Lecture 12 Aromatic Compounds Sources of Aromatic Hydrocarbons

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Lecture 12 Aromatic Compounds Sources of Aromatic Hydrocarbons Powered By Docstoc
					                                                               C 2004 Barry Linkletter, UPEI                                                                                C 2004 Barry Linkletter, UPEI




                                                                                                                                   Aromatic Compounds
                                                                                                           • Aromatic was used to described some fragrant
                                                                                                             compounds in early 19th century
                                                                                                                    – Not correct: later they are grouped by chemical behavior
                                Lecture 12                                                                            (unsaturated compounds that undergo substitution rather
                                                                                                                      than addition)
                                                                                                           • Current: distinguished from aliphatic compounds
                             Aromatic Compounds                                                              by electronic configuration

                                  Chapter 5




Chemistry 243 - Lecture 12                                                                1    Chemistry 243 - Lecture 12                                                                              2




                                                               C 2004 Barry Linkletter, UPEI                                                                                C 2004 Barry Linkletter, UPEI




             Sources of Aromatic Hydrocarbons                                                                                 Naming Aromatic Compounds

            • From high temperature distillation of coal tar                                               • Many common names (toluene = methylbenzene; aniline =
                                                                                                             aminobenzene)
            • Heating petroleum at high temperature and                                                    • Monosubstituted benzenes systematic names as
              pressure over a catalyst                                                                       hydrocarbons with –benzene
                                                                                                                    –       C6H5Br = bromobenzene
                                                                                                                    –        C6H5NO2 = nitrobenzene, and C6H5CH2CH2CH3 is
                                                                                                                            propylbenzene




Chemistry 243 - Lecture 12                                                                3    Chemistry 243 - Lecture 12                                                                              4
                                                                         C 2004 Barry Linkletter, UPEI                                                                          C 2004 Barry Linkletter, UPEI




                                The Phenyl Group                                                                                      Disubstituted Benzenes

            • When a benzene ring is a substituent, the term                                                           • Relative positions on a benzene ring
                                                                                                                                – ortho- (o) on adjacent carbons (1,2)
              phenyl is used (for C6H5_)
                                                                                                                                – meta- (m) separated by one carbon (1,3)
                     – You may also see “Ph” or “!” in place of “C6H5”                                                          – para- (p) separated by two carbons (1,4)
            • “Benzyl” refers to “C6H5CH2_”                                                                            • Describes reaction patterns (“occurs at the para position”)




Chemistry 243 - Lecture 12                                                                          5    Chemistry 243 - Lecture 12                                                                        6




                                                                         C 2004 Barry Linkletter, UPEI                                                                          C 2004 Barry Linkletter, UPEI




              Naming Benzenes With More Than Two
                                                                                                                              Structure and Stability of Benzene
                         Substituents
                 • Choose numbers to get lowest possible values                                                      • Benzene reacts with slowly with Br2 to give bromobenzene
                 • List substituents alphabetically with hyphenated numbers                                            (where Br replaces H)
                 • Common names, such as “toluene” can serve as root name (as                                        • This is substitution rather than the rapid addition reaction
                   in TNT)                                                                                             common to compounds with C=C, suggesting that in benzene
                                                                                                                       there is a higher barrier




Chemistry 243 - Lecture 12                                                                          7    Chemistry 243 - Lecture 12                                                                        8
                                                                     C 2004 Barry Linkletter, UPEI                                                                      C 2004 Barry Linkletter, UPEI




                Heats of Hydrogenation as Indicators
                                                                                                                                  Benzene’s Unusual Structure
                            of Stability
            • The addition of H2 to C=C normally gives off about                                                 • All its C-C bonds are the same length: 139 pm — between
                                                                                                                   single (154 pm) and double (134 pm) bonds
              118 kJ/mol – 3 double bonds would give off
                                                                                                                 • Electron density in all six C-C bonds is identical
              356kJ/mol                                                                                          • Structure is planar, hexagonal
                     – Two conjugated double bonds in cyclohexadiene add                                         • C–C–C bond angles 120°
                       2 H2 to                                                                                   • Each C is sp2 and has a p orbital perpendicular to the
            • Benzene has 3 unsaturations but gives off only 150                                                        plane of the six-membered ring
              kJ/mol on reacting with 3 H2 molecules
            • Therefore it has about 150 kJ more “stability”
              than an isolated set of three double bonds




Chemistry 243 - Lecture 12                                                                      9    Chemistry 243 - Lecture 12                                                                   10




                                                                     C 2004 Barry Linkletter, UPEI                                                                      C 2004 Barry Linkletter, UPEI




