Aromatic Compounds Early in the history of organic chemistry (late 18 th, early 19 th century) chemists discovered a class of compounds which were unusually stable A number of these compounds had a distinct odor Hence these compounds were called aromatic Today the term aromatic is used regardless of the odor of the compound Some aromatic compounds have little to no odor The parent aromatic compound was discovered to have a molecular formula of C 6 H 6 (called benzene) This 1:1 ratio of carbon to hydrogen is extremely low compared to other known compounds It was also quickly discovered that these aromatic compounds did not react like other alkene compounds
Structure of Benzene Before MR and other spectroscopic tools it was hard to determine the structure of organic compounds Ultimately the symmetry of the molecule revealed its structure All carbon atoms, and all carbon-carbon bonds, are symmetrically equivalent To account for these observations the proposed structure consisted of a cyclic compound stabilized by resonance Each resonance structure is equal in energy and thus each contributes equally to the overall structure
Stability The resonance structures imply an extra stability, but the amount of stability in benzene is much more than a typical resonance structure Consider reactivity: HBr Br Reaction is faster than 1- butene due to more stable carbocation intermediate HBr Br Br Having conjugation in ring somehow stabilizes compound HBr o reaction But the presence of a conjugated ring is not enough to cause this extra stability HBr Br What causes this extra stability in benzene and why is a 6-membered conjugated ring more stable than an 8-membered conjugated ring?
Stability of Aromatic Compounds Can measure stability by hydrogenation H 2 catalyst The energy required for this hydrogenation indicates the stability of the alkene 2 Kcal/mol Conjugation stability 49.8 Kcal/mol 55.4 Kcal/mol 57.4 Kcal/mol Almost double in energy? Kcal/mol 28.6 Kcal/mol How much energy should be in the hydrogenation of Benzene? Have three double bonds in conjugation, so therefore should expect ~79 Kcal/mol (~24 Kcal/mol more than 55 Kcal/mol for 1,3-cyclohexadiene) Benzene is ~ 30 Kcal/mol more stable than predicted!!
Aromatic Stabilization This ~30 Kcal/mol stabilization is called aromatic stabilization It is the cause of the difference in reactivity between normal alkenes It would cost ~30 Kcal/mol to break the aromaticity and thus the normal alkene reactions do not occur with benzene Somehow having these three double bonds in resonance in a cyclic system offers a tremendous amount of energy
Aromatic Stabilization Cyclic system alone, however, is not sufficient for aromatic stabilization Consider a four membered ring Cyclobutadiene also has a ring structure with conjugated double bonds that could resonate This compound however is highly reactive and does not exist with equivalent single and double bonds In solution it reacts with itself in a Diels-Alder reaction
Aromatic Stabilization Why the Difference in Stability? Can already see in electron density maps that cyclobutadiene is not symmetric Benzene 6-fold symmetry Cyclobutadiene ot symmetric
Molecular rbitals for Benzene For benzene there are 6 atomic p orbitals in conjugation, therefore there will be 6 M s -As the number of nodes increase, the energy increases For lowest energy M there are zero nodes, therefore bonding interactions between each carbon-carbon bond Benzene model Top view with orbitals Side view
Entire M Picture for Benzene 6 nodes 4 nodes 4 nodes E onbonded energy level 2 nodes 2 nodes Zero nodes
Molecular rbitals for Benzene otice all electrons are in bonding M s All the antibonding M s are unfilled With a cyclic system we obtain degenerate orbitals (orbitals of the same energy) verall this electronic configuration is much more stable than the open chain analog > This is now the definition of an aromatic compound (not aroma), Flat conjugated cyclic system is MRE stable than the open chain analog
Molecular rbitals for Cyclobutadiene E onbonded energy level Unlike benzene, cyclobutadiene has two electrons at the nonbonding energy level (these electrons do not stabilize the electronic structure)
Antiaromatic Cyclobutadiene is less stable than butadiene < If a cyclic conjugated system is less stable than the open chain analog it is called antiaromatic Part of the reason for cycobutadiene to be antiaromatic is the presence of two M s at the nonbonding level In butadiene all electrons are in bonding M s therefore the electrons are more stable in butadiene relative to cyclobutadiene
Frost Circle A simple method to determine the relative molecular orbital energy levels for a conjugated ring is called a Frost circle (or Frost Mnemonic) First just draw a circle ext draw a polygon with equal length of sides corresponding to the number of atoms in the ring being considered Place the polygon inside the ring having a vertex point directly at the bottom Wherever a vertex point of the polygon hits the ring corresponds to an energy level The electronic configuration would be obtained by placing the correct number of electrons in the molecular orbitals (the relative energy levels are also obtained as the ring drawn initially has a radius of 2β) Will work for any flat, conjugated ring system to determine energy levels
Hückel s Rule In order to determine if a system is aromatic or antiaromatic, without needing to determine the overall electronic energy of the closed form versus the open form, Hückel s rule was developed First the cyclic system must have a p orbital on all atoms in a continuous cyclic chain (if there is an atom without a p orbital in the cycle then the system is nonaromatic) In practice this means the cyclic system must be flat (to allow overlap of p orbitals) If these criteria are met then: If the system has 4n+2 π electrons, it is aromatic If the system has 4n π electrons, it is antiaromatic 6 π electrons, 4n+2 Therefore aromatic 4 π electrons, 4n Therefore antiaromatic o p orbital on one atom Therefore nonaromatic
Hückel s Rule What is the underlying cause for the symmetry in Hückel s rule? Ultimately the stabilization is due to the relative electronic configuration for a flat, conjugated ring system The symmetry is also observed with the Frost circle 4 π electron system 6 π electron system 8 π electron system btain 2 electrons at nonbonding level All electrons are at bonding level btain 2 electrons at nonbonding level 4n+2 systems allow all electrons to be in bonding molecular orbitals, therefore more stable 4n systems, however, will place 2 electrons at nonbonding level and thus be less stable
Hückel s Rule Remember that the cyclic ring must have overlap of p orbitals to be considered aromatic or antiaromatic by Hückel s rule Cyclooctatetraene If flat this molecule is antiaromatic with the 8 π electrons Molecule, however, adopts a non-flat low energy conformation top view side view This is an example of a rare case where delocalization is avoided to increase stability!
