CSUMB
ESSP 311 Organic Chemistry I
Ronald W. Rinehart, Ph.D.

Chapter 11 Arenes and Aromaticity

Chapter 11:  Arenes and Aromaticity 

I.  Benzene:  C₆H₆
            Prolog: benzene is unusual.
                        Isolated 1825 (Faraday); first synthesis & molecular formula determination 1834 (Mitscherlich);
                                    classic structure proposed by Kékulé (1866)
                        Formula:C₆H₆  ⇒ (SODAR = 4)  ⇒ highly unsaturated, but does not behave like alkenes or alkynes
                                    ►  can be hydrogenated, but not with the ease of C=C or C≡C
                                    ►  heat of hydrogenation is unusually low [see below], as is heat of combustion
                                    ► does not show addition reactions with HCl, HBr, HI
                                    ► does not react at all with Cl₂ or Br₂ in CCl₄; use of FeX₃ or AlCl₃(anh) catalyst with X₂ leads to
                                         substitution, not addition:  C₆H₆ + Br₂ (+ FeBr₃) à C₆H₅Br + HBr
                                    ► Further substitution studies show that all 6 C's and all 6 H's are equivalent, implying flaw in
                                                classic structure. Kékulé responds with "rapid isomer interconversion by shifting C=C"
                                                theory, foreshadowing concept of resonance.
            A.  Bonding
                        1.  sp² hybridization.
                        2.  The molecule is planar.
                        3.  The entire ring system is conjugated.
                                    a)  A continuous p system exists around the six carbons.
                        4.  All C-C bonds are equivalent in length:  1.397 Angstroms.
            B.  Resonance Stabilization in Benzene.       
                        1.  Benzene can be drawn in two major and numerous minor resonance structures.

 

            2.  Both major structures are of equal energy.
                        3.  Resonance stabilization is greatest when the contributing structures are of similar energy.
                        4.  Benzene is more stable than expected.
                                    a)  Evidence for this stabilization of benzene is its low heat of hydrogenation. 

                                     b)  Benzene is more stable by 152 kJ/mol [36 kcal/mol] than (hypothetical)  "1,3,5-cyclohexatriene".
                                                i)  This is referred to as "resonance energy"  (or stabilization energy, or delocalization energy).
                                    c)  Benzene is more stable than 1,3,5-hexatriene:

                                                i)  The heat of hydrogenation is 129 kJ/mol [30.7 kcal/mol] greater than that of benzene.
                                    d)  The resonance energy of benzene is ~5 times as great as that of 1,3,5-hexatriene.
                                                i)  Much larger than expected stabilization places benzene in the AROMATIC category.

II.  Aromatic Compounds
            A.  Criteria for aromaticity
                        1.  Aromatic compounds must be cyclic.
                        2.  They must be planar or close to planar.
                        3.  They must be conjugated with [4n + 2]  p e⁻ for monocyclic species.
                                    a)  Why [4n + 2] p electrons? 
                                                This number of p electrons leads to a CLOSED SHELL electron configuration.   
                                                i)  All bonding MO's are filled, and all nonbonding & antibonding orbitals are empty. (Fig. 11.5)
                                    b)  Use "Frost Circles" to help remember the types of molecular orbitals for monocyclic compounds.
                                                i)  Place the ring structure polygon with a vertex at the bottom within a circle.
                                                ii)  The points of contact with the circle represent molecular orbitals and their relative energies.
                                                iii)  Populate the orbitals with the appropriate number of electrons according to normal rules
                                                iv)  The center of the circle is at the energy level of an isolated p-orbital or nonbonding M.O.
                                                v)  Two orbitals with the same energy are said to be "degenerate"
                                    c)  Examples.

                                     d)  Use Frost Circles to show the MO's of cyclooctatetraene [draw in margin].                                      

                                    e)  Hückel's Rule:  Aromatic stability is associated with planar, monocyclic, fully conjugated
                                                 polyenes with [4n + 2] p electrons (n = 0, 1, 2, ...).