                   Drawing Benzene and Its Derivatives                                                                                  A Quick Aside…
            • The two benzene resonance forms can be represented by a
              single structure with a circle in the center to indicate the                                       • Aromatic compounds are an example of
              equivalence of the carbon–carbon bonds                                                               molecules with distributed !-systems
            • This does indicate the number of ! electrons in the ring but
              reminds us of the delocalized structure                                                            • What are the !-molecular orbitals of
            • We shall use one of the resonance structures to represent
              benzene for ease in keeping track of bonding changes in                                              molecules with more than 2 P orbitals in a
              reactions
                                                                                                                   row?
                                                                                                                          – E.g. conjugated !-systems




Chemistry 243 - Lecture 12                                                                     11    Chemistry 243 - Lecture 12                                                                   12
                                                   C 2004 Barry Linkletter, UPEI                                                        C 2004 Barry Linkletter, UPEI




                               Recall Ethylene                                                                     3 P Orbitals

            • Add or subtract the two P-orbitals                                               • Need 3 SP2 carbons so we must have a
                                                                                                 cation or anion.




Chemistry 243 - Lecture 12                                                   13    Chemistry 243 - Lecture 12                                                     14




                                                   C 2004 Barry Linkletter, UPEI                                                        C 2004 Barry Linkletter, UPEI




                             3-Carbon !-Systems                                                                 4-Carbon !-Systems




Chemistry 243 - Lecture 12                                                   15    Chemistry 243 - Lecture 12                                                     16
                                                                     C 2004 Barry Linkletter, UPEI                                                                 C 2004 Barry Linkletter, UPEI




                                             Benzene                                                                                   Benzene Orbitals

            • Benzene is a 6-carbon !-system
            • Benzene is cyclic and highly symmetrical
                     – A cyclic system can never have just 1 node
                     – A cyclic system can have 0,2,4… nodes
                     – There are 2 ways to align a set of nodes
                              • Node at a carbon
                              • Node between carbons




Chemistry 243 - Lecture 12                                                                     17    Chemistry 243 - Lecture 12                                                              18




                                                                     C 2004 Barry Linkletter, UPEI                                                                 C 2004 Barry Linkletter, UPEI




                             Molecular Orbital Description of
                                                                                                                                  Recall: Key Ideas on Benzene
                                        Benzene
            • The 6 p-orbitals combine to give                                                                   • Unusually stable - heat of hydrogenation
                     – Three bonding orbitals with 6 ! electrons,                                                  150!kJ/mol less negative than a cyclic triene
                     – Two nonbonding and two antibonding orbitals                                               • Planar hexagon: bond angles are 120°,
            • Orbitals with the same energy are degenerate                                                         carbon–carbon bond lengths 139 pm
                                                                                                                 • Undergoes substitution rather than electrophilic
                                                                                                                   addition
                                                                                                                 • Resonance hybrid with structure between two line-
                                                                                                                   bond structures
                                                                                                                 • One more important factor is the number of
                                                                                                                   electrons in the cyclic orbital


Chemistry 243 - Lecture 12                                                                     19    Chemistry 243 - Lecture 12                                                              20
                                                                       C 2004 Barry Linkletter, UPEI                                                                         C 2004 Barry Linkletter, UPEI




                                                                                                                              Compounds With 4n ! Electrons Are Not
                    Aromaticity and the 4n + 2 Rule                                                                              Aromatic (May be Antiaromatic)
            • Huckel’s rule, based on calculations – a planar cyclic                                               • Planar, cyclic molecules with 4 n ! electrons
              molecule with alternating double and single bonds has                                                  are much less stable than expected (anti-
              aromatic stability if it has 4n+ 2 ! electrons (n is                                                   aromatic)
              0,1,2,3,4)                                                                                           • They will distort out of plane and behave like
                                                                                                                     ordinary alkenes
                                                                                                                                                                       cyclobutadiene

            • For n=1: 4n+2 = 6; benzene is stable and the electrons are
                                                                                                                   • 4- and 8-electron compounds are not
              delocalized
                                                                                                                     delocalized (single and double bonds)
                                                                                                                   • Cyclobutadiene is so unstable that it
                                                                                                                     dimerizes at low temperature                     cyclooctatetraene

                                                                                                                   • Cyclooctatetraene has four double bonds,
                                                                                                                     reacting with Br2, KMnO4, and HCl as if it
                                                                                                                     were four alkenes




Chemistry 243 - Lecture 12                                                                       21    Chemistry 243 - Lecture 12                                                                      22