Aromatic Ions Benzene is a neutral aromatic compound Any compound with 4n+2 electrons in a continuous loop is considered aromatic regardless of the number of carbons in the loop There are many aromatic compounds with a different number of electrons than atoms in the loop Due to this difference usually these compounds are ions, hence aromatic ions
Cyclopentadienyl Anion Cyclopentadiene is nonaromatic since there is not a p orbital on one of the carbons in the ring nonaromatic base pk a ~15 Extremely low pka is due to aromatic stabilization H H Upon removal of a proton, however, there is now a p orbital on each carbon 6 electrons in system, therefore according to Hückel this is aromatic base pk a ~50-60 base pk a ~44 Unactivated alkanes have much higher pka Simple conjugation only explains small portion of stability
Aromatic Ions Any compound that will have 4n+2 electrons in a continuous loop for planar conjugated compound will be favored due to aromatic nature H H 2 S 4 Tropylium ion 6 π electrons, 4n+2 K 10 π electrons, 4n+2 eed correct number of conjugated electrons, not all conjugated ions are aromatic H H 2 S 4 base Does not form! High pka
Benzene Derivatives The IUPAC name of 1,3,5-cyclohexatriene is never used The common name of benzene dominates naming of these structures In addition, another common naming tool for benzene derivatives is for disubstituted compounds (ortho, meta, para) orthodimethylbenzene metadimethylbenzene paradimethylbenzene
Benzene Derivatives umber along ring to give lowest number First priority substituent is at the 1-position ther common names CH 3 H H toluene phenol benzoic acid If benzene group is being considered as a substituent, instead of root name, then use phenyl prefix, from phenol name Another common name used for a substituted toluene is called benzyl group H Br H 4-phenyl-2-butanol Benzyl bromide Benzyl alcohol
Heterocyclic Aromatic Compounds Compounds that contain atoms besides carbon can also be aromatic eed to have a continuous loop of orbital overlap and follow Hückel s rule for the number of electrons in conjugation Common noncarbon atoms to see in aromatic compounds include oxygen, nitrogen, and sulfur H S H pyridine pyrrole furan thiophene pyrimidine imidazole All of these compounds have 6 electrons conjugated in ring Consider where the lone pair(s) are located for each heteroatom
Pyridine ne common aromatic compound with nitrogen is pyridine ne carbon atom of benzene has been replaced with nitrogen Consider the placement of electrons Lone pair is orthogonal to conjugated electrons in ring The number of electrons in conjugation is 6 (don t include lone pair that is orthogonal to ring) therefore pyridine follows Hückel s rule and is aromatic Pyridine can be protonated in acidic conditions and it will still be aromatic, protonation occurs at lone pair
Pyrrole A similar aromatic compound is pyrrole H H With pyrrole the lone pair is included in the conjugated ring Have 6 electrons in loop and therefore this compound is aromatic If protonated, however, pyrrole will become nonaromatic since the nitrogen would thus be sp 3 hybridized without a p orbital for conjugation
Heterocyclic Aromatic Compounds Difference in electron placement affects properties pyridine pyrrole Excess electron density of lone pair is localized orthogonal to ring in pyridine while the electron density is conjugated in ring with pyrrole
Fused Rings Compounds with more than one fused ring can also be aromatic aphthalene The simplest two ring fused system is called naphthalene Like benzene, naphthalene is an aromatic compound with 10 electrons in a continuous ring around the cyclic system (one p orbital on each carbon is conjugated) Consider one electron Electron can resonate in p orbitals Will occur with all 10 electrons
Fused Rings The reactivity of naphthalene is similar to benzene It is unreactive toward normal alkene reactions because any addition would lower the aromatic stabilization If it did react, however, there would still be one benzene ring intact HBr Br Hypothetical reaction does not occur With larger fused ring systems normal alkene reactions start to occur
Anthracene Br Br 2 Two intact benzene rings Reactions occur at central ring due to large aromatic stabilization remaining Br Two intact benzene rings 2 2 Diels-Alder reactions can also occur about this central ring The dienophile approaches the central ring from top or bottom And then Diels-Alder reaction occurs to leave two intact benzene rings
Fused Heterocyclics Fused ring systems with heterocyclics can also be aromatic Extremely important compounds biologically and medicinally H 2 H H H H 2 Adenine (A) 10 π electrons Guanine (G) 10 π electrons Two of the four constituents of base pairs in DA consist of fused aromatic rings, the other two bases, cytosine (C) and thymine (T), are one ring aromatic base pairs H 2 H CH 3 H Thymine (T) H Cytosine (C)
