Aromatic Substitution Chemistry (Part of Chapter 2 and Chapter 11)

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1 Aromatic Substitution hemistry (Part of hapter 2 and hapter 11) Aliphatic ompounds: Aromatic ompounds: pentane 2-pentene 2-pentyne Aromatic compounds have closed loops of electrons that give rise to an aromatic ring current. Aromatic compounds are the arch examples of delocalized bonding. (March 4 th edition) Resonance or Kekule structures Resonance ybrid All aromatics are planar, conjugated, cyclic, unsaturated molecules. (see ückel Rule) The structure of benzene is not represented as two distinct structures as shown above, but is really thought of as a hybrid of the two forms. This form is the resonance hybrid. In benzene this means that the structure is not alternating double and single bonds but rather a bond order of 1.5 is determined. There are other possible, higher energy, zwitterionic forms, that do not significantly contribute to the resonance hybrid but may be taken into account if there are specific stabilizing groups on the aromatic ring. The basis set of p orbitals of the sp 2 hybridized carbons is shown below. So are some representative bond lengths that are pertinent.

2 an sp 2 hybridized carbon 1.53 Å 1.32 Å The basis set of p-orbitals Å The arrows shown in the resonance structures represent the electron movements. There are a number of arrows that are used in rganic hemistry, these are shown below. Please use the correct arrows for each specific use. Types of Arrows in use in rganic hemistry Resonance arrow, separates two resonance structures, do not confuse with equilibrium. In resonance structures only electron movement occurs, no atomic positions change. quilibrium arrow, separates sides of a chemical equilibium. Atomic positions change during this chemical reaction. ne way reaction arrow. Reaction is not reversible. Atomic positions change. Retrosynthetic arrow. This arrow is used during retrosynthetic analysis. This is beyond the scope of this course. Do not use this arrow as a reaction arrow. Two electron movement arrow. This arrow moves two electrons from the location of the tail to the location of the head. ne electron movement arrow. This arrow moves one electron from the location of hte tail to the location of the head. Arenes Polycyclic aromatic hydrocarbons that are derivatives of benzene with additional fused rings are called arenes. benzene naphthalene

3 anthracene pyrene The resonance energies of fused systems increase as the number of principal canonical forms increases. A couple of other arenes benzo[a]pyrene - a carcinogen a helicene Note that the helicenes are not exactly planar but must be twisted somewhat, although orbital overlap is decreased, it is still present. Recent research shows that large arrays of fused benzene rings can be bent significantly and still retain aromatic properties. ne important aromatic property that has impact in spectroscopy is the ring current. lectrons circulate if the ring is placed in a magnetic field in such a way as to create an opposing magnetic field. This causes shielding and deshielding regions around the molecule which create changes in the location of signals in NMR sepectra. The figure on the next page shows the behaviour of electrons in a strong applied magnetic field (B 0 ). Notice that while the magnetic field opposes the field in the centre of the electrons, on the periphery of the benzene the generated field augments the applied field. Thus protons on the outside of the ring observe a higher magnetic field that the applied field. This has implications in where the protons are observed in the NMR spectrum (they appear downfield in the region around 7.2 ppm). Aromatic protons are highly characteristic in the NMR and are easily observed in most cases.

4 Ring current B 0 Lines of magentic force shielding region deshielding region a strong applied magnetic field Stability Aromatic compounds have resonance stabilization in excess of that expected for closely related structures with localized π electrons. cyclohhexene kj -120 kj cyclohexadiene kj -240 kj benzene kj -360 kj In cyclohexaediene the enthalpy of the reaction is reduced slightly from the theoretical number due to the need to pay some energy in to account for a small amount of additional stability due to resonance interactions. In benzene this difference is very large due to the need to pay 152 kj in energy to break the additional stabilization due to aromaticity, thus reducing the energy given off in the reaction. Further below we discuss this matter further in some detail. Aromatic lectrophilic Substitution Most substitutions in aromatic systems involve the attack of an electron rich arene ring system on to an electron poor electrophyle. This is in contrast to most substitutions in aliphatic chemistry in which the electron rich nucleophile attacks the electron deficient carbon. The attacking electrophile is a positive ion (or positive end of a dipole or induced dipole). After the reaction the leaving group must depart without its electrons. The most common departing group is the proton, +. The mechanism is known as the arenium ion mechanism, and is characterized by the initial formation of the intermediate arenium ion, also known as the Wheland Intermediate. In the second step the leaving group departs. There is some resemblance of

5 this mechanism to the attack of nucleophiles on the carbonyls of esters or amides to give tetrahedral intermediates, except that the charges are reversed. The electrophile can be generated in various ways, examples are shown below. Generation of lectrophiles ( + ) l 2 + All 3 l l All l 3 Lewis active chloronium acid electrophile ion N S 4 N + N N 2 nitronium ion R 3 l + All 3 R 3 l All 3 R 3 carbonium ion R + All 3 l R l All 3 R + All 4 acyllium ion The Arenium Ion Mechanism B - + one canonical form of the arenium ion In the first step the pair of electrons from the double bond attacks the electrophile, breaking the aromatic stabilization of the benzene ring. The arenium ion is a high energy intermediate. The arenium ion is stabilized by resonance, as shown in a separate figure below. In the second step, some base present in the system (usually the solvent, but it can be other species with the ability to accept a proton) removes the proton to regenerate the aromatic stabilization.

6 We often simply show the proton falling off, but to be completely correct it must be recognized that the proton is never free in solution, it is always interacting with something. Usually we will indicate the loss or gain of atoms, ions or molecules over the arrow in the step to maintain the atom balance of the reaction. Three canonical forms (resonance structures) of the arenium ion exist and the ion is known as the Wheland Intermediate. The positive charge is stabilized by the resonance interaction with the two remaining double bonds in the ring. Wheland Intermediate or arenium ion ückel Rule onjugated monocyclic planer polyenes with 4n+2 π e - are aromatic. onversely, such compounds with 4n π e - are antiaromatic. Some xamples of Aromatic ompounds 4n+2 π electrons n = 1 n = 1 n = 1 n = 0 Some xamples of Antiaromatic ompounds 4n π electrons n = 1 n = 1 n = 2 yclooctatetrane is destabilized due to antiaromaticity if it is in a planar configuration, therefore it twists out of planarity in order to minimize the overall orbital energy.

7 rbital energies of cyclic planar conjugated systems can be estimated using Frost s ircle. Two examples are given below which compare both benzene and cyclobutadiene with their isolated double bond counterparts. ompound Frost's ircle rbital nergies rbital nergy Sketch Total lectron nergy Three isolated double bonds not applicable α β α + β 6α + 6β Bezene α 2β α β α 6α + 8β α + β α + 2β ompound Frost's ircle rbital nergies rbital nergy Sketch Total lectron nergy Two isolated double bonds not applicable α β α + β 4α + 4β ylcobutadiene α 2β α 4α + 2β α + 2β

8 Stabilization and Destabilization in Aromatic and Antiaromatic Systems It is observed that benzene has 2β of additional stabilization energy over that of three isolated double bonds. This is the aromatic stabilization. In the case of cyclobutadiene, we see that there is 2β of stabilization lost over the case of two isolated double bonds. This is the destabilization observed in antiaromatic systems.