UNIT I-PART-2.pptx
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The document discusses the structure of benzene. When benzene is cooled and x-rayed, all the bond lengths are found to be 0.139nm, making Kekule's alternating single-double bond proposal impossible. Benzene has a molecular formula of C6H6 but does not react like alkenes, undergoing substitution rather than addition reactions. Its stability comes from resonance energy arising from delocalized pi electrons within its hexagonal cyclic structure.
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1. Introduction
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3. Nomenclature
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Analytical Evidence of Benzene: Analytical evidence involves techniques used to study and confirm the structure of benzene. Key methods include:
1. Spectroscopy:
NMR (Nuclear Magnetic Resonance): Shows a characteristic proton NMR pattern with six protons in a symmetrical arrangement, supporting the idea of a regular hexagonal ring structure.
IR (Infrared Spectroscopy): Benzene shows a distinct absorption peak around 1500 cm^-1, corresponding to the C–C stretching vibrations typical of an aromatic ring.
2. Chemical Reactions:
Benzene undergoes electrophilic substitution reactions (e.g., halogenation, nitration) rather than addition reactions, which is a key feature of aromatic compounds. This behavior provides strong evidence of its stability and ring structures
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Benzene's structure was originally proposed to have alternating single and double bonds by Kekulé, but this posed several problems. Benzene is more stable than expected from the Kekulé structure and undergoes substitution reactions rather than addition reactions like ethene. The bonds in benzene are also all the same length, unlike what would be expected from alternating single and double bonds. Modern evidence shows benzene has a delocalized pi electron cloud above and below the ring that contributes to its unusual stability and reactivity.UNIT I-PART-6.pptx
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Benzene and its derivatives can undergo various reactions including nitration, sulphonation, and halogenation which introduce nitro, sulfo, and halo groups onto the benzene ring. The Friedel-Crafts alkylation reaction can add alkyl groups to benzene but has limitations. Friedel-Crafts acylation can similarly add acyl groups to benzene.UNIT I-PART-8.pptx
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UNIT I-PART-2.pptx
- 1. 1 UNIT I: Benzene and its derivatives Analytical, synthetic and other evidences in the derivation of structure of benzene
- 2. 2
- 3. 3 x-Ray Crystallography Laser Diffraction Electron Diffraction pattern Electron density map of benzene 3 4
- 4. 4 When benzene is cooled it crystallizes and x-ray diffraction can be used to measure the bond lengths, which in benzene are all found to be 0.139nm. Therefore, Kekule’s proposal of alternating single and double bonds is impossible. 5
- 5. 1 Not a Straight Chain
- 7. 3 C=C 3 3
- 8. 8 ï‚— Molecular formula of benzene is C6 H6 , it seemed clear that the molecule must be highly unsaturated ï‚— benzene does not show the chemical properties of alkenes ï‚— Benzene does undergo chemical reactions, but its characteristic reaction is substitution rather than addition. ï‚— When benzene is treated with bromine in the presence of ferric chloride as a catalyst, for example, only one compound with the molecular formula C6 H5 Br forms ï‚— Chemists concluded, therefore, that all six carbons and all six hydrogen's of benzene must be equivalent. 4
- 9. There are mainly three factors that decide the overall stability of benzene over other organic compounds. These factors are as follows: Resonance energy Aromatic stability Structural stability Resonance hybrid Resonance Resonance energy Stability 1 Presence of pi(4n+2)Ï€ electrons. Presence of delocalized electron Hexagonal in shape: less angle
- 10. 10 Stability 1
- 12. 12 Summary