Arenes and Aromaticity - Organic Chemistry - Lecture Notes, Study notes of Organic Chemistry

Download Arenes and Aromaticity - Organic Chemistry - Lecture Notes and more Study notes Organic Chemistry in PDF only on Docsity! 1 Arenes and Aromaticity Arenes Arenes are hydrocarbon derivatives of benzene. They are called aromatic systems due to their special stability (not due to their aroma!). The special stability results from the highly conjugated system with the electrons delocalized all the way around the ring. The name benzene comes from the Arabic luban jawi or “incense from Java” since it is isolated as a degradation product of gum benzoin. This is a balsam obtained from a tree that grows in Java and Sumatra. This degradation product is benzoic acid, which can be decarboxylated by heating with calcium oxide, CaO. C O OH benzoic acid CaO heat + CO2 benzene And tolu balsam from the South American tolu tree, when distilled, produces toluene or methylbenzene. The term aliphatic hydrocarbons, given to alkanes, alkenes, alkynes and benzene derivatives, comes from the Greek word aleiphar which means oil or unguent. Benzene Benzene has three double bonds that are conjugated and in a circular arrangement. Due to this conjugation and circular arrangement, the double bonds are much less reactive than normal, isolated alkenes. For example, benzene will not react with bromine or hydrogen gas in the presence of a metal catalyst, reactions that alkenes undergo readily. CH CH CH3CH3 Br2 C H H C CH3CH3 Br Brrt Br2 rt No Reaction docsity.com 2 CH CH CH3CH3 H2, Pd rt CH3CH2CH2CH3 H2, Pd rt No Reaction If we examine the structure of benzene we do not see a series of alternating double and single bonds as we would expect from the Lewis structure. We see that all the C-C bonds and all the C-H are exactly the same. The bond order for the C-C bond is 1.5, so it is a one and a half bond. We can see this if we compare the bond lengths in benzene with 1,3-butadiene. C C C C C C H H H H H H 120° sp2 carbon 1.08 A° 1.4 A° A normal sp2-sp3 bond length is 1.46 A° A norml sp2-sp2 bond length is 1.34 A° C C H H H C C H HH 1.34 A° 1.46 In benzene we have a series of conjugated dienes for which we can draw two resonance structures that are of the same energy. 1 2 3 4 5 6 1 2 3 4 5 6 There is a special stability that is associated with a cyclic conjugated system. We can get a measure of this special aromatic stability by looking at the heats of hydrogenation of cyclohexene, 1,3-cyclohexadiene, and benzene. It is much more difficult to hydrogenate benzene than alkenes or alkyne. Rhodium and platinum catalysts are more reactive than palladium catalysts and it is possible to hydrogenate benzene under moderate pressure at room temperature using rhodium or platinum catalysis. Using the less active nickel catalysts, high temperatures are needed (100-200°C) and high pressures (100 atmospheres). docsity.com 5 C O OH benzoic acid (benzenecarboxylic acid) CH CH2 styrene (vinylbenzene) C O CH3 acetophenone (methyl phenyl ketone) OH phenol (benzenol) NH2 aniline (benzenamine) OCH3 anisole (methoxybenzene) Disubstituted benzenes have a special nomenclature that describes the relation between the substituents. The term ortho describes two substituents that are in a 1,2-relationship. Note that there are two ortho positions. If one of the substituents is one that defines the parent name (i.e. the –OH for phenol, the –CO2H for benzoic acid, etc.) then this substituent gets the number one position and dominates the numbering. The term meta describes two substituents in a 1,3-relationship and para describes a 1,4-relationship. There are two meta positions and one para position. These terms are abbreviated as o-, m-, p-, respectively. Cl CH3 ortho-chlorotoluene or o-chlorotoluene (2-chlorotoluene) X ortho postitionortho postition There are two ortho positions. Br Br ortho-dibromobenzene (1,2-dibromobenzene) X meta positionmeta position There are two meta positions. CH3 Cl meta-chlorotoluene or m-chlorotoluene F F meta-difluorobenzene (3-chlorotoluene) (1,3-difluorobenzene) docsity.com 6 X para position There is only one para position. CH3 Cl para-chlorotoluene or p-chlorotoluene (4-chlorotoluene) Dimethyl derivatives of benzene are called xylenes. There are three possible isomers. We use the ortho, meta, para terms to describe them. CH3 CH3 ortho-xylene or o-xylene (1,2-dimethylbenzene) CH3 ortho-xylene or o-xylene (1,2-dimethylbenzene) CH3 para-xylene or p-xylene (1,4-dimethylbenzene) CH3 CH3 If there are three or more substituents use numerical locants. If there is a