                                                                       C 2004 Barry Linkletter, UPEI                                                                         C 2004 Barry Linkletter, UPEI




                                                                                                                          Aromaticity of the Cyclopentadienyl
                               Aromatic Ions
                                                                                                                                         Anion
            • The 4n + 2 rule applies to ions as well as neutral species                                           • 1,3-Cyclopentadiene contains
            •  Both the cyclopentadienyl anion and the cycloheptatrienyl                                             conjugated double bonds joined
              cation are aromatic                                                                                    by a CH2 that blocks
                                                                                                                     delocalization
            • The key feature of both is that they contain 6 ! electrons in
              a ring of continuous p orbitals                                                                      • Removal of H+ at the CH2
                                                                                                                     produces a cyclic 6-electron
                                                                                                                     system, which is stable
                                                                                                                   • Removal of H- or H• generate
                                                                                                                     nonaromatic 4 and 5 electron
                                                                                                                     systems
                                                                                                                   • Relatively acidic (pKa = 16)
                                                                                                                     because the anion is stable



Chemistry 243 - Lecture 12                                                                       23    Chemistry 243 - Lecture 12                                                                      24
                                                                      C 2004 Barry Linkletter, UPEI                                                                 C 2004 Barry Linkletter, UPEI




                                                                                                                         Aromatic Heterocycles: Pyridine and
                             Cycloheptatriene
                                                                                                                                      Pyrrole
            • Cycloheptatriene has 3 conjugated double bonds                                                      • Heterocyclic compounds contain elements other
              joined by a CH2                                                                                       than carbon in a ring, such as N,S,O,P
            • Removal of “H-” leaves the cation                                                                   • Aromatic compounds can have elements other
            • The cation has 6 electrons and is aromatic                                                            than carbon in the ring
                                                                                                                  • There are many heterocyclic aromatic
                                                                                                                    compounds and many are very common
                                                                                                                  • Cyclic compounds that contain only carbon are
                                                                                                                    called carbocycles (not homocycles)
                                                                                                                  • Nomenclature is specialized



Chemistry 243 - Lecture 12                                                                      25    Chemistry 243 - Lecture 12                                                              26




                                                                      C 2004 Barry Linkletter, UPEI                                                                 C 2004 Barry Linkletter, UPEI




                                    Pyridine                                                                                           Pyrrole
                                                                                                             • A five-membered heterocycle with one
            • A six-membered heterocycle with a nitrogen atom in its                                           nitrogen
              ring
                                                                                                             • ! electron system similar to that of
            • ! electron structure resembles benzene (6 electrons)                                             cyclopentadienyl anion
            • The nitrogen lone pair electrons are not part of the                                           • Four sp2-hybridized carbons with 4 p
              aromatic system (perpendicular orbital)                                                          orbitals perpendicular to the ring and
                                                                                                               4 p electrons
            • Pyridine is a relatively weak base compared to normal
              amines but protonation does not affect aromaticity                                             • Nitrogen atom is sp2-hybridized, and
                                                                                                               lone pair of electrons occupies a p
                                                                                                               orbital (6 ! electrons)
                                                                                                             • Since lone pair electrons are in the
                                                                                                               aromatic ring, protonation destroys
                                                                                                               aromaticity, making pyrrole a very
                                                                                                               weak base



Chemistry 243 - Lecture 12                                                                      27    Chemistry 243 - Lecture 12                                                              28
                                                                      C 2004 Barry Linkletter, UPEI                                                                 C 2004 Barry Linkletter, UPEI




                                                                                                                                   Polycyclic Aromatic Compounds:
                                 Why 4n +2?
                                                                                                                                             Naphthalene
            • When electrons fill the various molecular orbitals, it takes                                        • Aromatic compounds can have rings that share a
              two electrons (one pair) to fill the lowest-lying orbital and
              four electrons (two pairs) to fill each of n succeeding energy
                                                                                                                    set of carbon atoms (fused rings)
              level                                                                                               • Compounds from fused benzene or aromatic
            • This is a total of 4n + 2                                                                             heterocycle rings are themselves aromatic




Chemistry 243 - Lecture 12                                                                      29    Chemistry 243 - Lecture 12                                                              30




                                                                      C 2004 Barry Linkletter, UPEI                                                                 C 2004 Barry Linkletter, UPEI




                             Naphthalene Orbitals                                                                                         For Next Class

            • Three resonance forms and delocalized electrons                                                     • Finish Reading Chapter 5
                                                                                                                           – Benzene and aromaticity




Chemistry 243 - Lecture 12                                                                      31    Chemistry 243 - Lecture 12                                                              32

				
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