T. w. GRAHAM SOLOMONS CRAIG B. FRYHLE
`
`I ORGANIC CHEMISTRY I 10e
`
`
`
`
`
`SYNGENTA EXHIBIT 1010
`Syngenta v. FMC, PGR2020-00028
`
`

`

`solom_fm_i-xxxiv-hr2.qxd 14-10-2009 17:19 Page vi
`
`In memory of my beloved son, John Allen Solomons, TWGS
`To Deanna, in the year of our 25th anniversary. CBF
`
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`Library of Congress Cataloging-in-Publication Data
`Solomons, T. W. Graham.
`Organic chemistry/T.W. Graham Solomons.—10th ed./Craig B. Fryhle.
`p. cm.
`Includes index.
`ISBN 978-0-470-40141-5 (cloth)
`Binder-ready version ISBN 978-0-470-55659-7
`
`1. Chemistry, Organic—Textbooks.
`
`I. Fryhle, Craig B.
`
`II. Title.
`
`QD253.2.S65 2011
`547—dc22
`
`2009032800
`
`Printed in the United States of America
`10 9 8 7 6 5 4 3 2 1
`
`
`
`

`

`Ar matic om unds
`
`In ordinary conversation, the word "aromatic" conjures pleasant associations-the odor of freshly prepared cof(cid:173)
`fee, or of a cinnamon bun. Similar associations occurred early in the history of organic chemistry, when pleas(cid:173)
`antly "aromatic" compounds were isolated from natural oils produced by plants. As the structures of these
`compounds were elucidated, a number of them were found to contain a highly unsaturated six-carbon struc(cid:173)
`tural unit that is also found in benzene. This special ring structure became known as a benzene ring, and the
`aromatic compounds containing a benzene ring became part of a larger family of compounds now classified as
`aromatic on the basis of their electronic structure rather than their odor.
`The following are a few examples of aromatic compounds including benzene itself. In these formulas we
`foreshadow our discussion of the special properties of the benzene ring by using a circle in a hexagon to depict
`the six 1r electrons and six-membered ring of these compounds, whereas heretofore we have shown benzene
`rings only as indicated in the left-hand formula for benzene below.
`
`0
`
`~OCH3
`
`lV(OH
`
`Benzene
`
`Benzaldehyde
`(in oil of almonds)
`
`Methyl salicylate
`(in oil of wintergreen)
`
`~ ~~H o!H
`H,co~ u
`
`H<Y"'y
`OCH3
`
`Eugenol
`(in oil of cloves)
`
`Anethole
`(in oil of anise)
`
`Cinnamaldehyde
`(in oil of cinnamon)
`
`632
`
`HO~
`OCH3
`Vanillin
`(in oil of vanilla)
`
`

