Organic Chemistry - Meislich, Herbert - Nechamkin, Howard - Sharefkin, Jacob - Hademenos, George J. - 2013

188 CHAPTER 9 Cyclic Hydrocarbons

For each ring, the other ring can be viewed as 1,2-substituents. For the trans isomer, only the rigid (ee) con-
formation is possible structurally. As shown in Fig. 9.21, diaxial bonds point 180° away from each other and can-
not be bridged by only four C’s to complete the second ring. Cis fusion is (ea) and the bonds can be twisted to
reverse the (a) and (e) positions, yielding conformation enantiomers.

Figure 9.21

Problem 9.41 Explain the following facts in terms of the structure of cyclopropane. (a) The H’s of cyclo-
propane are more acidic than those of propane. (b) The Cl of chlorocyclopropane is less reactive toward SN2 and
SN1 displacements than the Cl in CH3CHClCH3.

(a) The external C ⎯H bonds of cyclopropane have more s character than those of an alkane (Fig. 9.1). The
more s character in the C ⎯ H bond, the more acidic the H.

(b) The C ⎯ Cl bond of chlorocyclopropane also has more s character, which diminishes the reactivity of the Cl.
Remember that vinyl chlorides are inert in SN2 and SN1 reactions. The Rϩ formed during the SN1 reaction
would have very high energy, since the C would have to use sp2-hybrid orbitals needing a bond angle of
120°. The angle strain of the Rϩ is much more severe (120°–60°) than in cyclopropane itself (109°–60°).

Problem 9.42 Use quantitative and qualitative tests to distinguish between (a) cyclohexane, cyclohexene,
and 1,3-cyclohexadiene; (b) cyclopropane and propene.

(a) Cyclohexane does not decolorize Br2 in CCl4. The uptake of H2, measured quantitatively, is 2 mol for 1
mol of the diene, but 1 mol for 1 mol of the cycloalkene.

(b) Cyclopropane resembles alkenes and alkynes, and differs from other cycloalkanes in decolorizing Br2 slowly,
adding H2, and reacting readily with H2SO4. However, it is like other cycloalkanes and differs from multi-
ple-bonded compounds in not decolorizing aqueous KMnO4.

Problem 9.43 (a) Give the structure of the major product, A, whose formula is C5H8, resulting from the dehy-
dration of cyclobutylmethanol. On hydrogenation, A yields cyclopentane. (b) Give a mechanism for this reaction.

(a) Compound A is cyclopentene, which gives cyclopentane on hydrogenation.

CH2OH +

+H+ CH2

(b) –H2O + –H+

ϩ

The side of a ring migrates, thereby converting a RC H2 having a strained four-membered ring to a much
more stable R2CHϩ with a strain-free five-membered ring.

C HCAHPATPETRE R1 01 0

Benzene and
Polynuclear

Aromatic
Compounds

10.1 Introduction

Benzene, C6H6, is the prototype of aromatic compounds, which are unsaturated compounds showing a low
degree of reactivity. The Kekulé structure (1865) for benzene has only one monosubstituted product:

(C6H5Y), since all six H’s are equivalent. There are three disubstituted benzenes—the 1,2-, 1,3-, and 1,4-position
isomers—designated as ortho, meta, and para, respectively.

Problem 10.1 Benzene is a planar molecule with bond angles of 120°. All six C-to-C bonds have the identical
length, 0.139 nm. Is benzene the same as 1,3,5-cyclohexatriene?

189

190 CHAPTER 10 Benzene and Polynuclear Aromatic Compounds

No. The bond lengths in 1,3,5-cyclohexatriene would alternate between 0.153 nm for the single bond and
0.132nm for the double bond. The C-to-C bonds in benzene are intermediate between single and double bonds.

Problem 10.2 (a) How do the following heats of hydrogenation (ΔHh, kJ/mol) show that benzene is not the
ordinary triene 1,3,5-cyclohexatriene? Cyclohexene, Ϫ119.7; 1,4-cyclohexadiene, Ϫ239.3; 1,3-cyclohexadiene,
Ϫ231.8; and benzene, Ϫ208.4. (b) Calculate the delocalization energy of benzene. (c) How does the delocaliza-
tion energy of benzene compare to that of 1,3,5-hexatriene (ΔHh ϭ Ϫ336.8 kJ/mol)? Draw a conclusion about the
relative reactivities of the two compounds.

In computing the first column of Table 10.1, we assume that in the absence of any orbital interactions, each
double bond should contribute Ϫ119.7 kJ/mol to the total ΔHh of the compound, since this is the ΔHh of an iso-
lated CϭC (in cyclohexane). Any difference between such a calculated ΔHh value and the observed value is
the delocalization energy. Since ΔHh for 1,4-cyclohexadiene is 7.5 kJ/mol less than that for 1,3-cyclohexadiene,
conjugation stabilizes the 1,3-isomer. [Remember that the smaller (more negative) the energy, the more stable
the structure.]

(a) 1,3,5-Cyclohexatriene should behave as a typical triene and have ΔHh ϭ Ϫ359.1 kJ/mol. The observed ΔHh
for benzene is Ϫ208.4 kJ/mol. Benzene is not 1,3,5-cyclohexatriene; in fact, the latter does not exist.

