Benzene Aromatic Character

Chapter 10

Benzene Aromatic Character

Aliphatic and aromatic compound

Chemists have found it useful to divide all organic compounds into two broad classes: aliphatic compounds and aromatic compounds. The original meanings of the words "aliphatic" (fatty) and "aromatic" (fragrant) no longer have any significance.

Aliphatic compounds are open-chain compounds and those cyclic compounds that resemble the open-chain compounds. The families we have studied so far alkanes, alkenes, alkynes, and their cyclic analogs are all members of the aliphatic class.

• Aromatic compounds are benzene and compounds that resemble benzene in chemical behavior. Aromatic properties are those properties of benzene that distinguish it from aliphatic hydrocarbons. Some compounds that possess aromatic properties have structures that seem to differ considerably from the structure of benzene: actually, however, there is a basic similarity in electronic configuration (Sec. 10.10)

• Aliphatic hydrocarbons, as we have seen, undergo chiefly addition and free radical substitution; addition occurs at multiple bonds, and free-radical substitution occurs at other points along the aliphatic chain. In contrast, we shall find that aromatic hydrocarbons are characterized by a tendency to undergo ionic substitution. We shall find this contrast maintained in other families of compounds (i.e., acids, amines, aldehydes, etc.); the hydrocarbon parts of their molecules undergo reactions characteristic of either aliphatic or aromatic hydrocarbons. 

Structure of benzene

It is obvious from our definition of aromatic Compounds that any study of their chemistry must begin with a study of benzene. Benzene has been known since 1825; its chemical and physical properties are perhaps better known than those of any other single organic compound. In spite of this, no satisfactory structure for benzene had been advanced until about 1931, and it was ten to fifteen years before this structure was generally used by organic chemists.

The difficulty was not the complexity of the benzene molecule, but rather the limitations of the structural theory as it had so far developed. Since an understanding of the structure of benzene is important both in our study of aromatic compounds and in extending our knowledge of the structural theory, we shall examine in some detail the facts upon which this structure of benzene.

Molecular formula. Isomer number. Kakuli structure

Benzene has the molecular formula C^H^. From its elemental composition and molecular weight, benzene was known to contain six carbon atoms and six hydrogen atoms. The question was: how are these atoms arranged?

• In 1858, August Kakuli (of the University of Bonn) had proposed that carbon atoms can join to one another to form chains. Then, in 1865, he offered an answer to the question of benzene: these carbon chains can. sometimes be closed to form right.

• Kakuli's structure of benzene was one that we would represent today as I.

• Benzene yields only one Mono substitution product, QH5Y. Only one bromobenzene, C$H5Br, is obtained when one hydrogen atom is replaced by bromine; similarly, only one chlorobenzene, C6H5C1, or one nitrobenzene, C6H5NO2 , etc., has ever been made. This fact places a severe limitation on the structure of benzene: each hydrogen must be exactly equivalent to every other hydrogen, since the replacement of any one of them yields the same product.

• Benzene yields three isomeric Di substitution products, C6H4Y2 or C6 H4YZ. Three and only three isomeric dibromo benzenes, C6H4Br2 , three chloronitrobenzenes, C$H4C1NO2, etc., have ever been made. This fact further limits our choice of a structure; for example, IV must now be rejected. (How many Di substitution products would .

At first glance, structure I seems to be consistent with this new fact; that is, we can expect three isomeric dibromo derivatives, the 1,2- the 1,3-, and the 1,4- dibromo compounds should be possible:

But Kakul visualized the benzene molecule as a dynamic thing: ". . . the form whirled mockingly before my eyes. V He described it in terms of two structures, VIII and IX, between which the benzene molecule alternates. As a consequence, the two 1,2-dibromobenzenes (VI and VID would be in rapid equilibrium and hence could not be separated. 

