Lecture 22 10

1. Lecture 22 – 10/18/2010Aromatic Chemistry Aromaticity and the Hückel 4n+2 rule Aromatic heterocycles and PAH’s Review of Electrophilic Aromatic Substitution Nucleophilic Aromatic Substitution Benzynes Oxidation reactions; benzylic halogenation Reduction reactions Arenediazonium salts

2. Benezenoid compounds are more stable and less reactive than alkenes and polyenes. The reactivity of benzenoid compounds is significantly different than alkenes and polyenes. Aromatic compounds react by substitution reactions rather than addition reactions. There are a large number of common names for simple aromatic compounds that have to be learned Since these compounds are planar the stereochemistry is positional:

3. During the 19th century there was confusion about the nature of benzene and there were several proposals for the structure of benzene to account for its properties. The Kekule proposal was closest to being correct. We now know that the bonding electrons in benzene and other aromatic compounds are delocalized. This delocalization results in stabilization that we call the resonance energy.

4. The value of the resonance stabilization has been determined a number of ways. One way was by carefully measuring the heats of hydrogenation (a calorimetric method) From these calculations the resonance energy of benzene is calculated to be 33 kcal/mol)

5. Early chemists struggled to understand the nature of this aromatic stabilization. Many polycyclic aromatic hydrocarbons (PAH) are known A number of aromatic heterocyclic compounds are also known.

6. It was thought that having alternating double and single bonds in a ring was sufficient to produce stabilization so a search was started to make other cyclic compounds with alternating single and double bonds to learn if they would be aromatic. Cyclooctatetraene (COT) was first made by Willstätter in 1911. They only were able to obtain a very small amount and it was not enough to test properly; however, it appeared to be more like an alkene. The Willstätter synthesis was repeated by Cope in the 1950’s and COT was shown to be non-planar – tub shaped. COT was made in Germany on an industrial scale during WWII by a process developed by Reppe to make synthetic gasoline from coal.

7. Cyclobutadiene was finally made in 1965. It was shown to be so highly reactive that it spontaneously dimerized (Diels-Alder reaction) at -78 °C. Cyclodecapentaene has not been made, but a bridged analog was made by Vogel in the 1960’s and it behaves like an aromatic compound.

8. Chemists also had difficulty understanding the remarkable acidity of cyclopentadiene and the unusual stability of the tropylium cation. It was the pioneering work of Hückel that provided the theoretical basis for understanding aromatic chemistry. Hückel’s 4n+2 Rule: a molecule can be aromatic only if it is planar and contains 4n+2 p-electrons (n is an integer). Systems with 4n p electrons cannot be aromatic even if they are planar and apparently conjugated.

9. The 4n+2 rule rationalized much chemistry: it showed why five-membered and six-membered ring hetereocyclic compounds were aromatic it explained the acidity of cyclopentadiene, and the stability of the tropylium ion: all these compounds were planar and had six p-electrons (n = 1). It explained the high reactivity of cyclobutadiene and the reactivity of COT (four p-electrons). It also initiated a wave of research activity to learn if other compounds would be aromatic if n ? 1, i.e. n = 0, 2, 3, 4……(2, 10, 14, 18 p-electrons).

10. Hückel’s system arranged the energy levels of aromatic orbitals in degenerate pairs except for the lowest energy level (and the highest energy level for even membered rings).

11. The energy levels for benzene, cyclopentadienide, and tropylium (4n+2 systems) are shown: The diagrams show why cyclobutadiene and the cyclopentadienyl cation (4n systems) are so unstable: they are like di-radicals

12. The Hückel rule explains why cyclopropenone is more stable than cyclopropanone even though it would appear to be more strained. It also explains why potassium metal dissolves in the presence of cyclooctatetraene to produce a stable red-colored solution – the red color is from the COT dianion.

13. Frost Circles – How not to memorize Rather than memorize the various energy levels of cyclic conjugated systems that may or may not be aromatic, a useful mnemonic exists called a Frost circle. Simply inscribe the cyclic system inside of a circle with one of the vertices pointing down. At each point a vertices touches the perimeter of the circle will be an energy level

14. Huckel MOs for Benzene

15. Heteroaromatics Heteroatom containing cyclic molecules that display aromatic reactivity. Follow a hybridization approach to understand reactivity. Consider furan:

16. Pyrrole & Pyridine

17. N-heteroaromatics: Purines

18. N-heteroaromatics: Pyrimidines