proton then experiences a +5 Hz spin-spin interaction on

some molecules, a net of zero spin-spin interaction on other molecules, and a -5 Hz spin-spin interaction on still others. Notice that the H 0 resonance in 100% ethanol in Figure 9-29 is quite broad in comparison with that in Figure 9-23 (p. 296), which is of ethanol containing a trace of HCI to make the exchange very fast. Calculate an approximate lifetime before exchange, At, for the hydroxyl proton in 100% ethanol that is in accord with the spectrum of Figure 9-29.

c. Explain why the CH, resonance in 100% ethanol in Figure 9-29, but not in Figure 9-23, is much less sharp than the CH, resonance.

27-3 WHY SPIN-SPIN SPLITTING?

In Section 9 - 1 0 6 , we outlined the structural features that lead to observation of spin-spin splitting in the nmr spectra of organic compounds. Rules for predicting the multiplicities and intensities of spin-spin splitting patterns also were discussed. However, we did not discuss the underlying basis for spin-spin splitting, which involves perturbation of the nuclear magnetic energy levels shown in Figure 9-21 by magnetic interactions between the nuclei. You may wish to understand more about the origin of spin-spin splitting than is provided by the rules for correlating and predicting spin-spin splitting given previously, but having a command of what follows is not necessary to the qualitative use of spin-spin splitting in structural analysis. However, it will provide you with an understanding of the origins of the line spacings and line multiplicities. We will confine our attention to protons, but the same considerations apply to other nuclei (13C, 15N, 19F, and "'P) that have the spin I -- l/2. T h e main differences between proton-proton splittings and those of other nuclei are in the magnitudes of the splitting constants (J values) and their variation with structure.

Why does splitting occur? Let us start by comparing the two-proton systems of 7 and 8: