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

I

Figure 11-4 Generalized valence bond (GVB) orbitals for one hydrogen of ethyne (left) and of ethane (right); see Section 6-7. The hydrogen and carbon nuclei are located in the X, Y plane of the coordinate system at the positions indicated by crosses, the hydrogen nucleus being on the left.

The long dashes indicate locations of change in orbital phase.The dotted lines are contour lines of electron amplitude of opposite phase to the solid lines. Notice how the contours of the ethyne hydrogen orbital are distorted toward carbon compared to those of the ethane hydrogen orbital. (Drawing courtesy W. A. Goddard Ill.)

bonding carbon orbital. Other things being equal, acidity increases with increasing s character in the carbon orbital.

On the average the s electrons are closer to the carbon nucleus than are p electrons. Therefore, the more s character there is to the C-H bond, the closer the electrons of the bond are, on the average, to the carbon nucleus. This makes it easier to remove the hydrogens as protons. This displacement of the electrons is clearly shown by the GVB orbitals (see Section 6-6) for the hydrogenbonding orbitals of ethane and ethyne (Figure 11-4).

11-8C Alkynide Anions as Nucleophiles

1-Alkynes are very weak acids, hence their conjugate bases, RC=C:@, are quite strong bases. These anions also are reactive carbon nucleophiles, and it is this property that makes them useful for organic syntheses. Recall from Chapter 8 that one of the most generally useful organic reactions is a displacement reaction in which an anionic nucleophile, Nu:@,attacks an alkyl de-

11-8D Coupling Reactions of Alkynes

rivative, RX, to displace X@and form a new bond between carbon and the nucleophile: -

Nu:R -t- :X@

The displaced group X often is a halide ion (chloride, bromide, or iodide), and if the entering nucleophile Nu:@is an alkynide anion, the reaction leads to formation of a carbon-carbon bond:

With those RX derivatives that undergo nucleophilic displacement readily, this is a general method of forming a C-C bond, thereby leading to substituted alkynes. The alkynide salts generally used are those of lithium, sodium, potassium, or magnesium.

11-8D Coupling Reactions of Al kynes

Another reaction of 1-alkynes that extends the carbon chain is a coupling reaction in which the alkyne dimerizes under the influence of a cuprous salt, usually cuprous ammonium chloride:

This addition of one molecule of alkyne to another is formally analogous to the dimerization of alkenes under the influence of sulfuric acid (see Section 10-9), but the mechanisms are quite different.

If the reaction is carried out in the presence of an oxidizing agent, such as 0, or a cupric salt dissolved in pyridine (a weak base), a different product is obtained. Under these conditions, oxidative coupling occurs to give a conjugated diyne:

butadiyne

Although the details of the mechanisms of these alkyne reactions are not known, it is likely that the ability of 1-alkynes to form carbon-metal bonds with metals such as copper is a key factor,

Other oxidative coupling reactions occur with transition metals, and this will be discussed in detail in Chapter 31.

Exercise 11-19 A serious contaminant in butenyne made by dimerization of ethyne with cuprous ion is 1,5-hexadien-3-yne. Show how this substance can be formed.

Additional Reading

G.E. Miller, "Hydrogenations with Diimide," J. Chem. Educ. 42, 254 (1965).

R.L. Burwell, Jr., "Deuterium as a Tracer in Reactions of Hydrocarbons on Metallic Catalysts," Accounts of Chemical Research 2, 289 (1969).

R.J. Kokes, "Characterization of Adsorbed Intermediates on Zinc Oxide by Infrared Spectroscopy," Accounts of Chemical Research 6, 226 (1973). This article is rather mistitled and actually is concerned mostly with the mechanism of heterogeneous hydrogenation of alkenes.

C. A. Tolman and J. P. Jesson, "Homogeneous Catalysis," Science 181, 501 (1973).

Supplementary Exercises

11-20 The following physical properties and analytical data pertain to two isomeric hydrocarbons, A and B, isolated from a gasoline:

Both A and B readily decolorize bromine and permanganate solutions and give the same products on ozonization. Suggest possible structures for A and B. What experiments would you consider necessary to further establish the structures and configurations of A and B?

11-21 It is possible to synthesize two isomeric cycloalkenes of formula C,H,,, Both of these compounds react with hydrogen in the presence of platinum to give cyclooctane, and each, on ozonization followed by reduction, gives:

a.What are the structures of the two compounds?

b.Would the two substances give the same compound on hydroxylation with potassium permanganate?

11-22 When 5-decyne is heated with diborane at 160" and the product is oxidized with hydrogen peroxide in basic solution, 1,lo-decanediol is obtained. Write equations to show the several reactions involved in these transformations. You need not show detailed mechanisms.

11-23 a. Draw the structures and configurations of the products that will be formed by the following reactions of cis-2-butene-2-D:

b. What is the stereochemical relationship between the products of the two reactions in Part a?

11-24* Show the structures of the products expected in each step of the following sequences. Be sure to indicate the stereochemistry of reactions where this is important. Remember that D is the hydrogen isotope of mass 2.

D NND

b. 1-methylcyclohexene -BH, -160" -H,O,, @OH

BH -160" CH3(CH2),CH=CH2 g. 3-methylcyclopentene --A

(as trapping agent) '

11-25 Two stable compounds of formula C,H, react with bromine and with KMnO,. On hydrogenation with a platinum catalyst at 25",both absorb two moles of hydrogen and form cyclohexane. Write possible structures for these substances and explain how electronic spectra may be used to tell which compound is which.

11-26 Suppose one added hydrogen as H, to the cyclopropene double bond of 1-methoxy-2-phenyl-3,3-dimethylcyclopropene. Explain how the proton nmr of the product can be used to infer whether the hydrogen added to the double bond in the suprafacial or antarafacial manner. (Review Section 9-10G and 9-1OH.)

11-27 In the hydrogenation of 1,2-dimethylcyclohexene over a platinum catalyst, the suprafacial addition product is formed. Assuming that the mechanism of this hydrogenation is as shown in Figure 11-2, what conditions must be put on the stereochemistry of each of the postulated steps in order that the overall reaction be suprafacial?

11-28* When optically active 3-methylhexane is shaken with a nickel catalyst in the presence of deuterium gas (D, = ,H,) it, like other alkanes, undergoes slow exchange of hydrogen for deuterium at the various carbons. The key observation is that substitution of D for H at C3 causes racemization, (i.e., formation of equal amounts of deuterated 3-methyl hexane with the two possible configurations at C3). Assuming that racemization and deuterium exchange occur only by way of the steps shown in Figure 11-2, determine how many steps backward the reaction has to go to produce the racemized, exchanged 3-methylhexane. To work this problem you will need to determine whether, when one starts with optically active 3-methylhexane, the various possible intermediate structures in Figure 11-2 would be chiral or not and how many steps back one would have to go to get to an achiral system. If you are uncertain about chirality we suggest that you review Section 5-1B.

Explain why the value of AH0 calculated from bond energies might be unreliable for the last reaction.

11-30 a. Write a mechanism for the sulfuric acid-induced dimerization of 2-methyl- 2-butene, indicating the products you expect to be formed. (It will be helpful to review Section 10-8B.)

b. Ozonization of the actual alkene mixture that is formed gives (along with a mixture

0

II

of aldehydes and ketones) substantial amounts of 2-butanone (CH3-C-CH,-CH,).

Write a structure and reaction mechanism for formation of a C,,-olefin that reasonably might be formed in the dimerization reaction and that, on ozonization, would yield 2-butanone and a C6-carbonylcompound. (Consider how sulfuric acid might cause the double bond in 2-methyl-2-butene to shift its position.)

11-31 How would you distinguish between the compounds in each of the following pairs using chemical methods (preferably test-tube reactions)?

11-32 How could you distinguish between the compounds in the previous exercise, using spectroscopic methods?

12-2 Spectroscopic Properties of Cycloalkanes

spectra (Section 9-7D), and that their lack of ultraviolet absorption at wavelengths down to 200 nm makes them useful solvents for the determination of ultraviolet spectra of other substances (Section 9-9B).

The proton nmr spectra of alkanes and cycloalkanes are characteristic but dificult to interpret because the chemical shifts between the various kinds of protons are usually small. Although proton spectra of simple cycloalkanes, (CH,),,, show one sharp line at room temperature, when alkyl substituents are present, small differences in chemical shifts between the ring hydrogens occur and, with spin-spin splitting, provide more closely spaced lines than normally can be resolved. The complexity so introduced can be seen by comparing the proton spectra of cyclooctane and methylcyclohexane shown in Figure 12-1. For methyl-substituted cycloalkanes the methyl resonances generally stand out as high-field signals centered on 0.9 ppm, and the area of these signals

Figure 12-1 Proton nmr spectra of cyclooctane and methylcyclohexane at 60 MHz with TMS as standard (0 ppm)

448 12 Cycloalkanes, Cycloalkenes, and Cycloalkynes

relative to the other C-H signals may be useful in indicating how many methyl groups there are (see Section 9-10K, especially Figure 9-46). However, in cyclopropanes the ring protons have abnormally small chemical shifts (6 =0.22 for cyclopropane), which often overlap with the shifts of methyl groups (6 = 0.9 ppm).

Although proton spectra are not very useful for identification purposes, 13C nmr spectra are very useful. Chain-branching and ring-substitution normally cause quite large chemical-shift changes, and it is not uncommon to observe 13Cshifts in cycloalkanes spanning 35 ppm. Some special features of application of 13C nmr spectra to conformational analysis of cycloalkanes are described in Section 12-3D.

12-3 CONFORMATIONS OF CYCLOALKANES

The equilibria (relative stabilities) and equilibration (rate of interconversion) of the rotational conformations of ethane and butane were discussed in Section 5-2. If you review this material, it will be clear that forming a ring from a hydrocarbon chain will greatly reduce the number of possible staggered and eclipsed conformations.

We will begin our discussion with cyclohexane because of its special importance, proceed to smaller rings, then give a brief exposition of the conformations of the larger rings.

12-3A Cyclohexane Conformations

If the carbons of a cyclohexane ring were placed at the corners of a regular planar hexagon, all the C-C-C bond angles would have to be 120". Because the expected normal C-C-C bond angle should be near the tetrahedral value of 109.5", the suggested planar configuration of cyclohexane would have angle strain at each of the carbons, and would correspond to less stable cyclohexane molecules than those with more normal bond angles. The actual normal value for the C-C-C bond angle of an open-chain -CH2-CH,-CH2-

unit appears to be about 112.5",which is 3" greater than the tetrahedral value. From this we can conclude that the angle strain at each carbon of a planar cyclohexane would be (120" - 112.5") = 7.5". Angle strain is not the whole story with regard to the instability of the planar form, because in addition to having C-C-C bond angles different from their normal values, the planar structure also has its carbons and hydrogens in the unfavorable eclipsed arrangement, as shown in Figure 12-2. How both of these factors can be taken into account is illustrated in Exercises 12-2 and 12-3.