Aromatic Compounds and Aromaticity Solomons 6th Edition Chapter 14 p 614 – 654 Chapter 15 p 655 – 703 (Reactions) You will by now be familiar with the structure of benzene C H 6 6 or Discovered in 1825 by Michael Faraday (RI). Molecular formula deduced by Mitscherlich in 1834. The fragrant odour of benzene and its derivatives led them to being classed as “aromatic”. This classification now has a chemical meaning – “aromaticity” is associated with a special stability resulting from structure. Elucidation of the structure posed a problem – the molecular formula C H indicated a 6 6 highly unsaturated compound (double and/or triple bonds) but benzene does not show this behaviour. Kekulé (1865) conceived a cyclic structure, H H C H C C C C H C H H but this would imply alternating single and double bonds (C-C = 1.47Å, C=C = 1.34Å). Kekulé suggested that two forms of benzene were in rapid equilibrium: Later spectroscopic evidence showed all bond lengths to be equal and intermediate between single and double bond lengths (1.39 Å). It was also found that benzene was a flat (planar) molecule. We now look at benzene using two different possible approaches to try to describe its stability. A. VALENCE BOND APPRAOCH Resonance hybrid, 2 canonical forms nb: not -Try to represent both single and double bond character of each bond. Remember with resonance structures, neither of the extremes actually exists – the structure is somewhere in between. Unicorn Dragon Rhinoceros Resonance structrures Real structure (hybrid) Further, all bond angles in benzene are 120º (revise 12.5), p electrons are delocalised. Resonance theory states that if more than one resonance form can be drawn for a molecule, then the actual structure is somewhere in between them. Furthermore, the actual energy of the molecule is lower than might be expected for any of the contributing structures. If a molecule has equivalent resonance structures it is much more stable than either canonical would be – hence the extra stability of benzene (called resonance energy). B. MOLECULAR ORBITAL REPRESENTATION OF BENZENE (MO THEORY) The bond angles of 120(cid:176) in benzene suggests that C atoms are sp2 hybridised. An alternative representation therefore starts with a planar framework and considers overlap of the p orbitals (p electrons). unused p orbital x6 2 delocalised p system sp hybridised C (Simple MO rules) Mix n x p atomic orbitals fi np molecular orbitals! Remember ethene? (p 26) 2p orbitals bonding p -orbital antibonding p -orbital (p * ) p * overlap Energy p 3 Higher energy p * In benzene there are 6 x overlapping p orbitals fi 6 p MO’s 3 Lower energy p The exact calculation of their position (shown below) is beyond our discussion. p * overlap Energy 6 isolated p orbitals p Each MO can accommodate 2 electrons, so for benzene we see all electrons are paired and occupy low energy MO’s (bonding MO’s). All bonding MO’s are filled. Benzene is therefore said to have a closed bonding shell of delocalised p electrons and this accounts in part for the stability of benzene. There is a simple “trick” for working out the orbital energies (625): Frost-Musulin diagrams - polygon in a circle. Draw the molecular framework of a cyclic system of overlapping p-orbitals, making sure you put an atom at the bottom. Atomic positions (positions of p-orbitals) then map on to the energy level diagram! feed in electrons! This leads to the very important Hückel’s Rule: The (4n + 2) pp Electron Rule For monocyclic planar compounds in which each atom has a p orbital (as in benzene) Hückel showed that compounds with (4n + 2) p electrons, where n = 0, 1, 2, 3 etc, would have closed shells of delocalised p electrons and should show exceptional stability (high resonance energy ” “aromatic”). i.e. planar monocycles with 2, 6, 10, 14….delocalised p electrons should be “aromatic”. i.e. p electrons are delocalised over the entire ring and the compound is thereby stabilised by the delocalisation. Compounds with 4n electrons Consider planar cyclooctatetraene (COT) (8 p electrons). Firstly construct the ‘polygon in a circle’. feed in electrons! No closed shell and 2 unpaired electrons in each of 2 non-bonding orbitals! Molecules with unpaired electrons are typically unstable and reactive. Therefore a planar form of COT should not be aromatic. Because no stability is gained by becoming planar it assumes a tub shape. COT is non-aromatic and in fact stability would be lost if it became planar. Monocyclic compounds with alternating single and double bonds are termed Annulenes. (CH=CH) n Thus: benzene is [6] annulene and COT is [8] annulene. Remember Hückel’s rule predicts that annulenes will be aromatic if i) they have (4n + 2) p electrons ii) they have a planar C skeleton A study of annulenes has verified Hückel’s rule. Consider [14] annulene and [16] annulene H H H H H H H H H H H H H H (4n)electrons (4n+2)electrons NOT