`

`As time passed, chemists found or synthesized many compounds with benzene rings that had no odor, such
`as benzoic acid and acetylsalicylic acid (aspirin).
`
`14.1 The Discovery of Benzene
`
`633
`
`Benzoic acid
`
`Acetylsalicylic acid
`(aspirin)
`
`14.1 The Discovery of Benzene
`
`In this chapter we shall discuss in detail the structural principles that underlie how the term
`"aromatic" is used today. We will also see how the structure of benzene proved so elusive.
`Even though benzene was discovered in 1825, it was not until the development of quan(cid:173)
`tum mechanics in the 1920s that a reasonably clear understanding of its structure emerged.
`e As we have seen above, two formula types are commonly used to depict benzene
`rings. The traditional bond-line representation allows easier depiction of mecha(cid:173)
`nisms involving the 7T' electrons, as we shall need to do in upcoming chapters,
`whereas the circle in the hexagon notation better suggests the structure and proper(cid:173)
`ties of benzene rings.
`
`One of the 1r molecular orbitals
`of benzene, seen through a
`mesh representation of its
`electrostatic potential at its van
`der Waals surface.
`
`The study of the class of compounds that organic chemists call aromatic compounds
`(Section 2.1D) began with the discovery in 1825 of a new hydrocarbon by the English
`chemist Michael Faraday (Royal Institution). Faraday called this new hydrocarbon "bicar(cid:173)
`buret of hydrogen"; we now call it benzene. Faraday isolated benzene from a compressed
`illuminating gas that had been made by pyrolyzing whale oil.
`In 1834 the German chemist Eilhardt Mitscherlich (University of Berlin) synthesized
`benzene by heating benzoic acid with calcium oxide. Using vapor density measurements,
`Mitscherlich further showed that benzene has the molecular formula C6H6:
`heat
`
`C6H5CO2H + CaO -
`Benzoic acid
`
`C6H6 + CaCO3
`Benzene
`
`The molecular formula itself was surprising. Benzene has only as many hydrogen atoms
`as it has carbon atoms. Most compounds that were known then had a far greater propor(cid:173)
`tion of hydrogen atoms, usually twice as many. Benzene, having the formula of C6H6, should
`be a highly unsaturated compound because it has an index of hydrogen deficiency equal
`to 4. Eventually, chemists began to recognize that benzene was a member of a new class of
`organic compounds with unusual and interesting properties. As we shall see in Section 14.3,
`benzene does not show the behavior expected of a highly unsaturated compound.
`During the latter part of the nineteenth century the Kekule-Couper-Butlerov theory of
`valence was systematically applied to all known organic compounds. One result of this effort
`was the placing of organic compounds in either of two broad categories; compounds were
`classified as being either aHphatic or arnmatk. To be classified as aliphatic meant then
`that the chemical behavior of a compound was "fatlike." (Now it means that the compound
`reacts like an alkane, an alkene, an alkyne, or one of their derivatives.) To be classified as
`aromatic meant then that the compound had a low hydrogen-to-carbon ratio and that it was
`"fragrant." Most of the early aromatic compounds were obtained from balsams, resins, or
`essential oils.
`
`

`

`634
`
`Chapter 14 Aromatic Compounds
`
`Kekule was the first to recognize that these early aromatic compounds all contain a six(cid:173)
`carbon unit and that they retain this six-carbon unit through most chemical transformations
`and degradations. Benzene was eventually recognized as being the parent compound of this
`new series.
`
`14.2 Nomenclature of Benzene Derivatives
`
`Two systems are used in naming monosubstituted benzenes.
`
`Iii In many simple compounds, benzene is the parent name and the substituent is sim(cid:173)
`ply indicated by a prefix.
`
`'
`
`.:·
`
`'.
`
`'.
`!'.
`~
`
`l 1 • l<il ~,.:
`,, .•
`
`We have, for example,
`
`F
`
`Cl
`
`Br 6
`6 6 6
`
`Fluorobenzene Chlorobenzene Bromobenzene Nitrobenzene
`
`0 For other simple and common compounds, the substituent and the benzene ring
`taken together may form a commonly accepted parent name.
`
`Methylbenzene is usually called toluene, hydroxybenzene is almost always called phenol,
`and aminobenzene is almost always called aniline. These and other examples are indicated
`here:
`
`.. ,_......H
`:0
`
`H.......,_ •• ,_......H
`
`N 6 6
`
`Toluene
`
`Phenol
`
`Aniline
`
`Benzenesulfonic acid
`
`Benzoic acid
`
`Acetophenone
`
`Anisole
`
`Iii When two substituents are present, their relative positions are indicated by the pre(cid:173)
`fixes ortho-, meta-, and para- (abbreviated o-, m-, and p-) or by the use of numbers.
`
`For the dibromobenzenes we have
`
`rArBr
`
`V-Br
`
`1,2-Dibromobenzene
`( o-dibromobenzene)
`ortho
`
`1,3-Dibromobenzene
`(m-dibromobenzene)
`meta
`
`Br ¢
`
`Br
`1,4-Dibromobenzene
`(p-dibromobenzene)
`para
`
`

