Aromatic Compounds Lecture 1 PDF

Lecture (1) AROMATIC COMPOUNDS BY / FIBY N. TAKLA LECTURER OF PHARMACEUTICAL ORGANIC CHEMISTRY THE PHENOMENON O F A RO M AT I C I T Y ❑ Benzene is not Benzine (petroleum ether) nor Benzyne nor Benzin. ❑ The term "aromatic" (Greek: aroma = fragr...

Lecture (1) AROMATIC COMPOUNDS BY / FIBY N. TAKLA LECTURER OF PHARMACEUTICAL ORGANIC CHEMISTRY THE PHENOMENON O F A RO M AT I C I T Y ❑ Benzene is not Benzine (petroleum ether) nor Benzyne nor Benzin. ❑ The term "aromatic" (Greek: aroma = fragrant smell). Now the term "aromaticity" denotes specific chemical and physical properties and does not refer to the odor of the compounds. I M P O R TA N C E O F A R O M A T I C C O M P O U N D S THE KEKULÉ STRUCTURE OF BENZENE ❑ Kekulé (1865) was the first to suggest a cyclic structure for benzene with the carbon atoms bonded to each other by alternating single and double bonds and only one hydrogen atom is attached to each carbon. ❑All bonds will be identical (1.39 Ao) (neither single nor double) STRUCTURE OF BENZENE RING  REQUIREMENTS FOR AROMATICITY 1. Molecule must be cyclic 3. A molecule must be completely conjugated. 2. Molecule must be planar (Sp2 hybridization) To be aromatic, each p orbital must overlap with p orbitals on adjacent atoms.  REQUIREMENTS FOR AROMATICITY 4) A molecule must satisfy Hückel’s rule and contain a particular number of  electrons. Hückel's rule: Considering aromaticity, a compound can be classified in one of three ways Aromatic Antiaromatic Nonaromatic Compound that lacks one (or more) of the following requirements for aromaticity: being cyclic, planar, Cyclic Cyclic and completely conjugated. Planar Planar Completely Completely conjugated conjugated 4n+2  4n  electrons. electrons. Examples  cyclic Aromatic, Antiaromatic or Non- Planar aromatic compounds conjugated Huckle’s  cyclic cyclic cyclic Planar Planar Planar conjugated conjugated conjugated Huckle’s Huckle’s Huckle’s X 4n+2=10 4n+2=14 n=2 n=3 Aromatic Antiaromatic Aromatic Nonaromatic cyclic cyclic cyclic cyclic Planar Planar Planar Planar X conjugated conjugated conjugated conjugated X Huckle’s Huckle’s Huckle’s Huckle’s X X 4n+2=2 4n=4 4n+2=6 4n=4 n=0 n=1 n=1 n=1 cycloheptatrienyl Anthracene(Aromatic) (Aromatic) cation (Aromatic) cyclic cyclic cyclic Planar Planar Planar conjugated conjugated conjugated Huckle’s Huckle’s Huckle’s 4n+2=6 4n+2=14 4n+2=6 n=1 n=3 n=1 ANNULENES( CONJUGATED CYCLOPOLYALKENES) The and annulenes, cyclic on the other hand, are anti aromatic. Why?? Planar conjugated Huckle’s anulene annulene (aromatic) (aromatic) 4n+2=14,18 n=3, 4  Exception: and annulene are not aromatic because its rings are not planar Angle strain Tub conformation HETEROCYCLIC  NOMENCLATURE The most common monosubstituted benzene parent compounds More examples of monosubstituted benzene compounds: 4-phenyl heptane Nomenclature of disubstituted benzenes Nomenclature of polysubstituted benzenes 1. Identify and name the parent. 2. Identify and name the substituents. 3. Assign a locant to each substituent. 4. Arrange the substituents alphabetically Tert butyl R Make sure to alphabetize the substituents. In the example above, bromo is listed before chloro. Isobutyl 2-Isopropyl benzaldhyde