1. Aromatic compounds Dr AKM Shafiqul Islam

2. hydrocarbons aliphaticaromatic alkanes alkenes alkynes Aromaticity

3.   Aliphatic compounds: open-chain compounds and ring compounds that are chemically similar to open-chain compounds. Alkanes, alkenes, alkynes, dienes, alicyclics, etc.   Aromatic compounds: unsaturated ring compounds that are far more stable than they should be and resist the addition reactions typical of unsaturated aliphatic compounds. Benzene and related compounds.

4. Aromatic Compounds  Aromatic was originally used to described some fragrant compounds in early 19th century.  Later, aromatic was used for specific chemical behavior – cyclic unsaturated compounds that undergo substitution rather than addition.  Current: aromatic compounds distinguished from aliphatic compounds by a special electronic configuration.

5. Benzene  This aromatic hydrocarbon was first discovered in 1825 but its structure was not generally agreed upon until 1946. Facts about benzene:  Formula = C 6 H 6  Isomer number: one monosubstituted isomer C 6 H 5 Y known two disubstituted isomers C 6 H 4 Y 2 known  Benzene resists addition reaction, undergoes substitution reactions.  Heats of hydrogenation and combustion are far lower than they should be.  From X-ray, all of the C—C bonds in benzene are the same length and intermediate in length between single and double bonds.

6. KMnO 4 oxidation no reaction Br 2 /CCl 4 addition no reaction HIaddition H 2 /Ni reduction no reaction Reagent Cyclohexene Benzene

7. Benzene + 3 H 2, Ni, room temp.  NR Benzene + 3 H 2, Ni, 200 o C, 1500 psi  cyclohexane Although highly unsaturated, benzene does not react like alkenes, dienes, cycloalkenes, or alkynes (addition reactions) rather it undergoes substitution reactions instead.

8. Kekule Structure of Benzene  Open chain structure not possible  Cyclic structure of benzene proposed by Kekule (1872)

9. Benzene The Lewis representation of benzene suggests that we deal with a six-membered ring of carbon atoms that are held together by alternating single and double bonds. C: 6 4 = 24 valence electrons H: 6 1 = 6 valence electrons ---------------------------------------- Total = 30 electrons = 15 bonds

10. Benzene This implies that we should observe alternating short (1.33 A) and long (1.54 A) bond lengths. Measurements indicate that all bond lengths are the same (1.39 A). Again, we need to draw resonance structures.

11. Aromatic Compounds  Aromatic compound: a hydrocarbon that contains one or more benzene-like rings –arene: a term used to describe aromatic compounds –Ar-: a symbol for an aromatic group derived by removing an -H from an arene –Kekulé structure for benzene (1872)

12.  But experiments show that the Kekulé structure is not correct. All C-C bonds are identical and benzene does not undergo addition reactions typical of double bonds.  A correct description is given by resonance theory or by orbital models – valence bond or molecular orbital.

13. Benzene  Resonance structure for benzene (1930s) –the theory of resonance, developed by Linus Pauling, provided the first adequate description of the structure of benzene –according to the theory of resonance, certain molecules and ions are best described by writing two or more Lewis structures; the real molecule or ion is a resonance hybrid of these structures –each individual Lewis structure is called a contributing structure –we show that the real molecule is a resonance hybrid of the two or more Lewis structures by using a double- headed arrow between them

14. Benzene –here are two contributing structures for benzene –the resonance hybrid has some of the characteristics of each Lewis contributing structure –the length of a carbon-carbon bond in benzene, for example, is midway between that of a carbon- carbon single bond and a double bond

15. Delocalized electrons are not confined between two adjacent bonding atoms, but actually extend over three or more atoms.

16. Hybrid Resonance Structure

17. The Criteria for Aromacity: Hückel's Rule  A molecule must be cyclic, planar, completely conjugated and contain a particular number of  electorn. That is, they are the kind of hydrocarbons treated by Hückel's rule.

18. Hückel's rule 1.A molecule must be cyclic 2.a molecule must be planer 3.a molecule must be conjugated 4.a molecule must satisfy Hückel's rule and contain a particular number of  electron

19. Aromaticity and the 4n + 2 Rule  Huckel’s rule (based on calculations) – a planar cyclic molecule with conjugated double bonds has aromatic stability if it has 4n+ 2  electrons, where n is 0,1,2,3,4 (any integer)  For n=1: 4n+2 = 6; benzene is stable and the electrons are delocalized

20. among planar, monocyclic, completely conjugated polyenes, only those with 4n + 2  electrons possess special stability (are aromatic) n 4n+2 0 2 1 6benzene! 210 314 418 Hückel's Rule

21.  Hückel restricted his analysis to planar, completely conjugated, monocyclic polyenes  he found that the p molecular orbitals of hese compounds had a distinctive pattern  one  orbital was lowest in energy, another was highest in energy, and the others were arranged in pairs between the highest and the lowest

22. Sources of Aromatic Hydrocarbons - from high temperature distillation of coal tar - heating petroleum at high temp and pressure w/ catalyst

23. The Phenyl Group  When a benzene ring is a substituent, the term phenyl is used (for C 6 H 5  ) –You may also see “Ph” or “  ” in place of “C 6 H 5 ”  “Benzyl” refers to “C 6 H 5 CH 2 ”

24. Disubstituted Benzene Names To indicate relative position on a benzene ring: –ortho- (o) on adjacent carbons (1,2) –meta- (m) separated by one carbon (1,3) –para- (p) separated by two carbons (1,4)

25. Pyridine N O S N H PyrroleFuranThiophene Heterocyclic Aromatic Compounds

26. Exercise

27. Thank you very much