Lecture 1: Aromatic Organic Chemistry PDF

‫للا َويُعَ ِّلّ ُم ُك ُم‬‫َّ‬ ‫ْ‬ ‫ا‬‫و‬‫ُ‬ ‫ق‬‫َّ‬ ‫( َوات‬ ‫ش ْي ٍء‬ ‫للاُ ِّب ُك ِّ ّل َ‬‫للاُ َو ّ‬ ‫ّ‬ ‫ع ِّليم)‬ ‫َ‬ ‫[البقرة‪]282:‬‬ ‫‪1‬‬  Prepared by Dr Marwa Mahmo...

‫للا َويُعَ ِّلّ ُم ُك ُم‬‫َّ‬ ‫ْ‬ ‫ا‬‫و‬‫ُ‬ ‫ق‬‫َّ‬ ‫( َوات‬ ‫ش ْي ٍء‬ ‫للاُ ِّب ُك ِّ ّل َ‬‫للاُ َو ّ‬ ‫ّ‬ ‫ع ِّليم)‬ ‫َ‬ ‫[البقرة‪]282:‬‬ ‫‪1‬‬  Prepared by Dr Marwa Mahmoud Abdel-hakeem  Organic compounds are classified into either aliphatic compounds as alkanes, alkenes, alkynes, etc... or aromatic compounds  Benzene and benzaldehyde were first isolated from naturally occurring products that have a pleasant smell 3  isolated by Michael Faraday in 1825, has molecular formula C6H6.  it was expected to be highly unsaturated.  Benzene is remarkably stable, it does not undergo addition and oxidation reactions characteristic of alkenes. 4  Benzene can be formulated as two hypothetical cyclohexatriene structures (I) and (II) which have equal bond lengths and bond angles and are in rapid equilibrium. 5  Benzene is a planar symmetrical hexagon. All C-C bonds are equivalent in length, bond length =1.39 A° (i.e. intermediate between C-C (1.54 Å), and C=C (1.33 Å). bond lengths). All bond angles (C-C-C and H-C-C) = 120°  Such a structure is possible only if all the carbon atoms have the same electron density, with π electrons delocalized over the entire skeleton of ring carbons. 6  Each C is (sp2) hybridized and uses 2 sp2 orbitals to form 2 sigma (σ) bonds with neighboring carbons and use the third orbital to form a sigma bond with 1s orbital of hydrogen and all C's are planar.  Each carbon has in addition a p orbital perpendicular to the sp orbitals and planarity of the molecule allows these orbitals to overlap sideways i.e. C1 can overlap with each of C2 and C6. 7 Overlap of all 6 p orbitals occurs leading to formation of 6 delocalized MO containing 6 π electrons. The electron density is greatest above and below the plane of the benzene ring. 8  Accordingly, the exact structure of benzene is a resonance hybrid of two possible Kekulé structures (I) and (II) which are identical except for the position of π electrons 9  The heat of hydrogenation (ΔH) of cyclohexene = - 28.6 kcal / mole.  Calcd. ΔH for benzene based on the cydohexatriene: Calcd. ΔH = 3 x -28.6 = -85.8 Kcal / mole.  The actual heat of hydrogenation (ΔH found) of benzene is - 49.8 Kcal / mole.  The difference between (ΔH calcd. & ΔH found) is: resonance energy. - Resonance energy of benzene = ΔH calculated - ΔH found = - 85.8 - (-49.8) = -36 Kcal / mole. 10  The high resonance energy of benzene means that more energy is required for reactions in which aromaticity is lost (e.g. addition reactions) as alkenes are hydrogenated at room temperature while benzene requires high temperature and pressure.  Explain??? 11 Requirements of aromaticity 1- Cyclic. 2- Planar. 3- Fully conjugated, i.e. all ring carbons are sp2 hybridized. 4- It should have 4n+2 π electrons (Huckel's rule) 12  Huckel's rule aromatic compound must have an odd number of pairs of electrons, which can mathematically be written as 4n+2 (n = 0,1,2,3 etc). 13 compounds Cyclic Planar Fully Huckel's congugate rule d Aromatic √ √ √ √ Antiaromatic √ √ √ x Non x x x ---- aromatic One of these not found 14  Examples  Benzene (C6H6) Cyclic, conjugated, planar, It has 6π electrons (4n+2). It is stable” aromatic” system. 15  Cyclopropene (C3H4)  Cyclobutadiene (C4H4)  Cyclopentadiene (C5H6)  Cyclophetatiene (tropilidine, C7H8)  Cycloctatetraene (C8H8), Annulene 16  Annulene Cyclic, conjugated, but not planar due to stric hinderance of the internal hydrogen----non aromatic  Annulene  Cyclic, conjugated, planar & n=4 So ----aromatic 17  A- Monosubstituted benzenes  prefix the name of the substituent group to the word benzene.  Special names of benzene derivatives 18  B- Disubsubstituted Benzenes  When two substituents occur on a benzene ring three isomers are possible. 19  If the two groups are different, and neither is a group that gives a special name to the molecule, we simply name the two groups successively and end the word with benzene 20  If one of the two groups gives a special name to the molecule, the compound is named as a derivative of that special compound. 21  If all the groups are the same , each is given a number the sequence being the one that gives the lowest combination of numbers  if the groups are different  example: 3-bromo- 5- chloronitrobenzene. 22  If one of the groups that give a special name is present, then the compound is named as having a special group in position 1. 23