Download to read offline
More Related Content
lect 2.pptx
- 1. Organic Chemistry (II) (Code:PE 213) Instructor: Dr.Ehssan Nassef e-mail: Ehssan.Nassef@Pua.edu.org. Office Hours: Monday 1:30-3:30 Pharos University in Alexandria Faculty of Engineering. Petrochemical Department
- 2. Hydrocarbons • Hydrocarbons canbe classified to: • Aliphatic Hydrocarbons (Alkane,Alkene and alkynes) Organic I • Alyciclic Hydrocarbons. • Aromatic Hydrocarbons. Organic II
- 3.
- 4.
- 5. 5 The Term Aromatic Arenesand Aromaticity • Aromatic term is used to describe a fragrant substance. • Today the ward aromatic refers to benzene and its structural relatives. • Benzaldehyde, toluene, and benzene are all aromatic compounds. • Arenes are hydrocarbons based on the bz ring as the structural unit. H O CH3 Benzene Benzaldehyde Toluene
- 6. 6 Structure and Stabilityof Benzene • Benzene is more stable than typical alkenes. • All C-C-C bond angles are 120 degrees, all six carbon atoms are sp2 hybridized, and each carbons has a p orbital perpendicular to the plane of the six- member ring.
- 7. 7 We can seewhy benzene is stable: According to resonance theory , the more resonance forms a substance has ,the more stable it is. Benzene with two resonance forms of equal energy ,is therefore more stable and less reactive than alkene.
- 8. Structure of benzene (Thekekule Proposal) • August Kekule proposed that: 1- benzene contains a ring of carbon atoms and can be formulated as cyclohexa-1,3,5-triene. 2- Benzene is a planner structure. BUT the proposal failed in explaining : 1- why benzene is uncreative compared to alkene. 2- why does benzene give a substitution product rather than an addation product on reaction with broumine(Br2)
- 9. 9 Bonds of BenzeneRings • Each π bond overlaps equally well with both neighboring p orbitals; six π electrons are delocalized around the ring. 1.5 bonds on average
- 10. 10 Molecular Orbital Descriptionof Benzene • Planar molecule with the shape of a regular hexagon. • All C-C-C bond angles are 120 degrees • All six carbons are sp2 hybridized. • Each carbon atom has a p orbital perpendicular to the plane of the six- membered ring. • All six carbon atoms and six p orbitals in benzene are equivalent.
- 11. 11 The resonance Proposalof benzene and the orbital structure The six pi electrons are completely delocalized around the ring.
- 12. 12 Let’s review… • Benzeneis a cyclic conjugated molecule • It is unusually stable, with a heat of hydrogenation of - 206 kJ/mol • It is planar with the shape of a regular hexagon; bond angles 120 • It undergoes substitution reactions that retain the cyclic conjugation • It is a resonance hybrid • Generally less dense than water • Insoluble in water but soluble in many organic solvents
- 13. 13 The Huckel 4n+ 2 Rule To be aromatic, the molecule must also follow the Huckel 4n + 2 rule: A molecule must have 4n + 2 pi electrons where n is an integer (0, 1, 2, 3, etc…). The number of pi electrons must be odd.
- 14.
- 15. 15 1. Destructive Distillationof Coal • Gives coal gas, ammoniacal liquor, coal tar and coke as products • The coal tar produced is a mixture of many organic compounds (mainly aromatic ones) • benzene and methylbenzene can be obtained by fractional distillation
- 16. New Way Chemistryfor Hong Kong A-Level 3A 16 1. Destructive Distillation of Coal A laboratory set-up of the destructive distillation of coal
- 17. 17 Industrial Preparation 2. Catalytictrimerization of ethyne HC CH 3 organonickel catalyst 70o C, under pressure
- 18. 18 Industrial Preparation 3. CatalyticReforming of Petroleum • Converts alkanes and cycloalkanes into aromatic hydrocarbons C6H14 C6H6 + 4H2 500 oC, 10 – 20 atm Pt
- 19. 19 Laboratory Synthesis 1. Decarboxylationof Sodium Salt of Benzoic Acid • When sodium benzoate is fused with sodium hydroxide the carboxylate group is removed
- 20. 20 2. Reduction ofPhenol • Passing phenol vapour over heated zinc dust produce benzene and zinc(II) oxide Benzene is separated by fractional distillation
Editor's Notes
- #5 ,mkk