Aromatic Base Pairing The four bases shown in the preceding page (A, G, C, T) are the bases used in DA The bases are attached to a sugar through the H group on each ring and the sugars are linked through a phosphate backbone π stacking P Sugar P Sugar P Hydrogen bonding Base Base Base Base P Sugar P Sugar P Sugar H H Sugar H H H G C The bases are complementary to each other and bind through hydrogen bonding (C binds with G and A binds with T) This complementarity allows genetic information to be passed along as the DA is replicated Things that disrupt this complementarity can cause cell death or possibly cancer
Polycyclic Aromatic Hydrocarbons Polycyclic aromatic hydrocarbons (PAH s) have been shown to disrupt this base pairing The PAH is first oxidized by enzymes (these enzymes are essential to remove hydrophobic compounds in the body) Sugar H H 2 P450 P450 eed PAH to react, Benzene or naphthalene would not undergo this reaction H H H H Guanine can react with this epoxide, however, which will destroy its hydrogen-bonding complementarity in the DA base pairing, thus causing cells to eventually die
Benzyl Group The benzyl group behaves similar to the allyl group seen previously, the orbitals on this group are stabilized through resonance with the adjacent benzene group Cl CH 3 S 1 CH 3 H UC Intermediate S 2 CH 3 a I CH 3 Transition State Cl Br S 1 Intermediate: Cl 2 cation 2 cation resonance S 2 T.S.: 1 I I I 1 in resonance 1 in resonance Relative rate: 1 100,000 Relative rate: 1 33 78
Benzyl Group A unique reaction of benzyl groups is that the benzyl carbon can be oxidized with either potassium permanganate (KMn 4 ) or dichromic acid (H 2 Cr 2 7 ) to a carboxylic acid 1. KMn 4 2. H+,H 2 1. KMn 4 2. H+,H 2 CH 3 1. KMn 4 2. H+,H 2 H 1. KMn 4 2. H+,H 2 1. KMn 4 2. H+,H 2 C 2 H H 2 C If alkyl chain is longer, then carbon-carbon bonds are broken and left with benzoic acid Must have hydrogen on benzylic carbon, though, as a t-butyl group will not be oxidized Realize also this reaction is not selective, any alkyl chain on benzene will be oxidized
Benzyl Group As seen in chapter 12, halogenation reactions can occur with either chlorine or bromine under photolytic conditions Reaction proceeds through a radical intermediate The benzylic radical is more stable due to resonance with aromatic ring CH 2 Remember that chlorination was more reactive, bromination though occurred selectively Cl 2, h! Cl Cl Br 2, h! Br Realize reaction does not occur on aromatic ring, do not obtain radical at sp 2 hybridized carbon
Birch Reduction While typical alkene reactions do not occur on benzene, the aromatic ring can be reduced by adding electrons to the system (in essence a nucleophilic addition) The reduction is similar to the dissolving metal reduction of alkynes to E-alkenes The electrons need to be generated in situ H 3 (l) a H 3 (l) e a+ This electron is called a solvated electron
Birch Reduction In the presence of an aromatic ring this electron will react e Addition of one electron thus generates a radical anion This strongly basic anion will abstract a proton from alcohol solution RH H H
Birch Reduction The radical will then undergo the same operation a second time H H e RH H H H H H H The final product has thus been reduced from benzene to a 1,4-cyclohexadiene (always obtain a 1,4 relationship of the dienes in a Birch reduction they are not conjugated) The aromatic stabilization has been lost
Birch Reduction What happens if there is a substituent on the aromatic ring before reduction? X H 3 (l), a RH X X Which regioisomer will be obtained? Similar to every other reaction studied need to ask yourself, What is the stability of the intermediate structure? The preferred product is a result of the more stable intermediate
Birch Reduction The intermediate in a Birch reduction is the radical anion formed after addition of electron With electron withdrawing substituent: H 3 (l), a CH 3 H Placing negative charge adjacent to carbonyl allows resonance With electron donating substituent: CH 3 H 3 (l), a CH 3 H CH 3 CH 3 Want negative charge as far removed from donating group as possible
Spectroscopy of Aromatic Compounds We have already seen how aromatic benzene compounds have a relatively large downfield MR shift due to aromatic ring current Therefore any of these aromatic systems, which by definition have a ring current, have a large downfield shift Can use as a characteristic of aromaticity S
Mass Spectrometry A characteristic peak in a MS for a benzenoid compound is the presence of a peak at m/z 91 (if formation is possible) Due to resonance stabilized benzyl cation