substituent that determines the parent name, this substituent gets number one. Then choose the direction of numbering so as to give the next substituted position the lowest number. List the substituents in alphabetical order. When there is no parent name other than benzene, number the positions so as to give the lowest locant at the first point of difference, just like in substituted cyclohexanes. F OCH3 CH2CH3 1 2 4 4-ethyl-2-fluoro-anisole Cl NO2 NO2 1 2 4 1-chloro-2,4-dinitrobenzene When the benzene ring is a substituent, use “phenyl”. CH2CH2CH2OH 123 3-phenyl-1-propanol We use the term “aryl” to refer to any benzene ring derivative. The “benzyl” group is a C6H5CH2- group. Biphenyl is two benzene rings joined directly together. docsity.com 7 CH2 benzyl group CH2 Br benzylbromide biphenyl We can have polycyclic aromatic rings. Many of these are known and found in coal tar, which is produced when coal is converted to coke by heating to ~1000°C. naphthalene resonance energy 255 KJ/mol anthracene 347 KJ/mol phenanthracene 381 KJ/mol Naphthalene has a resonance energy of 255 KJ/mol. This is more than the resonance energy of benzene (152 KJ/mol) but it is less that twice the benzene resonance energy. The second benzene ring does not quite have the same amount of stabilization as the first. This can be seen from the resonance structures. In general, the most stable resonance structure for a polycyclic aromatic hydrocarbon is the one with the greatest number of rings that that correspond to Kekule formulations in which there are six π−electrons delocalized around the ring. For naphthalene only resonance structure B fits this criteria. It is therefore the most stable structure and is most representative of the actual structure of the molecule. Only the ring on the left corresponds to the Kekule formulation in which there are 6 π−electrons fully delocalized. Most stable form since both rings have the fully aromatic sturcture with the 6 π− electrons fully delocalized. Only the ring on the right corresponds to the Kekule formulation in which there are 6 π−electrons fully delocalized. A B C Note that phenanthracene has a larger stabilization energy than anthracene, even though both have three aromatic rings joined together and both have 14 π−electrons. This is because the arrangement of π−electrons is more favorable in phenanthracene than anthracene in that anthracene has no Lewis structure in which all three rings are fully aromatic with the 6 π−electrons fully delocalized around all three rings but phenanthracene does. This is the one shown above. docsity.com 10 CO2H H H H CO2HNa, NH3 (CH3)3COH Free Radical Halogenation A benzylic C-H bond is weaker than a C-H bond of a tertiary alkane or allylic alkane. This can be determined by comparing bond dissociation energies. CH2 H C H H ΔH = +356 KJ/mol CH2 CH CH2 CH2 CH C H HH ΔH = +368 KJ/mol CH3 C CH3 CH3 H CH3 C CH3 CH3 ΔH = +380 KJ/mol The benzylic radical is stabilized by resonance, just like the allylic radical. Note that the odd electron is delocalized on the benzylic position and the ortho and para carbon. C H H C H H C H H C H H Most stable Lewis structure since the ring retains its aromaticity. In free radical halogenations there is excellent selectivity for the benzylic position because the benzylic C-H bond is the weakest bond due to the fact that it forms a resonance stabilized radical. CH3 Cl2 hv or heat or peroxides CH2 Cl The mechanism is a free radical chain reaction. docsity.com 11 Inititation: Cl Cl hv 2 Cl Propagation CH2 H + Cl CH2 + H Cl CH2 Cl Cl+ CH2 Cl + Cl A good reagent for benzylic bromination is NBS. This provides a low level concentration of Br2. Again, only the benzylic position is brominated CH2CH3 + N O O Br ROOR CCl4, 80°C CHCH3 Br N-bromosuccinimide (NBS) + NH O O Oxidation of Alkylbenzenes The side chains of alkylbenzene, alkenyl and alkynyl derivatives can be oxidized by strong oxidizing reagents such as chromic acid or potassium permanganate. The oxidizing agents do not react with the benzene ring itself but react at the benzylic position to fully oxidize the benzylic carbon. The result is a benzoic acid derivative in which a carboxylic acid attached directly to the benzene ring. C R' H R Na2Cr7O7, H2SO4 or KMnO4 heat C O OH In order for this reaction to occur there must be at least one benzylic hydrogen. Tertiary alkyl groups do not react. C CH3 CH3 CH2CH3 Na2Cr7O7, H2SO4 H2O, heat No Reaction docsity.com 12 It does not matter what functional groups are contained in the side chain; the end result of the oxidation is the benzylic carboxylic acid. If there are