                        4.  Annulenes are completely conjugated monocyclic hydrocarbons.
                                    a)  Annulenes may or may not be aromatic.
                                    b)  Cyclobutadiene is also known as [4]-annulene.
                                                i)  It is not aromatic.
                                    c)  Benzene is [6]-annulene.
                                                i)  Benzene is aromatic.
                                    d)  Annulenes with [4n + 2] p electrons but which are nonplanar lack stability and are not aromatic. 
                                                i)  10-annulene is an example of  this.

                        5.  Questions.
                                    a)  Is cycloheptatriene aromatic?
                                                Answer: No.  The sp³ hybridized carbon prevents delocalization about the entire ring.
                                    b)  Is cycloheptatrienyl cation (tropylium ion) aromatic?

                        Answer: Yes.  It has [4n + 2] p electrons and a 7^th  p orbital allowing delocalization about the entire ring.
                                    c)  Which is more stable, cycloheptatriene or the cycloheptatriene cation?
Answer: The neutral molecule.  The aromaticity of the tropylium ion means that it forms easily for a carbocation.
                        6.  Other aromatic species include the following:

                        7.  Would cyclopentadiene be a relatively strong or weak acid (for a hydrocarbon?)
                                    a)  To answer this write a chemical equation showing cyclopentadiene losing a proton, forming the
                                                            conjugate base, and analyze the stability of the conjugate base.
                                    Answer:  It is a (relatively) strong acid due to the aromaticity of its conjugate base.  (pKa = 16)

                         8.  Would cycloheptatriene be a relatively strong or weak acid?

Answer:  pKa = 36.  It is a weak acid: Even though the electrons are delocalized by resonance over the entire ring,
                         it has 8 p electrons and therefore is NOT aromatic. 
                        (It is a stronger acid than an alkane, but much weaker than cyclopentadiene.

 III.  Physical Properties of Arenes.
            A.  Arenes Are Similar To Alkanes:
                        1.  Arenes are nonpolar.
                        2.  They are insoluble in water.
                        3.  They are less dense than water.

IV.  Heterocyclic Aromatic Compounds.
            A.  Heterocyclic Compounds
                        1.  Heterocyclic compounds are organic compounds with ring atoms other than C.
                        2.  O and N can contribute an electron pair in cyclic p-systems.

 V.  Polycyclic Aromatic Hydrocarbons
            A.  Polycyclic aromatic compounds consist of two or more rings.
                        1.  Most are aromatic since they are made up of inter-connected benzene rings.
            B.  Hückel's Rule does not apply (it is only for monocyclics).
            C.  Examples

                        3.  Benzo[a]pyrene.
                                    a)  Benzo[a]pyrene is a carcinogen.
                                    b)  Oxidized in the liver to an epoxydiol, which induces mutations leading to uncontrolled cellular
                                                growth.                        

 VI.  Nomenclature of Substituted Derivatives of Benzene.
            A.  The Benzene Ring as a Substituent Group
                        1.  Two important groups are the phenyl and benzyl groups.

 

             B.  Common Compounds [learn these names/structures!]

 VII.  Benzylic Carbocations and Radicals 
            A.  Benzylic carbocations and radicals are similar to but more stable than their allylic counterparts.
                        1.  Recall that the allylic cation, for example, can delocalize the positive charge over three carbons:

                        2.Benzyl cation (& free radical): extra stabilization due to delocalization through resonance into the ring.
                                    a)  The p orbital of the benzylic carbon can be parallel to ring's p             orbitals.               

            B.  An alkenyl benzene is stabilized to about the same extent as a conjugated diene:

VIII.  Reactions of Aromatic Compounds and Alkenylbenzenes
            A.  Reactions of Aromatic Compounds
                        1.  The Birch Reduction
                                    a)  The Birch Reduction is a metal-ammonia-alcohol reduction of aromatic rings.
                                    b)  It is an example of a dissolving metal reduction.
                                    c)  A group I metal, liquid ammonia, and alcohol reduces arenes to nonconjugated dienes.

