Benzene Ring and Aromaticity

Short Seminar on Basic Description of Benzene Ring and Aromaticity

1. TOPIC: Benzene Ring and Aromaticity 3/14/2018 Slide no:1 Presented by- Chiranjib Acharjee Roll-1299 B.Sc. 2 nd Semester Chemistry(M) Bholanath College, Dhubri

2. 3/14/2018 SLIDE NO: 2 Statistics Structure Resonance in benzene Bonding Nomenclature Reactions Industry uses and preparation. CONTENTS :

3. 3/14/2018 SLIDE NO: 3 Discovered by Michael Faraday in 1825 Molecular formula:C6H6 Molar mass: 78.11 g/mol Most Basic Aromatic Molecule Bond length: C-C 139 pm and C-H 109pm Colorless Liquid Boiling Point: 8o.08 o C Melting Point: 5.48 o C Density: 0.8765g/cm Insoluble in Water Soluble in Oils & Fats & organic compounds Explosive Vapours Flammable Liquid STATISTICS ON BENZENE:

4. SLIDE NO 4 These structures given below are known as Kekul é structures . Although benzene is still drawn as a six- membered ring with alternating ᴨ bonds, in reality there is no equilibrium between the two different kinds of benzene molecules. Current descriptions of benzene are based on resonance and electron delocalization due to orbital overlap. In the nineteenth century, many other compounds having properties similar to those of benzene were isolated from natural sources. Since these compounds possessed strong and characteristic odors, they were called aromatic compounds. It should be noted, however, that it is their chemical properties, and not their odor, that make them special . Benzene has a cyclic,planar,hexagonal structure. STRUCTURE:

5. SLIDE NO 5 The resonance description of benzene consists of two equivalent Lewis structures, each with three double bonds that alternate with three single bonds. The true structure of benzene is a resonance hybrid of the two Lewis structures, with the dashed lines of the hybrid indicating the position of the  bonds. We will use one of the two Lewis structures and not the hybrid in drawing benzene. This will make it easier to keep track of the electron pairs in the  bonds (the  electrons). RESONANCE IN BENZENE:

6. 3/14/2018 SLIDE NO 6 BONDING IN BENZENE: As per the Molecular Orbital Theory(MOT), all the six carbon atoms in benzene ring are sp 2 hybridized. The sp 2 hybrid orbitals overlap with each other and with s orbitals of the 6 hydrogen atoms to form c-c and c-h sigma bonds All the sigma bonds in benzene lie in one plane and all the bond angles are 120 ◦ . Each carbon atom in benzene possesses an unhybridized p-orbital containing one electron .The p- orbitals are perpendicular the plane of sigma bond. The lateral overlap of these p- orbitals produces a ᴨ molecular orbital containing 6 electrons. One half is above and other half lies below the plane of sigma-bonds.

7. 3/14/2018 SLIDE NO 7 BONDING IN BENZENE CONTINUED: The six electrons of the p orbitals cover all the six carbon atoms, and are said to be delocalized. As a result of which a strong ᴨ bond is formed and the benzene molecule is stabilized.

8. SLIDE NO 8 Four structural criteria must be satisfied for a compound to be aromatic. THE CRITERIA FOR AROMATICITY— H ü ckel’s Rule: [1] A molecule must be cyclic. To be aromatic, each p orbital must overlap with p orbitals on adjacent atoms.

9. SLIDE NO 9 [2] A molecule must be planar. All adjacent p orbitals must be aligned so that the  electron density can be delocalized. . [3] A molecule must be completely conjugated Aromatic compounds must have a p orbital on every atom

10. SLIDE NO 10 [4] A molecule must satisfy H ü ckel’s rule , and contain a particular number of  electrons. Benzene is aromatic and especially stable because it contains 6  electrons. Cyclobutadiene is antiaromatic and especially unstable because it contains 4  electrons. Hückel's rule:

11. 3/14/2018 SLIDE NO 11 NOMENCLATURE : Monosubstituted Benzene:These are named by prefixing the name of the substitutent to the word benzene. For eg , benzene in which one of the hydrogen is replaced with chlorine is called chlorobenzene . Disubstituted benzene:When there are two substituents on the ring, three positional isomers are possible. The relative positions are indicated by prefixes ortho (o-) meta(m-) and para (p-) At position 2 3 4 respectively.

12. 3/14/2018 SLIDE NO 12 NOMENCLATURE CONTINUED: Polysubstituted benzene:When three or more substitutents are present on the benzene ring, numbers must be used to designate their positions. One of the substitutent is put on the top of the hexagon and numbered 1. The numbering of the remaining positions is done clockwise or anticlockwise, which gives the lowest possible group of numbers to the other substitutents .

13. 3/14/2018 SLIDE NO 13 Nitration: Hydrogen in the benzene ring is replaced by nitrogen (NO2- ) to give nitro-benzene Halogenation : : Hydrogen Replaced by Halogen(X- ) to give halobenzenes . Sulphonation : : Hydrogen Replaced by (SO3H-) Friedel -craft alkylation: Alkyl group (R-) Attaches to ring to make a alkyl-benzenes Friedel -craft acylation : : Acyl group (RCO-) Attaches to Ring to Make a Ketone which on Clemention reduction gives n-alkyl benzene. REACTIONS OF BENZENE:

14. 3/14/2018 SLIDE NO 14 Made from Coal Tar (a liquid left over when coal is heated) and Petroleum Used as a Source for Almost all Aromatic Compounds Make Complicated Aromatic Compounds by Taking Benzene and Add on Other Rings Piece by Piece Solvent for: Sulfur, Phosphorus and Iodine,Gums,Fats , Waxes and Resins,Simple Organic Compounds One of the Most Commonly Used Solvents in Organic Chemistry INDUSTRY USES AND PREPARATION:

15. 3/14/2018 SLIDE NO: 15 Aniline: Dye (Aniline Oil) Plastics Drugs (Sulfanilamide) Explosives Phenol: Disinfectant Antiseptic (Diluted) Insecticide Explosives Detergent Raw Material for Aspirin INDUSTRY USES AND PREPARATION:

16. 3/14/2018 SLIDENO: 16 THANK YOU “The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge.” --STEPHEN HAWKING