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Aromatic Compounds and Aromaticity

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    Aromatic Compounds and Aromaticity

    Solomons 6thEdition

    Chapter 14 p 614 654Chapter 15 p 655 703 (Reactions)

    You will by now be familiar with the structure of benzeneC6H6

    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. Thisclassification now has a chemical meaning aromaticity is associated with a special stability resultingfrom structure.

    Elucidation of the structure posed a problem the molecular formula C6H6indicated a highly unsaturatedcompound (double and/or triple bonds) but benzene does not show this behaviour.

    Kekul (1865) conceived a cyclic structure,

    but this would imply alternating single and double bonds (C-C = 1.47, C=C = 1.34).

    Kekul suggested that two forms of benzene were in rapid equilibrium:

    Later spectroscopic evidence showed all bond lengths to be equaland intermediate between single anddouble bond lengths (1.39 ). It was also found that benzene was a flat (planar) molecule.

    We now look at benzene using two different possible approachesto try to describe its stability.

    A. VALENCE BOND APPROACH

    Resonance hybrid, 2 canonical forms

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    -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 inbetween.

    Further, all bond angles in benzene are 120 (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 actualstructure is somewhere in between them. Furthermore, the actual energy of the molecule is lower thanmight be expected for any of the contributing structures. If a molecule has equivalent resonance structuresit is much more stable than either canonical would be hence the extra stability of benzene (calledresonance energy).

    B. MOLECULAR ORBITAL REPRESENTATION OF BENZENE (MO THEORY)

    The bond angles of 120 in benzene suggests that C atoms are sp2hybridised. An alternative representationtherefore starts with a planar framework and considers overlap of the p orbitals (p electrons).

    (Simple MO rules)

    Mix n x p atomic orbitals np molecular orbitals!

    Remember ethene? (p 26)

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    The exact calculation of their position (shown below) is beyond our discussion.

    Each MO can accommodate 2 electrons, so for benzene we see all electrons are paired and occupy lowenergy MOs (bonding MOs). All bonding MOs are filled. Benzene is therefore said to have a closedbonding shellof 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 ofoverlapping 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!

    This leads to the very important

    Hckels Rule: The (4n + 2) p Electron Rule

    For monocyclic planar compoundsin which each atom has a p orbital (as in benzene) Hckel showed thatcompounds with (4n + 2) p electrons, where n = 0, 1, 2, 3 etc, would have closed shells of delocalised pelectrons 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

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    delocalisation.

    Compounds with 4n electrons

    Consider planarcyclooctatetraene (COT) (8 p electrons).

    Firstly construct the polygon in a circle.

    No closed shell and 2 unpaired electrons in each of 2 non-bonding orbitals! Molecules with unpairedelectrons are typically unstableand reactive.

    Therefore a planarform of COT should notbe aromatic.

    Because nostability is gained by becoming planar it assumes a tubshape.

    COT is non-aromaticand in fact stability would be lostif it became planar.

    Monocycliccompounds with alternating single and double bonds are termed Annulenes.

    Thus: benzene is [6] annulene and COT is [8] annulene.

    RememberHckels rule predicts that annulenes will be aromatic ifi) they have (4n + 2) p electronsii) they have a planar C skeleton

    A study of annulenes has verified Hckels rule.

    Consider [14] annulene and [16] annulene

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    What about [10] annulene? - predictit would be a stable aromaticcompound. However, Hs interferepreventing planarity therefore it is not aromatic.

    (Note: naphthalene. Not reallya test of Hckels rule since it is bicyclicbut we can regard it as a similarcase 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 unstablethan

    it is Anti-aromatic.

    The definitions:-

    If, on ring closure, the p electron energy of an open chain polyene (alternating single and doublebonds) decreasesthe molecule is classified as aromatic.

    1.

    e.g.

    If, on ring closure, the p electron energy increases, the molecule is classified as antiaromatic.2.

    If, on ring closure, the p electron energy remains the samethe molecule is classified asnon-aromatice.g. COT (just a polyene).

    3.

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    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: Has a single unsplit signal for H at d 7.27 ppm. This tells us that all H are equivalent.

    Importantly the signal appears at a low field strength so the nuclei are deshieldedcompared to normalalkene protons.

    How is this explained/understood in terms of electron delocalisation?

    Inducedmagnetic field tries to oppose (neutralise) applied filed B0. But (since magnetic lines of forceare continuous) at the position of the protons of benzene the applied field is reinforcedby the fieldproduced by the circulation of p electrons.

    This causes the H nuclei to be strongly deshielded the protons sense the sum of the two fields andtherefore the applied field B0does not have to be as high (strong).

    Thus delocalised p electrons cause peripheral protons to absorb at very low magnetic field strengths.

    Used as a criterion for Aromaticity.

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    Consider [18] annulene (4n + 2 electrons with n = 4)

    12 outer protons d 9.36 inner protons d -3.0 ppmX-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 asingle conjugated system.

    Aromatic Ions

    Cyclopentadiene is unusually acidic (pKa 16)

    In contrast, pKa of cycloheptatriene is 36. Loss of HYDRIDE is unusually easy, however, because it leadsto an aromatic cation tropyllium ion.

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    Benzenoid Aromatic Compounds

    We have seen that benzene exhibits unusual stability compared to cyclohexatriene structure.

    Also

    Difference (357 207 = 150 kJ/mol) is called the Resonance Energy of benzene.

    Benzenoid Compounds(fused benzene rings) have similar aromatic properties to benzenee.g.

    An interesting non-benzenoidaromatic compound is Azulene, which has large resonance energy and a large

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    dipole moment.

    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 thancarbon in the ring. These are called Heterocycliccompounds. Further, some are aromatic compounds - can be termed heteroaromatic.

    However, the degree of aromaticity (extra stability) may vary as the heteroatom changes.

    In electronicterms pyridineis related to benzene.

    Pyrrole has electrons arranged differently related to the cyclopentadienyl anion.

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    (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 a6-membered ring.

    Favoured by electron withdrawing groups on the dienophile and electron donating groups on the diene e.g.

    An indication of the stabilityof benzene over that indicated by is that it does notundergo a Diels

    Alder reaction, despite the fact that we can locate a diene fragment in its structure.

    Butinterestingly

    Exhibits diene behaviour note product still has 2 benzenoid rings.

    Note. Anthracene often undergoes normal SEAr reactions.

    In the heteroaromatic systems

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    Thiophene has more aromatic character than furan.

    Electrophilic aromatic substitution in heteroaromatics compared to benzene.

    We know

    And that

    Pyridine contains electron withdrawing N in place of CH\less reactive towards electrophiles (E+) - pdeficient system.

    (Protonation of pyridine further reduces reactivity.)

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    The 5-membered heteroaromatics furan, pyrroles and thiophene can be regarded as p excessive systems (6 p

    electrons over 5 atoms).

    Back to top

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