`

`14.2 Nomendaturn of Benzene Derivatives
`
`635
`
`and for the nitrobenzoic acids
`
`2-Nitrobenzoic acid
`(o-nitrobenzoic acid)
`
`3-Nitrobenzoic acid
`(m-nitrobenzoic acid)
`
`4-Nitrobenzoic acid
`(p-nitrobenzoic acid)
`
`The dimethylbenzenes are often called xylenes:
`
`lyCH,
`
`1,2-Dimethylbenzene
`(o-xylene)
`
`CH3
`1,3-Dimethylbenzene
`(m-xylene)
`
`1,4-Dimethylbenzene
`(p-xylene)
`
`e If more than two groups are present on the benzene ring, their positions must be
`indicated by the use of numbers.
`
`As examples, consider the following two compounds:
`
`:¢:"'
`
`1,2,3-Trichlorobenzene
`
`Br
`1,2,4-Tribromobenzene
`(not 1,3,4-tribromobenzene)
`
`€Ill The benzene ring is numbered so as to give the lowest possible numbers to the
`substituents.
`
`€Ill When more than two substituents are present and the substituents are different,
`they are listed in alphabetical order.
`e When a substituent is one that together with the benzene ring gives a new base name,
`that substituent is assumed to be in position 1 and the new parent name is used.
`
`' 9 OH
`
`ON~
`
`N02
`3,5-Dinitrobenzoic acid
`
`2,4-Difluorobenzenesulfonic acid
`
`e When the C6H5-
`group is named as a substituent, it is called a phenyl group.
`The phenyl group is often abbreviated as C6H5- , Ph-, or <p-.
`A hydrocarbon composed of one saturated chain and one benzene ring is usually named
`as a derivative of the larger structural unit. However, if the chain is unsaturated, the
`
`Helpful Hint
`Note the abbreviations for com(cid:173)
`mon aromatic groups.
`
`

`

`636
`
`Chapter 14 Aromatic Compm.1nds
`
`compound may be named as a derivative of that chain, regardless of ring size. The fol(cid:173)
`lowing are examples:
`
`Butyl benzene
`
`( E)-2-Phenyl-2-butene
`
`~
`
`C6Hs
`2-Phenylheptane
`
`it Benzyl is an alternative name for the phenylmethyl group. It is sometimes abbrevi(cid:173)
`ated Bn.
`
`©("Cl
`
`The benzyl group
`(the phenylmethyl
`group)
`
`Benzyl chloride
`(phenylmethyl chloride
`or BnCI)
`
`Provide a name for each of the following compounds.
`
`(a)
`
`(c)
`
`Cl
`STRATEGY AND ANSWIE~ In each compound we look first to see if a commonly named unit containing a ben(cid:173)
`zene ring is present. If not, we consider whether the compound can be named as a simple derivative of benzene,
`or if the compound incorporates the benzene ring as a phenyl or benzyl group. In (a) we recognize the common
`structural unit of acetophenone, and find a tert-butyl group in the para position. The name is thus p-tert-butylace(cid:173)
`tophenone or 4-tert-butylacetophenone. Compound (b), having three substituents on the ring, must have its sub(cid:173)
`stituents named in alphabetical order and their positions numbered. The name is 1,4-dimethyl-2-nitrobenzene. In
`(c) there would appear to be a benzyl group, but the benzene ring can be considered a substituent on the alkyl chain,
`so it is called phenyl in this case. The name is 2-chloro-2-methyl-l-phenylpentane. Because (d) contains an ether
`functional group, we name it according to the groups bonded to the ether oxygen. The name is benzyl ethyl ether,
`or ethyl phenylmethyl ether.
`
`Review Problem 14. 1
`
`Provide a name for each of the following compounds.
`
`(b)(YlQ]
`
`(d )~Q~
`
`ifoH
`(c)J:
`
`(a)
`
`0
`
`Br}V
`
`Cl
`
`