two alkyl groups on the benzene ring both will react, providing that they each have at least one benzylic hydrogen. CH2CH3 C C CH2CH3 CH3 H Na2Cr7O7, H2SO4 H2O, heat C C O OH O OH Nucleophilic Substitution of Benzylic Halides Primary benzylic halides are very reactive in bimolecular nucleophilic substitution (SN2) reactions. The partial positive charge that develops in the transition states is stabilized by the adjacent benzene ring, accelerating the reaction. And they cannot undergo competing substitution reactions. C H H ClNO2 O C O CH3 Na C H H ONO2 C O CH3 + Na Cl Secondary benzylic halides are very subject to elimination reactions since the resulting double bond is stabilized by conjugation with benzene ring. C H Br CH2 H Na OCH3 C H CH2 conjugated π−system SN1 reactions are also very favorable because the highly stabilized carbocation. A tertiary benzylic carbocation is much more stable than a tertiary carbocation. For example, in a solvolysis reaction in aqueous acetone, chlorodimethylphenylmethane reacts six hundred times faster than 2-chloro-2-methylpropane. docsity.com 15 C H C H H C H CH3 H2, Pd C C H C H CH3 H HH H In the presence of peroxides, HBr will add so as to form the more stable benzylic radical (also in accordance with Markovnikov’s rule). CH CH2 H Br ROOR, heat CH2 CH2 Br The reaction follows a free radical chain mechanism in which the weak O-O bond of the peroxide breaks homolytically when heated RO OR heat 2 RO RO + H Br ROH + Br CH CH2 + Br C CH2 Br H H Br benzylic radical C CH2 Br H H The Polymerization of Styrene Polystyrene or styrofoam is a very important material that has many uses as packing material, insulation, beverage cups, etc. It is very light and very strong. There are several ways to polymerize styrene including free radical polymerization, cationic polymerization with acid, Ziegler-Natta polymerization and other methods. Free radical polymerization is what is called a head-to-tail polymerization. We need a radical initiator, RO, which adds to the terminal end of the styrene to start the radical chain process. The ensuing benzylic radical then adds to another molecule of styrene to give a benzylic radical, which in turn adds to the terminal carbon of another styrene molecule. This process repeats itself thousands of times until the desired chain length is produced. Then a radical terminator, X, is added to stop the reaction. In a final purification process, the radical initiator and radical terminator are removed to give polystyrene. The length of the chains has a large effect on the properties of the final material and the chain lengths can be tailored to the desired application. docsity.com 16 RO OR heat 2 RO CH CH2 + C CH2 OR H benzylic radical C CH2 OR CH2 H RO C CH CH2 H C CH2 OR CH2 H C HCH2C H CH2 CH repeat ORCH2CHCH2CHCH2CHX Add X to terminate the reaction n Remove the initiator OR and terminator, X, to get polystyrene. polystyrene Aromaticity As we have seen, benzene with its circular arrangement of six π−electrons, is very stable due to the delocalization of the π−electrons over all of the six carbons of the benzene ring. There literally is a small electric current flowing around the ring. When we examine other systems that might also have this special aromatic stability, we find some interesting results. For example, look at cyclobutadiene, a molecule that on first glance looks as if it should be aromatic. It is a cyclic molecule and has two π−bonds that are conjugated. It looks as if it should be particularly stable in the same way that benzene is particularly stable but in fact it is not. It has never actually been isolated in pure form since it decomposes immediately on formation so in effect it is not a known molecule. It turns out that 1,3-butadiene is destabilized by the circular arrangement of π−electrons. It is less stable than expected. This is also true of cyclooctatetraene. This molecule can be isolated and heats of hydrogenation experiments have been done to determine that the circular, conjugated arrangement of double bonds is less stable than isolated double bonds. docsity.com 17 cyclobutadiene cyclooctatetraene + H2 Pd ΔH=-96 KJ/mol + 4H2 Pd ΔH = -410 KJ/mol Expect 4 X -96 = -384 KJ/mol We see that cyclooctatetraene is less stable than expected by 26 KJ/mol. The circular arrangement of electrons, rather than adding stability, as it does in benzene, actually makes cyclobutadiene and cyclooctatetraene less stable. These compounds are called anti- aromatic. We can account for this using an empirical rule developed by Huckel, called Huckel’s Rule. Huckel’s Rule: