Lecture materials for the Introductory Chemistry course for Forensic Scientists, University of Lincoln, UK. See http://forensicchemistry.lincoln.ac.uk/ for more details.

1. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Chemical Bonding 3 POLAR BONDS University of Lincoln presentation

3. Homonuclear & Heteronuclear bonds This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Homonuclear bonds Hetronuclear bonds Ethane (C 2 H 6 ) Hydrazine (N 2 H 4 ) Hydrogen peroxide (H 2 O 2 )

5. Bond Energies This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Bond Dissociation Energy (kJmol -1 ) 298 294 151 436 I H 366* 315 193 436 Br H 432* 339 242 436 Cl H 570* 298 159 436 F H Exptl X–Y ½ (X–X + Y–Y) Y–Y X–X Y X

6. Anomalous Bond Energies This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License 4 298 294 H–I 51 366 315 H–Br 93 432 339 H–Cl 272 570 298 H–F  E Measured Bond Energy (kJmol -1 ) Expected Bond Energy (kJmol -1 ) Molecule

7. Why are some heteronuclear bonds much stronger than expected? ? This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

10. Why does this happen? This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

12. Electronegativity This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License The higher the electronegativity, the stronger the ‘pulling’ power of the atom within a bond O 3.4 F 4.0 N 3.0 C 2.6 Cl 3.2 H 2.2 Li 1.0 Na 0.9 K 0.8 Rb 0.8 Cs 0.8 Mg 1.3 Be 1.6 Ca 1.0 Sr 0.9 Ba 0.9 S 2.6 P 2.2 B 2.0 Si 1.9 Al(III) 1.6 Se 2.6 Br 3.0 As(III) 2.2 Ge(IV) 2.0 I 2.7 Te 2.1 Sb 2.1 Ga(III) 1.8 Sn(IV) 2.0 In(III) 1.8 At 2.2 Po 2.0 Bi 2.0 Pb(IV) 2.3 Tl(III) 2.0

13. … When electrons are held tightly by an atom in a bond, due to the high electronegativity of that atom, the bond is much harder to break This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License So, why are some heteronuclear bonds much stronger than expected?

14. Examples of Polar Bonds This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License  –  +  +  +  -  +  - The slight charges on each end of the molecule lead to electrostatic attraction between adjacent molecules – HYDROGEN BONDING

16. HYDROGEN BONDING ( ) This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License H – F H – F H – F H – F H – F

17. Hydrogen bonding affects the physical properties of molecules with polar bonds This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License NH 3 , H 2 O and HF all have anomalously HIGH boiling points, since extra energy is needed to break the hydrogen bonds

18. Can Molecular Orbital Theory account for polar bonds? ? This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

19. A quick recap… This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License ATOMIC Orbitals MOLECULAR Orbitals H + H H 2

22. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

23. Consider the MO diagram of F 2 This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Energy 2p 2p σ * (2 p Z ) σ (2 p Z ) π * (2p y ) π * (2p x ) π (2p y ) π (2p x ) 2s 2s σ * (2 s ) σ (2 s ) F F

24. Heteronuclear Diatomic molecule MO This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Homonuclear MO diagrams are symmetrical . Heteronuclear MOs are asymmetrical – the energies of equivalent atomic orbitals are DIFFERENT Energy 2s 2s σ * (2s) σ * (2s) X Y

25. LiH molecule This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Energy 2s 2s σ * (2s) σ * (2s) Li H Only valence orbitals shown. The 1s (H) and 2s (Li) overlap to form the  and  * molecular orbitals

26. HF This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License The 2p z (F) can overlap with the 1s(H). T he orbitals that do not overlap form NON-BONDING MOs Energy 1 s 2p σ * σ 2s H F Non-bonding Non-bonding HF

27. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License The 1s orbital on the H overlaps with the 2p z on the F to form a  -bond. No overlap can occur between the 1s and the 2p x or 2p y , as these are pointing in the wrong direction 1 s 2 p z 1 s 2 p x H F H F Bonding Anti- Bonding

28. HF This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License The electrons are sat closer to the F atomic orbitals than the H atomic orbitals. Therefore it is predicted that the H–F bond would be POLAR Energy 1 s 2p σ * σ 2s H F Non-bonding Non-bonding HF H–F  +  -

29. LiF This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Energy 2 s 2p σ * σ 2s Li F Non-bonding Non-bonding LiF Li–F  +  -

30. Hence, the MO theory can predict POLAR bonds This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

31. Summary This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License