(b) See Table 10.1.
(c) The delocalization energy of benzene (Ϫ150.7 kJ/mol) is much smaller than that of 1,3,5-hexatriene

(Ϫ22.3 kJ/mol). Three conjugated double bonds engender a large negative delocalization energy only when

TABLE 10.1

CALCULATED OBSERVED

ΔHh, ΔHh, DELOCALIZATION
kJ/mol kJ/mol ENERGY

+ H2 –119.7

Cyclohexene Cyclohexane

+ 2H2 2(–119.7) –239.3 0.0
= –239.4 –7.6
1,4-Cyclohexadiene
2(–119.7) –231.8
+ 2H2 = –239.4

1,3-Cyclohexadiene

+ 3H2 3(–119.7) –208.4 –150.7
= –359.1

Benzene

HH
H

H + 3H2 C6H14 3(–119.7) –336.8 –22.3
H = –359.1
H

HH

cis-1,3,5-Hexatriene

CHAPTER 10 Benzene and Polynuclear Aromatic Compounds 191

they are in a ring. Since the ground-state enthalpy of benzene is much smaller in absolute value than that of the
triene, the ΔH ‡ for addition of H2 to benzene is much greater, and benzene reacts much slower. Benzene is less
reactive than open-chain trienes toward all electrophilic addition reactions.

Problem 10.3 (a) Use the ΔHh’s for complete hydrogenation of cyclohexene, 1,3-cyclohexadiene, and
benzene, as given in Problem 10.2, to calculate ΔHh for the addition of 1 mol of H2 to (i) 1,3-cyclohexadiene,
(ii) benzene. (b) What conclusion can you draw from these values about the rate of adding 1 mol of H2 to these
three compounds? (The ΔH of a reaction step is not necessarily related to ΔH ‡ of the step. However, in the cases
being considered in this problem, ΔHreaction is directly related to ΔH ‡.) (c) Can cyclohexadiene and cyclohexene
be isolated on controlled hydrogenation of benzene?

Equations are written for the reactions so that their algebraic sum gives the desired reactant, products, and
enthalpy.

(a) (i) Add reactions (1) and (2):

Note that reaction (1) is a dehydrogenation (reverse of hydrogenation) and that its ΔH is positive. (ii) Add
the following two reactions:

(b) The reaction with the largest negative ΔHh value is the most exothermic and, in this case, also has the fastest
rate. The ease of addition of 1 mol of H2 is:
cyclohexene (Ϫ119.7) Ͼ 1,3-cyclohexadiene (Ϫ112.1) ϾϾ benzene (ϩ23.4)

(c) No. When one molecule of benzene is converted to the diene, the diene is reduced all the way to cyclohexane
by two more molecules of H2 before more molecules of benzene react. If 1 mol each of benzene and H2 are
reacted, the product is –13 mol of cyclohexane and –23 mol of unreacted benzene.

192 CHAPTER 10 Benzene and Polynuclear Aromatic Compounds

Problem 10.4 The observed heat of combustion (ΔHc ) of C6H6 is Ϫ3301.6 kJ/mol.* Theoretical values are
calculated for C6H6 by adding the contributions from each bond obtained experimentally from other compounds;
these are (in kJ/mol) Ϫ492.4 for CϭC, Ϫ206.3 for C— C and Ϫ225.9 for C—H. Use these data to calculate
the heat of combustion for C6H6 and the difference between this and the experimental value. Compare the dif-
ference with that from heats of hydrogenation.

The contribution is calculated for each bond, and these are totaled for the molecule:
Six C—H bonds = 6(–225.9) = –1355.4 kJ/mol

Three C—C bonds = 3(–206.3) = –618.9
Three C==C bonds= 3(–492.4) = –1477.2

TOTAL = –3451.5 (calculated ΔHc for C6H6 )
Experimental = –3301.6

DIFFERENCE = –149.9 kJ/mol
This difference is the delocalization energy of C6H6; essentially the same value is obtained from ΔHh (Table 10.1).
Problem 10.5 How is the structure of benzene explained by (a) resonance, (b) the orbital picture, and
(c) molecular orbital theory?
(a) Benzene is a hybrid of two equal-energy (Kekulé) structures differing only in the location of the double bonds:

(b) Each C is sp2-hybridized and is σ bonded to two other C’s and one H (Fig. 10.1). These σ bonds compose
the skeleton of the molecule. Each C also has one electron in a p orbital at right angles to the plane of the
ring. These p orbitals overlap equally with each of the two adjacent p orbitals to form a π system parallel
to and above and below the plane of the ring (Fig. 10.2). The six p electrons in the π system are associated
with all six C’s. They are therefore more delocalized, and this accounts for the great stability and large res-
onance energy of aromatic rings.

Figure 10.1

(c) The six p AO’s discussed in part (b) interact to form six π MO’s. These are indicated in Fig. 10.3, which
gives the signs of the upper lobes (cf. Fig. 8.3 for butadiene). Since benzene is cyclic, the stationary waves
representing the electron clouds are cyclic and have nodal planes, shown as lines, instead of nodal points.
See Problem 9.28 for the significance of a 0 sign. The six p electrons fill the three bonding MO’s, thereby
accounting for the stability of C6H6.

* Some books define heat of combustion as Ϫ⌬Hc, and values are given as positive numbers.