Stability of the benzene ring. Reactions of benzene

Kekune's structure, then, accounts satisfactorily for facts (a), (b), and (c) in Sec. 10.3. But there are a number of facts that are still not accounted for by this structure; most of these unexplained facts seem related to unusual stability of the benzene ring. The most striking evidence of this stability is found in the chemical reactions of benzene.

Stability of the benzene ring. Heats of hydrogenation and combustion

• Besides the above qualitative indications that the benzene ring is more stable than we would expect cyclohexatriene to be, there exist quantitative data which show how much more stable.

• Heats of hydrogenation and combustion of benzene are lower than expected, We recall (Sec. 6.3) that heat of hydrogenation is the quantity of heat evolved when one mole of an unsaturated compound is hydrogenated. In most cases the value is about 28-30 kcal for each double bond the compound contains. It is not surprising, then, that cyclohexene has a heat of hydrogenation of 28.6 kcal and cyclohexadiene has one about twice that (55.4 kcal).

• We might reasonably expect cyclohexatriene to have a heat of hydrogenation about three times as large as cyclohexene, that is, about 85.8 kcal. Actually, the value for benzene (49.8 kcal) is 36 kcal less than this expected amount.

• This can be more easily visualized, perhaps, by means of an energy diagram (Fig. 10.1), in which the height of a horizontal line represents the potential energy content of a molecule. The broken lines represent the expected values, based upon three equal steps of 28.6 kcal. The final product, cyclohexane, is the same in all three cases.

 The fact that benzene evolves 36 kcal less energy than predicted can only mean that benzene contains 36 kcal less energy than predicted; in other words, benzene is more stable by 36 kcal than we would have expected cyclohexatriene to be. The heat of combustion of benzene is also lower than that expected, and by about the same amount.

Carbon-carbon bond lengths in benzene

All carbon- carbon bonds in benzene are equal and are intermediate Length between single and double bonds. Carbon-carbon double bonds in a wide variety of compounds are found tribe about 1.34 A long. Carbon-carbon single bonds, in which the nuclei are held together by only one pair of electrons, are considerably longer: 1.53 A in ethane, for example, 1.50 A in propylene, i.48 A in 1,3-butadiene. 

If benzene actually possessed three single and three double bonds, as in a Kakuli structure, we would expect to find three short bonds (1.34 A) and three long bonds (1.48 A, probably, as in 1,3-butadiene). Actually, x-ray diffraction studies show that the six carbon-carbon bonds in benzene are equal and have a length of 1.39 A, and are thus intermediate between single and double bonds.

Resonance structure of benzene 

The Kekute structure of benzene, while admittedly unsatisfactory, was generally used by chemists as late as 1945. The currently accepted structure did not arise from the discovery of new facts about benzene, but is the result of an extension or modification of the structural theory; this extension is the concept of resonance. 

• The Kakuli structures I and II, we now immediately recognize, meet the clodazons* for -resonance: structures that differ only in the arrangement of electrons Benzene is a hybrid offhand II. Since, Tarictic a? ^HacTlT^quTvalent, and hence of exactly the same stability, they make equal contributions to the hybrid. And also since I and II are exactly equivalent, stabilization due to resonance should be large.

• The puzzling aspects of benzene's properties now fall into place. The six bond lengths are identical because the six bonds are identical: they are one-and-a half bonds and their length, 1.39 A, is intermediate between the lengths of single and double bonds.

• But addition would convert benzene into a less stable product by destroying the resonance-stabilized benzene ring system; for example, according to Fig. 10.1 the first stage of hydrogenation of benzene requires 5.6 kcal to convert benzene into the less stable cyclohexadiene. As a consequence, it is easier for reactions of benzene to take an entirely different course, one in which the ring system is retained : substitution.

Orbital picture of benzene

A more detailed picture of the benzene molecule is obtained from a consideration of the bond orbitals in this molecule.