`

`14.3 Reactions of Benzene
`
`637
`
`14.3 Reactions of Benzene
`
`In the mid-nineteenth century, benzene presented chemists with a real puzzle. They knew
`from its formula (Section 14.1) that benzene was highly unsaturated, and they expected it
`to react accordingly. They expected it to react like an alkene by decolorizing bromine in
`carbon tetrachloride through addition of bromine. They expected that it would change the
`color of aqueous potassium permanganate by being oxidized, that it would add hydrogen
`rapidly in the presence of a metal catalyst, and that it would add water in the presence of
`strong acids.
`Benzene does none of these. When benzene is treated with bromine in the dark or with
`aqueous potassium permanganate or with dilute acids, none of the expected reactions occurs.
`Benzene does add hydrogen in the presence of finely divided nickel, but only at high tem(cid:173)
`peratures and under high pressures:
`
`CC14, dark, 25°C
`KMn04
`Hp, 25°C
`
`Benzene-
`
`No addition of bromine
`
`Hp, heat
`
`H,!Ni 0
`
`No hydration
`
`Slow addition
`at high temperature
`and pressure
`
`Benzene does react with bromine but only in the presence of a Lewis acid catalyst such
`as ferric bromide. Most surprisingly, however, it reacts not by addition but by substitution(cid:173)
`benzene substitution.
`
`Substitution
`
`6 6 +
`C H
`
`B
`r2
`
`FeBrs C H B
`6 s r + HBr
`
`Addition
`
`When benzene reacts with bromine, only one monobromobenzene is formed. That is, only
`one compound with the formula C6H5Br is found among the products. Similarly, when ben(cid:173)
`zene is chlorinated, only one monochlorobenzene results.
`Two possible explanations can be given for these observations. The first is that only
`one of the six hydrogen atoms in benzene is reactive toward these reagents. The second
`is that all six hydrogen atoms in benzene are equivalent, and replacing any one of them
`with a substituent results in the same product. As we shall see, the second explanation
`is correct.
`
`Review Problem 14.2
`
`(a)
`
`Listed below are four compounds that have the molecular formula C6H6. Which of these
`compounds would yield only one monosubstitution product, if, for example, one hydrogen
`were replaced by bromine?
`
`(b) t!:J
`
`(c) -0 (d) rn
`
`

`

`638
`
`Chapter 14 Aromatic Compounds
`
`14.4 The Kekule Structure for Benzene
`
`In 1865, August Kekule, the originator of the structural theory (Section 1.3), proposed the
`first definite structure for benzene,* a structure that is still used today (although as we shall
`soon see, we give it a meaning different from the meaning Kekule gave it). Kekule sug(cid:173)
`gested that the carbon atoms of benzene are in a ring, that they are bonded to each other
`by alternating single and double bonds, and that one hydrogen atom is attached to each car(cid:173)
`bon atom. This structure satisfied the requirements of the structural theory that carbon atoms
`form four bonds and that all the hydrogen atoms of benzene are equivalent:
`
`H
`I
`H, ,,,,.c, ..,...-H
`C
`C
`I
`II
`C
`C
`H',,.. 'c ..,...-
`'-H
`I
`H
`The Kekule formula for benzene
`
`or 0
`
`A problem soon arose with the Kekule structure, however. The Kekule structure pre(cid:173)
`dicts that there should be two different 1,2-dibromobenzenes, but there are not. In one of
`these hypothetical compounds (below), the carbon atoms that bear the bromines would be
`separated by a single bond, and in the other they would be separated by a double bond.
`
`~B r
`
`~B r
`
`and (XBr
`
`Br
`
`These 1,2-clibromobenzenes do not exist
`as isomers.
`
`8 Only one 1,2-dibromobenzene has ever been found, however.
`
`To accommodate this objection, Kekule proposed that the two forms of benzene ( and of
`benzene derivatives) are in a state of equilibrium and that this equilibrium is so rapidly estab(cid:173)
`lished that it prevents isolation of the separate compounds. Thus, the two 1,2-dibro(cid:173)
`mobenzenes would also be rapidly equilibrated, and this would explain why chemists had
`not been able to isolate the two forms:
`
`There is no such equilibrium between
`benzene ring bond isomers.
`
`8 We now know that this proposal was also incorrect and that no such equilibrium
`exists.
`
`Nonetheless, the Kekule formulation of benzene's structure was an important step forward
`and, for very practical reasons, it is still used today. We understand its meaning differently,
`however.
`The tendency of benzene to react by substitution rather than addition gave rise to another
`concept of aromaticity. For a compound to be called aromatic meant, experimentally,
`that it gave substitution reactions rather than addition reactions even though it was highly
`unsaturated.
`Before 1900, chemists assumed that the ring of alternating single and double bonds was
`the structural feature that gave rise to the aromatic properties. Since benzene and benzene
`derivatives (i.e., compounds with six-membered rings) were the only aromatic compounds
`
`*In 1861 the Austrian chemist Johann Josef Loschmidt represented the benzene ring with a circle, but he made
`no attempt to indicate how the carbon atoms were actually arranged in the ring.
`
`