among planar, monocyclic, fully conjugated polyenes, only those having 4n+2 π−electrons, where n is a whole number (n = 0, 1, 2, 3, …) will be aromatic. This is an empirical rule, meaning that Huckel developed the rule simply by looking at many, many cyclic conjugated polyenes and grouping those are aromatic and looking for a common property. He found that only certain number of π−electrons lead to Aromaticity and other numbers lead to anti-aromatic compounds. The number n has no physical significance. Using Huckel’s rule we can predict that compounds having two π−electrons (n = 0), six π−electrons (n = 1), ten π−electrons (n = 2) and so on will have the special stability associated with aromaticity but those that have four π−electrons (cyclobutadiene), eight π−electrons (cyclooctatetraene), etc. do not have this special stability and are anti- aromatic. We can explain the underlying theoretical basis for Huckel’s rule by looking at the π−molecular orbitals that are present in these monocyclic, planar, conjugated polyenes. To do this we use a device known as Frost’s circle, another empirical device. For a particular ring size, (3, 4, 5, 6, etc.) inscribe a polygon for that ring size inside a circle so that one of its vertices lies at the bottom of the circle. The points where the polygon touches the circle give the energy levels of the π−molecular orbitals. The number of p-orbitals involved equals the number of sixes of the polygon. For example, for cyclobutadiene, there are 4 p- orbitals (and it is a four-membered ring) so we inscribe a square inside a circle with one corner of the square at the bottom of the square. The center of the circle represents the non-bonding energy level. docsity.com 20 Aromatic Ions We can also have aromatic stabilization of certain anions or cations, provided they have the continuous, circular overlap of p-orbitals and they have the requisite 4n+2 π−electrons. For example, there is the cycloheptatrienyl cation, the so-called tropylium ion, is aromatic. It is completely conjugated and it has six π−electrons spread out over seven carbon atoms. H H H H HH H H sp2 carbon with empty p-orbital We see continuous overlap of the π−orbitals. Cyclopentadienyl anion is also resonance stabilized and is aromatic. Counting the two lone pair of the anion and the four electrons in the two double bonds it has six π−electrons spread out over five carbon atoms. C H C H H H H H We can see the very large amount of resonance stabilization of cyclopentadienyl anion by looking at the pKa of 1,3-pentadiene. It is very acidic (pKa 16) when compared to cyclopentane (pKa > 60). This is due to the resonance stabilization of the resulting anion. C HH pKa 16 + OHNa C H Keq = ~ 1 HH OHNa+ pKa 60 No Reaction docsity.com 21 We also see resonance stabilization for cyclopropylium cation and cyclooctatetraene dianion. These are not stable species but are more stable than would be expected due to their resonance stabilization. H Heterocyclic Aromatic Compounds We can also have aromatic compounds that have a heteroatom in the ring. In organic chemistry a heteroatom is any atom other than carbon. The most common heteroatoms are nitrogen, oxygen and sulfur. Many of these molecules are important building blocks in biochemical systems. They may be monocyclic five or six-membered rings. N pyridine N H pyrrole O furan S thiophene Many bicyclic heterocycles are known. N N quinoline isoquinoline N H indole O S benzofuran benzothiophene Many heterocycles contain two or more heteroatoms. N N H N O imidazole oxazole N S thiazole docsity.com 22 N H N benzimidazole N N NH N purine N N NH N NH2 Adenine (a DNA base) N N pyrimidine These systems are all aromatic but they all have less aromatic stabilization than benzene itself since the heteroatom distorts the equal distribution of the π−electrons since, in all of the examples presented, it is more electronegative than carbon. Pyridine, for example, has six π−electrons, not counting the lone pair on the sp2-nitrogen ring. N The six π−electrons come from the two electrons in each of the three π−bonds. The lone pair is not part of the aromatic system. pyridine But in pyrrole, the lone is part of the aromatic system. The lone pair of pyrrole, therefore, is partially delocalized around the pyrrole ring and is therefore less available for sharing. Pyrrole is a much weaker base than pyridine. N H Lone pair is part of the aromatic system. N H sp2 nitrogen The lone of of electrons is partially delocalized around the ring and not available for sharing. N + H O H N H HO pKa 5.2 N H + H O H N HH pKa -3 docsity.com