Since each carbon is bonded to three other atoms, it uses spa 2 orbitals (as in ethylene, Sec. 5.2). These lie in the same plane, that of the carbon nucleus, and arc directed toward the corners of an equilateral triangle. If we arrange the six carbons and six hydrogens of benzene to permit maximum overlap of these orbitals, we obtain the structure shown in Fig. 10.2a.

Benzene is afloat molecule, with every carbon and every hydrogen lying in the same plane. It is a very symmetrical molecule, too, with each carbon atom lying at the angle of a regular hexagon; every bond angle is 120. Each bond orbital is cylindrically symmetrical about the line joining the atomic nuclei and hence, as before, these bonds are designated as a bonds.

The molecule is not yet complete, however. There are still six electrons to be accounted for. In addition to the three orbitals already used, each carbon atom has a fourth orbital, a p orbital. As we know, this p orbital consists of two equal lobes, one lying above and the other lying below the plane of the other three orbitals, that is, above and below the plane of the ring; it is occupied by a single electron.

Representation of the benzene 

• For convenience we shall represent the benzene ring by a regular hexagon containing a circle (I); it is understood that a hydrogen atom is attached to each angle of the hexagon unless another atom or group is indicated.

• There is no complete agreement among chemists about how to represent the benzene ring. The student should expect to encounter it most often as one of the Kakuli formulas. The representation adopted in this book has certain advantages, and its use seems to be gaining ground. It is interesting that very much the same representation was advanced as long ago as 1899 by Johannes Thiele (of the University of Munich), who used a broken circle to stand for partial bonds ("partial valences").

Aromatic character. The Hatchel 4n + 2 rule

We have defined aromatic compounds as those that resemble benzene. But just which properties of benzene must a compound possess before we speak of it as being aromatic? Besides the compounds that contain benzene rings, there are many other substances that are called aromatic; yet some of these superficially bear little resemblance to benzene.

From the experimental standpoint, aromatic compounds are compounding whose molecular formulas would lead us to expect a high degree of unsaturation, and yet which are resistant to the addition reactions generally characteristic of unsaturated compounds. Instead of addition reactions, we often find that these aromatic compounds undergo electrophilic substitution reactions like those of benzene. Along with this resistance toward addition and presumably the cause of it we find evidence of unusual stability: low heats of hydrogenation and low heats of combustion. Aromatic compounds are cyclic generally containing five-, six-, or seven-membered rings and when examined by physical methods, they are found to have flat (or nearly fiat) molecules. Their protons* show the same sort of chemical shift in Namr spectra (Sec. 13.8) as the protons of benzene and its derivatives.

Each molecule is a hybrid of either five or seven equivalent structures, with the charge or odd electron on each carbon. Yet, of the six compounds, only two give evidence of unusually high stability: the cyclopentadienyl anion and the cycloheptatrienylium cation. 

Industrial source  

We have already mentioned (Sec. 3.13) that petroleum from certain areas, (in particular California) is rich in cycloalkanes, known to the petroleum industry as naphthene's. Among these are cyclohexane, methylcyclohexane, methyl cyclopentane, and 1,2-dimethylcyclopentane.

These cycloalkanes are converted by catalytic reforming into aromatic hydrocarbons, and thus provide one of the major sources of these important compounds (Sec. 12.4). For example

Just as elimination of hydrogen from cyclic aliphatic compounds yields aromatic compounds, so addition of hydrogen to aromatic compounds yields cyclic aliphatic compounds, specifically cyclohexane derivatives. An important example of this is the hydrogenation of benzene to yield pure cyclohexane.

Preparation

• Preparation of alicyclic hydrocarbons from other aliphatic compounds generally involves two stages: (a) conversion of some open-chain compound or both cis and trans products are formed. (Problem: Using the approach of Sec. 7.12, assure yourself that this is so.

• Methylene can insert itself into every carbon-hydrogen bond of most kinds of molecules. We cannot take time to say more here about this remarkable reaction, except that when addition is the desired reaction, insertion becomes an annoying side-reaction.