`

`14.5 The Thermodynamic Stability of Benzene
`
`639
`
`known, chemists naturally sought other examples. The compound cyclooctatetraene seemed
`to be a likely candidate:
`
`0
`
`Cyclooctatetraene
`
`In 1911, Richard Willstatter succeeded in synthesizing cyclooctatetraene. Willstatter
`found, however, that it is not at all like benzene. Cyclooctatetraene reacts with bromine by
`addition, it adds hydrogen readily, it is oxidized by solutions of potassium permanganate,
`and thus it is clearly not aromatic. While these findings must have been a keen disap(cid:173)
`pointment to Willstatter, they were very significant for what they did not prove. Chemists,
`as a result, had to look deeper to discover the origin of benzene's aromaticity.
`
`14.5 The Thermodynamic Stability of Benzene
`
`We have seen that benzene shows unusual behavior by undergoing substitution reactions
`when, on the basis of its Kekule structure, we should expect it to undergo addition. Benzene
`is unusual in another sense: It is more stable thermodynamically than the Kekule structure
`suggests. To see how, consider the following thermochemical results.
`Cyclohexene, a six-membered ring containing one double bond, can be hydrogenated
`easily to cyclohexane. When the b.H0 for this reaction is measured, it is found to be -120
`kJ mol- 1, very much like that of any similarly substituted alkene:
`
`Cyclohexene
`
`Cyclohexane
`
`!iH° = -120 kJ mo1-1
`
`j
`~ ·,1
`' !
`
`We would expect that hydrogenation of 1,3-cyclohexadiene would liberate roughly twice
`as much heat and thus have a b.H0 equal to about -240 kJ mol- 1. When this experiment
`is done, the result is b.H0 = -232 kJ mol- 1
`. This result is quite close to what we calcu(cid:173)
`lated, and the difference can be explained by taking into account the fact that compounds
`containing conjugated double bonds are usually somewhat more stable than those that con(cid:173)
`tain isolated double bonds (Section 13.8):
`
`2H,PI 0 Calculated
`
`!iH 0 = 2 x (-120) = -240 kJ mo1-1
`Observed
`!iH0 = -232 kJ mo1- 1
`
`1,3-Cyclohexadiene
`
`Cyclohexane
`
`If we extend this kind of thinking, and if benzene is simply 1,3,5-cyclohexatriene, we
`would predict benzene to liberate approximately 360 kJ mol- 1 [3 X (-120)] when it is
`hydrogenated. When the experiment is actually done, the result is surprisingly different.
`The reaction is exothermic, but only by 208 kJ mol- 1:
`
`0 Calculated
`
`!iH 0 = 3 x (-120) = -360 kJ mo1-1
`!iH 0 = -208 kJ mo1- 1
`Observed
`152 kJ mo1-1
`Difference
`
`Cyclohexane
`
`Benzene
`
`When these results are represented as in Fig. 14.1, it becomes clear that benzene is much
`more stable than we calculated it to be. Indeed, it is more stable than the hypothetical 1,3,5-
`cyclohexatriene by 152 kJ mol- 1
`. This difference between the amount of heat actually
`released and that calculated on the basis of the Kekule structure is now called the reso(cid:173)
`mm.ce energy of the compound.
`
`