           
                                    d)  It provides a way to prepare nonconjugated dienes from arenes.
                                    e)  kinetically controlled reaction forms the less stable nonconjugated diene, not conjugated product.
                                    f)  Alkyl-substituted arenes yield 1,4-cyclohexadienes where the alkyl group is a substituent of a
                                                carbon bearing a double bond.
                                                i)  Complete the equation:

                                                ii)  Draw the structure of the other reasonable possible product which is not formed:

                                    g)  Mechanism: similar to metal-ammonia reduction of alkynes -- a four-step sequence where:
                                                (i)  steps 1 and 3 involve a transfer of an electron from the metal and
                                                (ii)  steps 2 and 4 involve the alcohol transfering a proton.
                                    h)  Mechanism:

                         2.  Free Radical Halogenation of Alkylbenzenes.
                                    a)  The benzylic C-H bond is weaker than the C-H bond of an alkane, and therefore easier to break.
                                                i)  Homolytic cleavage of the benzylic C-H bond is relatively easy since the resulting benzylic
                                                            free radical is stabilized via delocalization into the ring (similar to the resonance structures
                                                            for the benzylic carbocation-  a good exercise would be to draw these structures.)
                                                ii)  Free-radical halogenation as a  result occurs at the benzylic position.
                                    b)  Examples.

                                    b)  Mechanism:  the usual initiation, propagation, and termination [not shown] processes

 

                        3.    Oxidation of Alkylbenzenes by Cr(VI) and Mn(VII).
                                    a)  Mn(VII) as in KMnO₄ cleaves alkenes, but not alkanes and benzene.
                                    b)  A benzene ring has an activating effect on the benzylic position.
                                                i)  Cr(VI) and Mn(VII) oxidize alkyl side chains [with >1 benzylic H] to arylcarboxylic acids.
                                                ii)  the tert-butyl group is not affected under these conditions
                                    c)  Cr(VI) as in Na₂Cr₂O₇/H₂O/H₂SO₄  does the same thing
                                    d)  Example.

                                       e)  Side-chain oxidation of alkylbenzenes important for elimination of certain substances from body.
                                                i)  In the liver, toluene (a foreign substance) is oxidized by the action of an enzyme
                                                            (O₂       with cytochrome P₄₅₀) to benzoic acid and readily eliminated. 

                                                            Why is benzoic acid more easily eliminated than toluene from the body?

Answer:  Benzoic acid has the polar       carboxyl group, is more polar than toluene and thus is more soluble in water.
                                                ii)  Benzene has no side chain to oxidize; instead it is converted to a carcinogenic substance.

                        4.  Nucleophilic Substitution at the Benzylic Position
                                    a)  1^o benzylic halides are excellent substrates for S_N2 substitution; they can't undergo elimination.

                                    b)  2^o benzylic halides undergo both substitution and elimination reactions.  
                                                i)  Strong bases favor E2 reactions.
                                                ii)  Good nucleophiles which are weak bases favor S_N2 reactions.

                                    c)  Since the benzylic cation is stable, 3^o benzylic halides undergo S_N1 reaction (> 3^o alkyl halides).
                                                Medium to strong bases will still cause E2 reactions if there is a b-H

                                                i)  S_N1 hydrolysis: 2-chloro-2-phenylpropane is 600 times as fast as 2-chloro-2-methylpropane            

            B.  Preparation and Reactions of Alkenylbenzenes
                        1.  Preparation of Alkenylbenzenes
                                    a)  Dehydration of Benzylic Alcohols

                                    b)  Dehydrohalogenation (E2)

                        2.  Reactions of Alkenylbenzenes
                                    a)  These reactions are similar to those of alkenes.
                                    b)  Examples.

                                    c)  The addition of HX is regioselective.
                                                i)  The reason for the regioselectivity can be understood by looking at the cation intermediates
                                                            involved in the reaction:

Many thanks to Rod Oka of MPC for generously sharing his "Lecture Companion" outline,
reproduced here in extensively modified form by permission, with web references and other goodies added by me.
Structures drawn with  CS ChemOffice ChemDraw™, and MDL IsisDraw™ .