• In the gas phase, with low alkene concentration and in the presence of an inert gas, addition of methylene to the 2-butenes is, we have seen, non stereospecific. If, however, there is present in this system a little oxygen, addition becomes completely stereospecific (syn). Account in detail for the effect of oxygen. (Hint: See Sec. 

Substituted carbenes. a-Elimination

A more generally useful way of making cyclopropanes is illustrated by the reaction of 2-butene with chloroform in the presence of potassium /ire/-butoxide (f-Bu Ter/-butyl):

The dichlorocyclopropanes obtained can be reduced to hydrocarbons or hydrolyzed to ketones, the starting point for many syntheses (Chap. 19)

Analysis of alicyclic hydrocarbons

• A cyclopropane readily dissolves in concentrated sulfuric acid, and in this resembles an alkene or alkyne. It can be differentiated from these unsaturated hydrocarbons, however, by the fact that it is not oxidized by cold, dilute, neutral permanganate.

• Similarly, the absorption of only one mole of hydrogen shows that cyclohexane contains only one carbon-carbon double bond; yet its molecular formula is Q>Hoi which in an open-chain compound would correspond to two carbon carbon double bonds or one triple bond. Again, only a cyclic structure.

Cleavage products of cycloalkenes and cycloalkanes also reveal the cyclic structure. Ozonolysis of cyclohexene, for example, does not break the molecule into two aldehydes of lower carbon number, but simply into a single six-carbon compound containing two aldehyde.

Nomenclature of benzene derivatives

In later chapters we shall consider in detail the chemistry of many of the derivatives of benzene. Nevertheless, for our present discussion of the reactions of the benzene ring it will be helpful for us to learn to name some of the more important of these derivatives.

For many of these derivatives we simply prefix the name of the substituent group to the word -benzene, as, for example, in chlorobenzene, bromobenzene, iodobenzamide, or nitrobenzene. Other derivatives have special names which may.

Quantitative elemental analysis: nitrogen and sulfur

This chapter has dealt with the structure of benzene and with some of its reactions. It is well to remind ourselves again that all this discussion has meaning only because it is based upon solid facts. As we saw earlier (Sec. 2.24), we can discuss the structure and reactions of a compound only when we know its molecular formula and the molecular formulas of its products.

To know a molecular formula, we must know what elements are present in the compound, and in what proportions. In Sec. 2.25 we saw how various elements can be detected in an organic compound, and in Sec. 2.26 how the percentage of carbon, hydrogen, and halogen can be measured.

In the Dumas method, the organic compound is passed through a tube containing, first, hot copper oxide and, next, hot copper metal gauze. The copper oxide oxidizes the compound (as in the carbon-hydrogen combustion, Sec. 2.26), converting combined nitrogen into molecular nitrogen. The copper gauze reduces any nitrogen oxides that may be formed, also to molecular nitrogen. The nitrogen gas is collected, and its volume is measured. For example, an 8.32-mg sample of aniline yields 1.11 cc of nitrogen at 21- and 743-mm pressure (corrected for "the vapor pressure of water). We calculate the volume at standard temperature and pressure,

Why is the nitrogen in the Dumas analysis collected over 50% aqueous KOH rather than, say, pure water, aqueous NaCl, or mercury?

• In the Kjeldahl method, the organic compound is digested with concentrated sulfuric acid, which converts combined nitrogen into ammonium sulfate. The solution is then made alkaline. The ammonia thus liberated is distilled, and its amount is determined by titration with standard acid. For example, the ammonia formed from a 3.51-mg sample of aniline neutralizes 3.69 ml of 0.0103 N acid. For every milliequivalent of acid there is a milliequivalent of ammonia, and

Sulfur in an organic compound is converted into sulfate ion by the methods used in halogen analysis (Sec. 2.26) : treatment with sodium peroxide or with nitric acid (Carius method). This is then converted into barium sulfate, which is weighed.