`

`640
`
`Chapter 14 Aromatic Compounds
`
`>,
`Cl
`~
`C:
`Q)
`
`~ C:
`
`Q)
`
`6H° = -120
`kJ mo1-1
`
`0 a. O+H2
`T
`!
`
`tiH° =-232
`kJ mo1-1
`
`Figure 14.1 Relative stabilities of
`cyclohexene, 1,3-cyclohexadiene, 1,3,
`5-cyclohexatriene (hypothetical), and
`benzene.
`
`=j=
`I
`I
`I
`I
`I
`I
`I
`I
`
`t:,H° ~ _
`
`Resonance
`(stabilization)
`energy= 152 kJ
`mo1-1
`
`senzeTne + 3 H2
`360
`kJ mo1-1
`I
`I
`I
`I
`I
`I
`I
`I
`
`tiH° =-208
`kJ mo1-1
`
`l
`
`I *
`
`l
`Cyclohexane 0
`
`14.6 Modern Theories of the Structure of Benzene
`
`It was not until the development of quantum mechanics in the 1920s that the unusual behav(cid:173)
`ior and stability of benzene began to be understood. Quantum mechanics, as we have seen,
`produced two ways of viewing bonds in molecules: resonance theory and molecular orbital
`theory. We now look at both of these as they apply to benzene.
`
`14.6A The Resonance Explanation of the Structure
`of Benzene
`A basic postulate of resonance theory (Sections 1.8 and 13.5) is that whenever two or more
`Lewis structures can be written for a molecule that differ only in the positions of their elec(cid:173)
`trons, none of the structures will be in complete accord with the compound's chemical and
`physical properties. If we recognize this, we can now understand the true nature of the two
`Kekule structures (I and II) for benzene.
`
`fP Kekule structures I and II below differ only in the positions of their electrons; they
`do not represent two separate molecules in equilibrium as Kekule had proposed.
`
`Instead, structures I and II are the closest we can get to a structure for benzene within the
`limitations of its molecular formula, the classic rules of valence, and the fact that the six
`hydrogen atoms are chemically equivalent. The problem with the Kekule structures is that
`they are Lewis structures, and Lewis structures portray electrons in localized distributions.
`(With benzene, as we shall see, the electrons are delocalized.) Resonance theory, fortunately,
`does not stop with telling us when to expect this kind of trouble; it also gives us a way out.
`ti According to resonance theory, we consider Kekule structures I and II below as
`resonance contributors to the real structure of benzene, and we relate them to each
`other with one double-headed, double-barbed arrow (not two separate arrows,
`which we reserve for equilibria).
`
`Resonance contributors, we emphasize again, are not in equilibrium. They are not struc(cid:173)
`tures of real molecules. They are the closest we can get if we are bound by simple rules of
`valence, but they are very useful in helping us visualize the actual molecule as a hybrid:
`
`0 (oota==) 0
`
`II
`
`

`

`14.6 Modern Theories of the Stnicture of Benzene
`
`641
`
`Look at the structures carefully. All of the single bonds in structure I are double bonds
`in structure II.
`
`8 A hybrid (average) of Kekule structures I and II would have neither pure single
`bonds nor pure double bonds between the carbons. The bond order would be
`between that of a single and a double bond.
`
`Experimental evidence bears this out. Spectroscopic measurements show that the molecule
`of benzene is planar and that all of its carbon-carbon bonds are of equal length. Moreover,
`the carbon-carbon bond lengths in benzene (Fig. 14.2) are 1.39 A, a value in between that
`for a carbon-carbon single bond between sp2-hybridized atoms (1.47 A) (see Table 13.1)
`and that for a carbon-carbon double bond (1.34 A).
`
`H
`H
`'-..
`/
`c--c 120°
`12o)~C~v H
`H--. C/
`'\, ~ c--. C
`1.09A
`120°
`'\,
`/
`H
`H
`
`1.39A
`
`Figure 14.2 Bond lengths and
`angles in benzene. (Only the u
`bonds are shown.)
`
`e The hybrid structure of benzene is represented by inscribing a circle inside the
`hexagon as shown in formula III below.
`
`III
`
`There are times when an accounting of the 7T electron pairs must be made, however, and
`for these purposes we use either Kekule structure I or II. We do this simply because the elec(cid:173)
`tron pairs and total 7T electron count is obvious in a Kekule structure, whereas the number of
`7T electron pairs represented by a circle can be ambiguous. As we shall see later in this chap(cid:173)
`ter, there are systems having different ring sizes and different numbers of delocalized 7T elec(cid:173)
`trons that can also be represented by a circle. In benzene, however, the circle is understood to
`represent six 7T electrons that are delocalized around the six carbons of the ring.
`e An actual molecule of benzene ( depicted by the resonance hybrid III) is more sta(cid:173)
`ble than either contributing resonance structure because more than one equivalent
`resonance structure can be drawn for benzene (I and II above).
`
`The difference in energy between hypothetical 1,3,5-cyclohexatriene (which if it existed
`would have higher energy) and benzene is called resonance energy, and it is an indication
`of the extra stability of benzene due to electron delocalization.
`
`If benzene were 1,3,5-cyclohexatriene, the carbon-carbon bonds would be alternately long
`and short as indicated in the following structures. However, to consider the structures here
`as resonance contributors (or to connect them by a double-headed arrow) violates a basic
`principle of resonance theory. Explain.
`
`Review Problem 14.3
`
`14.68 The Molecular Orbital Explanation of the Structure
`of Benzene
`The fact that the bond angles of the carbon atoms in the benzene ring are all 120° strongly
`suggests that the carbon atoms are sp2 hybridized. If we accept this suggestion and con(cid:173)
`struct a planar six-membered ring from sp2 carbon atoms, representations like those shown
`
`