aromatic n = 3 aromatic What about [10] annulene? - predict it would be a stable aromatic compound. However, H’s interfere preventing planarity therefore it is not aromatic. H H (Note: naphthalene. Not really a test of Hückel’s rule since it is bicyclic but we can regard it as a similar case if we look at periphery!) What about [4] annulene (cyclobutadiene)? (Draw polygon in a circle for yourself) It was eventually made in 1965 but has a very short lifetime. It is highly unstable – more unstable than it is “Anti-aromatic”. The definitions:- 1. If, on ring closure, the p electron energy of an open chain polyene (alternating single and double bonds) decreases the molecule is classified as aromatic. e.g. 2. If, on ring closure, the p electron energy increases, the molecule is classified as antiaromatic. 3. If, on ring closure, the p electron energy remains the same the molecule is classified as non-aromatic e.g. COT (just a polyene). Evidence for electron delocalisation in aromatic compounds. NMR as a test for aromaticity. (p 627) Key evidence for electron delocalisation is provided by NMR. Fact: H H H Has a single unsplit signal for H at d 7.27 ppm. This tells us that all H are equivalent. H H H Importantly the signal appears at a low field strength – so the nuclei are deshielded compared to normal alkene protons. How is this explained/understood in terms of electron delocalisation? deshielded shielded 5.7-4.6 7.27 0 (TMS) normal range for H Induced magnetic field tries to ‘oppose’ (‘neutralise’) applied filed B . But (since 0 magnetic lines of force are continuous) at the position of the protons of benzene the applied field is reinforced by the field produced by the circulation of p electrons. Circulation of p -electrons give rise to ring current opposing field H H B o This causes the H nuclei to be strongly deshielded – the protons sense the sum of the two fields and therefore the applied field B does not have to be as high (strong). 0 Thus delocalised pp electrons cause peripheral protons to absorb at very low magnetic field strengths. Used as a criterion for Aromaticity Consider [18] annulene (4n + 2 electrons with n = 4) H H H H H H H H H H H H H H H H H H 12 outer protons d 9.3 6 inner protons d -3.0 ppm X-ray structure of [18] annulene shows that it is very nearly planar – no bond alternation (double / single) supports delocalisation. Possible definition of Aromatic Compounds “Cyclic systems which exhibit diamagnetic ring current and in which all of the ring atoms are involved in a single conjugated system.” Aromatic Ions Cyclopentadiene is unusually acidic (pKa 16) " " or H H caynciolonpentadienyl Base total 6e therefore aromatic In contrast, pKa of cycloheptatriene is 36. Loss of HYDRIDE is unusually easy, however, because it leads to an aromatic cation – tropyllium ion. cycloheptatrienyl -H+ anion -H- H (easy) H H tropyllium ion H Benzenoid Aromatic Compounds We have seen that benzene exhibits unusual stability compared to “cyclohexatriene” structure. Br Br Br2, AlBr3 NOT Br electrophilic addition (normal for substitution (SEAr) alkenes) Also H2 H2 BUT Pt Pt D H= -119kJ/mol D H= -207kJ/mol (Expect 3x-119=357kJ/mol) Difference (357 – 207 = 150 kJ/mol) is called the “Resonance Energy” of benzene. Benzenoid Compounds (fused benzene rings) have similar “aromatic” properties to benzene e.g. naphthalene anthracene RE 350kJ/mol RE 254kJ/mol An interesting non-benzenoid aromatic compound is Azulene, which has large resonance energy and a large dipole moment. etc RE 204kJ/mol azulene Heterocyclic Aromatic Compounds So far we have only considered carbon skeleton compounds. However, many compounds we find in nature are cyclic compounds with an element other than carbon in the ring. These are called Heterocyclic compounds. Further, some are aromatic compounds - can be termed heteroaromatic. N N O S H pyridine pyrrole furan thiophene However, the degree of aromaticity (extra stability) may vary as the heteroatom changes. In electronic terms pyridine is related to benzene. 1 electron contributed to p -system by N N lone pair in sp2 orbital -leads to basic properties of pyridine sp2 hybridised N Pyrrole has electrons arranged differently – related to the cyclopentadienyl anion. 2 electrons contributed to p -system by N. Not available for protonation. N H sp2 hybridised N (Similar electronic configurations for furan and thiophene) The Diels Alder Reaction as an indicator of aromaticity In the Diels Alder reaction a double bond adds to a 1,3 conjugated diene (4+2 cycloaddition) to give a 6-membered ring. Favoured by electron withdrawing groups on the dienophile and electron donating groups on the diene e.g. O O O O O O maleic anhydride An indication of the stability of benzene over that indicated by is that it does not undergo a Diels Alder reaction, despite the fact that we can ‘locate’ a diene fragment in its structure. O O NO REACTION O