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Structures and Shapes of Hydrocarbons

Chapter 25. Structures and Shapes of Hydrocarbons. 25.1 Saturated Hydrocarbons 25.2 Unsaturated Hydrocarbons 25.3 Aromatic Hydrocarbons. 25.1 Saturated Hydrocarbons (SB p.17). sp 3 Hybridization. 25.1 Saturated Hydrocarbons (SB p.17). 25.1 Saturated Hydrocarbons (SB p.19).

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Structures and Shapes of Hydrocarbons

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  1. Chapter 25 Structures and Shapes of Hydrocarbons 25.1Saturated Hydrocarbons 25.2Unsaturated Hydrocarbons 25.3Aromatic Hydrocarbons

  2. 25.1 Saturated Hydrocarbons (SB p.17) sp3 Hybridization

  3. 25.1 Saturated Hydrocarbons (SB p.17)

  4. 25.1 Saturated Hydrocarbons (SB p.19) Methane (CH4)

  5. 25.1 Saturated Hydrocarbons (SB p.19) 109.5° sp3 hydrid orbital

  6. 25.1 Saturated Hydrocarbons (SB p.20) 109.5° 109.5° Ethane (C2H6)

  7. 25.1 Saturated Hydrocarbons (SB p.20) Electron Pair Repulsion Theory • predict the geometry of arrangement of atoms in the molecules • Procedure:1. Focus on the central atom of the molecule • 2. Consider all of the valence electron pairs of the central atom, i.e. bond pairs and lone pairs • 3. Electron pairs tend to stay as far apart as possible (electronic repulsion between lone pairs is generally greater than that between bond pairs) • 4. The shape of the molecule is referred to the positions of the atoms (the shape has minimum repulsion between the electron pairs)

  8. 25.1 Saturated Hydrocarbons (SB p.20) e.g. Methane • tetrahedral orientation • electron pairs to have the maximum separation of 109.5°

  9. 25.1 Saturated Hydrocarbons (SB p.21) (a) Check Point 25-1 (a) Draw a three-dimensional structure for propane (C3H8). Answer

  10. 25.1 Saturated Hydrocarbons (SB p.21) (b) (i) 10  bonds (ii) 13  bonds (iii) 13  bonds Check Point 25-1 (b) How many  bonds are there in a molecule of (i) propane? (ii) butane? (iii) methylpropane? Answer

  11. 25.2 Unsaturated Hydrocarbons (SB p.22) sp2 Hybridization

  12. 25.2 Unsaturated Hydrocarbons (SB p.22)

  13. 25.2 Unsaturated Hydrocarbons (SB p.23) Ethene (C2H4)

  14. 25.2 Unsaturated Hydrocarbons (SB p.23)

  15. 25.2 Unsaturated Hydrocarbons (SB p.24) Cyclohexene (C6H10)

  16. 25.2 Unsaturated Hydrocarbons (SB p.24) Example 25-1 The bond enthalpy of the carbon-carbon double bond in ethene is +612 kJ mol–1 whilst the bond enthalpy of the carbon-carbon single bond in ethane is +348 kJ mol–1. Explain briefly why the bond enthalpy of the carbon-carbon double bond is less than twice that of the carbon-carbon single bond. Solution: A carbon-carbon double bond is composed of a  bond and a  bond. The  bond is formed by the head-on overlap of orbitals. The electrons of the  bond are distributed along the internuclear axis of the two bonded atoms.  bond is a strong bond and has a high bond enthalpy. On the other hand, a  bond is formed by the side-way overlap of orbitals. As the side-way overlap of orbitals is less effective than the head-on overlap of orbitals, a  bond is weaker than a  bond. Hence, the bond enthalpy of the carbon-carbon double bond (i.e. a  bond and a  bond) is less than twice that of the carbon-carbon single bond (i.e. a  bond) Answer

  17. 25.2 Unsaturated Hydrocarbons (SB p.25) Check Point 25-2 (a) State the difference between the  bond and the  bond in the carbon-carbon double bond. Answer

  18. 25.2 Unsaturated Hydrocarbons (SB p.25) • Differences between the  bond and the  bond in the carbon-carbon double bond:

  19. 25.2 Unsaturated Hydrocarbons (SB p.25) (b) (i) 8  bonds and 1  bond (ii) 11  bonds and 1  bond (iii) 11  bonds and 1  bond Check Point 25-2 (b) How many  and  bonds are present in a molecule of (i) propene? (ii) but-1-ene? (iii) but-2-ene? Answer

  20. 25.2 Unsaturated Hydrocarbons (SB p.25) sp Hybridization

  21. 25.2 Unsaturated Hydrocarbons (SB p.25)

  22. 25.2 Unsaturated Hydrocarbons (SB p.26) Ethyne (C2H2)

  23. 25.2 Unsaturated Hydrocarbons (SB p.27) (b) (i) 6  bonds and 2  bonds (ii) 9  bonds and 2  bonds (iii) 9  bonds and 2  bonds Check Point 25-3 (a) How many  and  bonds are present in a molecule of (i) propyne? (ii) but-1-yne? (iii) but-2-yne? Answer

  24. 25.2 Unsaturated Hydrocarbons (SB p.27) (b) Carbon 1 is sp3-hybridized, whereas carbon 2 is sp-hybridized. Check Point 25-3 (b) What is the hybridization of each carbon atom in ethanenitrile? 1 2 CH3CN Answer

  25. 25.2 Unsaturated Hydrocarbons (SB p.27) (c) x: 109.5° y: 120° z: 180° Check Point 25-3 (c) State the bond angles indicated in the compound below: Answer

  26. 25.2 Unsaturated Hydrocarbons (SB p.28) Benzene (C6H6) • From X-ray crystallography, • all carbon-carbon bonds are equivalent • between length of C = C and C – C bond

  27. 25.2 Unsaturated Hydrocarbons (SB p.29) • electrons are not localized as shown above. They are delocalized and represented in a better way as follows:

  28. 25.3 Aromatic Hydrocarbons (SB p.29) Example 25-2 Classify the following compounds as saturated, unsaturated or aromatic hydrocarbons. (a) (b) Solution: (a) Unsaturated hydrocarbon (b) Aromatic hydrocarbon Answer

  29. 25.3 Aromatic Hydrocarbons (SB p.29) Example 25-2 Classify the following compounds as saturated, unsaturated or aromatic hydrocarbons. (c) (d) (e) Solution: (c) Unsaturated hydrocarbon (d) Saturated hydrocarbon (e) Saturated hydrocarbon Answer

  30. 25.3 Aromatic Hydrocarbons (SB p.30) Example 25-3 Give the approximate values of the indicated bond angles of the following compound: Answer Solution: a = 180° ; b = 109.5° ; c = 120° ; d = 120°

  31. 25.3 Aromatic Hydrocarbons (SB p.30) A carbon-carbon double bond is stronger than a carbon-carbon single bond, as the carbon-carbon double bond contains one  bond and one  bond whereas the carbon-carbon single bond contains only one  bond. Check Point 25-4 Which is a stronger bond, a carbon-carbon single bond or a carbon-carbon double bond? Explain your answer. Answer

  32. The END

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