`

`642
`
`Chapter 14 Aromatic Compounds
`
`in Figs. 14.3a and b emerge. In these models, each carbon is sp2 hybridized and has a p
`orbital available for overlap with p orbitals of its neighboring carbons. If we consider
`favorable overlap of these p orbitals all around the ring, the result is the model shown in
`Fig. 14.3c.
`
`(a} Six sp2-hybridized carbon atoms joined in
`Figure 14.3
`a ring (each carbon also bears a hydrogen atom}. Each
`carbon has a p orbital with lobes above and below the
`plane of the ring. (b} A stylized depiction of the p orbitals
`in (a}. (c} Overlap of the p orbitals around the ring results
`in a molecular orbital encompassing the top and bottom
`faces of the ring. (Differences in the mathematical phase
`of the orbital lobes are not shown in these
`representations.}
`
`H
`
`H
`
`(a)
`
`(b)
`
`(c)
`
`€ff> As we recall from the principles of quantum mechanics (Section 1.11 ), the number
`of molecular orbitals in a molecule is the same as the number of atomic orbitals
`from which they are derived, and each orbital can accommodate a maximum of
`two electrons if their spins are opposed.
`
`If we consider only the p atomic orbitals contributed by the carbon atoms of benzene, there
`should be six 1r molecular orbitals. These orbitals are shown in Fig. 14.4.
`
`..i ..i ..i ..i ..i ..i
`
`Six isolated p orbitals
`(with six electrons)
`
`Atomic orbitals
`
`Antibonding
`MOs
`
`Bonding
`MOs
`
`Figure 14.4 How six p atomic orbitals (one from each carbon of the benzene ring} combine to
`form six 1r molecular orbitals. Three of the molecular orbitals have energies lower than that of an
`isolated p orbital; these are the bonding molecular orbitals. Three of the molecular orbitals have
`energies higher than that of an isolated p orbital; these are the antibonding molecular orbitals.
`Orbitals ,fl2 and ,fi3 have the same energy and are said to be degenerate; the same is true of
`orbitals ,fi4 and i/Js-
`
`The electronic configuration of the ground state of benzene is obtained by adding the
`six 1r electrons to the 1r molecular orbitals shown in Fig. 14.4, starting with the orbitals of
`lowest energy. The lowest energy 1r molecular orbital in benzene has overlap of p orbitals
`with the same mathematical phase sign all around the top and bottom faces of the ring. In
`this orbital there are no nodal planes (changes in orbital phase sign) perpendicular to the
`atoms of the ring. The orbitals of next higher energy each have one nodal plane. (In gen(cid:173)
`eral, each set of higher energy 1r molecular orbitals has an additional nodal plane.) Each
`of these orbitals is filled with a pair of electrons, as well. These orbitals are of equal energy
`
`Cj
`
`AM.·'
`
`.:a .,,
`
`

