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Structure and shape of organic molecules

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1. Structure and shape of organic molecules Hybridization sp3 : no double bond ? no pi bond formation, all carbon in saturated hydrocarbons are sp3 hybridized sp2 : one double bond, C=C, C=O, etc. sp : two double bonds or one triple bond,C C (alkyne), C N (acid nitrile), etc.

2. Structure and shape of organic molecules Shapes of allene :

3. Structure and shape of organic molecules Allene : CH2=C=CH2 Hybridization of the carbon atoms : sp2,sp and sp2 respectively

4. Structure and shape of organic molecules

5. Drawing 3 ? D structure

6. Structure of aromatic compounds - benzene Formula vapour density = 39 molecular mass = vapour density x 2 ? molecular mass = 39 x 2 = 78 Let molecular formula of benzene be (CH)n n(12+1) = 78 ? n = 6 molecular formula of benzene = C6H6

7. Kekule?s suggestion : Evidence of Kekule? structure 1.????? All benzene derivatives will react and change to benzene (6 carbon atoms) ? 6 carbon atoms forms a suitable (and stable) core of nucleus. ? 2. X-ray diffraction of hexamethylbenzene shows a regular hexagon with CH3 groups Structure of aromatic compounds - benzene

8. 3. Chemicals evidences a.?????? Hydrogenation of benzene leads to the formation of cyclohexane. 6 univalent hydrogen atoms are added instead of 8 ? benzene is a ring structure (alicyclic compound) NOT a unsaturated straight chain (acyclic compound). C6H6 + 3H2 ? C6H12 Structure of aromatic compounds - benzene

9. BUT benzene behaves in a strange way in some other areas: b.?????? Benzene behaves as a saturated compound towards conc. H2SO4, halogen ? But alkenes have reaction with halogen and conc. H2SO4 . Structure of aromatic compounds - benzene

10. c.???? In substitution reaction: If Kekule? structure, one compound would be formed : ? and no isomeric compounds were found. ? all the hydrogen atoms are in exactly equivalent positions relative to the rest of the molecule. ? all the six hydrogen atoms are identical. Structure of aromatic compounds - benzene

11. d. Disubstitution of X2 Structure of aromatic compounds - benzene

12. Based on the about information, Kekule? hence suggested that double and single bonds in benzene ring were continually oscillating between the two arrangements. No carbon ? carbon bond could be identified as definitely single or double. This phenomenon is known as resonance. Resonance in benzene

13. Ozonolysis reaction / Ozonization Evidence of resonance in benzene

14. One mole of this structure would give one mole of : Ethanedial And two moles of Oxopropanal But in fact, butanedione is also found as one of the product, because :

15. That is, both canonical forms of 1,2-dimethylbenzene exist in resonance : Evidence of resonance in benzene

16. Bond length C-C 1.54? C=C 1.34? Carbon ? carbon bond in benzene in 1.39? and all six connecting bonds in the ring are identical in length. Therefore, the two structures are in resonance. Evidence of resonance in benzene

17. Enthalpy of formation of benzene from gaseous carbon and hydrogen atoms : Evidence of resonance in benzene

18. Experimental value is more negative than calculated value ? the real structure of benzene is more stable than the proposed structure (I.e one of the canonical form). The difference in energy between the canonical form and the real structure is called resonance energy. The resonance energy of benzene is 154 kJ/mol, which is very large ? the real structure is much stable than the proposed canonical form. This also explain the extra stability shown by benzene, e.g. no addition reaction as alkene. Evidence of resonance in benzene

19. Establishment of double bond by removing 2 H atoms This is usually an endothermic reaction, I.e. unfavourable process / the product is less stable than the reactant. The conversion of cyclohexa-1,3-diene to benzene is weakly exothermic : Evidence of resonance in benzene

20. Evidence of resonance in benzene

21. Representation of benzene Usually use one of the Kekule? formula OR a resonance hybrid of the Kekule? structure to represent the structure of benzene.

22. Delocalization of electron Orbital picture of benzene

23. Delocalization of electron Singly filled p-orbital is left on each carbon atom. These p-orbitals are all perpendicular to the plane of C ? C and C ? H sigma bonds.

24. The p-orbitals then overlap laterally with each other to form pi ? molecular orbital below and above the plane of the ring. Six electrons are delocalized in this pi ? molecular orbital. This delocalization of electron cause extra stability to the benzene ring. Delocalization of electron


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