`

`14.7 Hikkel's Ru!e: The 4n + 2 1r Electron Rule
`
`643
`
`(degenerate) because they both have one nodal plane. Together, these three· orbitals com(cid:173)
`prise the bonding 1T molecular orbitals of benzene. The next higher energy set of 1T mole(cid:173)
`cular orbitals each has two nodal planes, and the highest energy 1r molecular orbital of
`benzene has three nodal planes. These three orbitals are the antibonding 1T molecular orbitals
`of benzene, and they are unoccupied in the ground state. Benzene is said to have a closed
`bonding shell of delocalized 1T electrons because all of its bonding orbitals are filled with
`electrons that have their spins paired, and no electrons are found in antibonding orbitals.
`This closed bonding shell accounts, in part, for the stability of benzene.
`Having considered the molecular orbitals of benzene, it is now useful to view an elec(cid:173)
`trostatic potential map of the van der Waals surface for benzene, also calculated from quan(cid:173)
`tum mechanical principles (Fig. 14.5). We can see that this representation is consistent with
`our understanding that the 1r electrons of benzene are not localized but are evenly distrib(cid:173)
`uted around the top face and bottom face (not shown) of the carbon ring in benzene.
`It is interesting to note the recent discovery that crystalline benzene involves perpen(cid:173)
`dicular interactions between benzene rings, so that the relatively positive periphery of one
`molecule associates with the relatively negative faces of the benzene molecules aligned
`above and below it.
`
`,I
`
`Figure 14.5 Electrostatic
`potential map of benzene.
`
`i
`
`~ I i
`
`)
`,)
`
`14.1 Hiickel 1s Rule: The 4n + 2 1r Electron Rule
`
`In 1931 the German physicist Erich Htickel carried out a series of mathematical calcula(cid:173)
`tions based on the kind of theory that we have just described. Hilckel's n.de is concerned
`with compounds containing one planar ring in which each atom has a p orbital as in
`benzene. His calculations show that planar monocyclic rings containing 4n + 2 1T electrons,
`where n = 0, l, 2, 3, and so on (i.e., rings containing 2, 6, 10, 14, ... , etc., 1T electrons),
`have closed shells of delocalized electrons like benzene and should have substantial reso(cid:173)
`nance energies.
`
`ii In other words, Htickel's rule states that planar monocyclic rings with 2, 6, 10,
`14, ... , delocalized electrons should be aromatic.
`
`14.7A How to Diagram the Relative Energies of 1'f
`Molecular Orbitals in Monocydk Systems Based
`on Huckel's Rule
`There is a simple way to make a diagram of the relative energies of orbitals in monocyclic
`conjugated systems based on Htickel's calculations. To do so, we use the following procedure.
`
`1. We start by drawing a polygon corresponding to the number of carbons in the ring,
`placing a corner of the polygon at the bottom.
`2. Next we smrnund the polygon with a circle that touches each corner of the polygon.
`3. At the points where the polygon touches the circle, we draw short horizontal lines
`outside the circle. The height of each line represents the relative energy of each 1T
`molecular orbital.
`4. Next we draw a dashed horizontal line across and halfway up the circle. The energies
`of bonding 1T molecular orbitals are below this line. The energies of antibonding 1T
`molecular orbitals are above, and those for non bonding orbitals are at the level of the
`dashed line.
`5. Based on the number of 1T electrons in the ring, we then place electron arrows on the
`lines corresponding to the respective orbitals, beginning at the lowest energy level
`and working upward. In doing so, we fill degenerate orbitals each with one electron
`first, then add to each unpaired electron another with opposite spin if it is available.
`
`

`

`644
`
`Chapter 14 Aromatic Compounds
`
`Applying this method to benzene, for example (Fig. 14.6), furnishes the same energy lev(cid:173)
`els that we saw earlier in Fig. 14.4, energy levels that were based on quantum mechanical
`calculations.
`
`,,J. !
`.,,
`,.
`
`Figure 14.6 The polygon-and-circle
`method for deriving the relative
`energies of the 7T molecular orbitals of
`benzene. A horizontal line halfway up
`the circle divides the bonding orbitals
`from the antibonding orb