BAB 2

1. BAB 2 Orbital dan perannya pada Ikatan Kovalen

2. FROM LEWIS DIAGRAMS TO MOLECULAR SHAPE VSEPR THEORY

3. IKATAN KOVALEN . . H H . . H H H H ( Model tumpang tindih orbital) Atom yang terpisah atoms move closer Tumpang tindih orbital Pembentukan Ikatan

4. .. : H O H QUESTION Can we predict the shapes of molecules simply by combining the atomic orbitals available on each atom? ….. LET’S TRY IT FOR H2O

5. 2p 2p . . oxygen = [He]2s22p4 OXYGEN ORBITALS z y 2p22p12p1 O x .. 2s 2p Orbital saling tegak lurus (90o) [ cartoon ]

6. unpaired unpaired oxygen = [He]2s22p4 OXYGEN ORBITALS z 2p y 2p . O x .. .. 2s . 2p

7. . H . Incorrectly predicts a 90o angle. H hydrogen = 1s1 z Combining atomic orbitals to form H2O. oxygen = [He]2s22p4 2p y 2p . O x .. .. 1s 2s . 2p 1s

8. EXPERIMENTAL RESULT H O The actual H-O-H angle in water (measured by electron diffraction) is 105o H 105o This is not very good agreement with the atomic orbital model!

9. VSEPRTheory Valence Shell Electron Pair Repulsion • Based on the simple idea that groups of electrons repel each other • Predicts molecular shapes quite well

10. A better result is predicted by VSEPR theory nucleus consider the completed valence shell to be a spherical volume around the nucleus electron pairs (4 pair) repel each other try to minimize repulsions by maximizing the distance between all pairs of electrons in the final solution, they should all be equidistant valence shell TETRAHEDRAL

11. Basic Shapes of Molecules

12. 6 pair octahedral 90o d2sp3 5 pair trigonal bipyramid 120o, 90o dsp3 4 pair tetrahedral 109o28’ sp3 O R G A N I C (pyrimidal, angular ) 3 pair trigonal planar 120o sp2 2 pair linear 180o sp VALENCESHELL ELECTRON PAIR REPULSION VSEPR THEORY pairs geometry angles hybridization For most molecules, these predictions are correct to within a few degrees (± 5o).

13. Orbitals • The region of space around an atom in which an electron is likely to be found is an orbital. • The shape and size of the orbital are determined by a mathematical equation called a wave function.

14. Orbitals • When atoms combine to form molecules, they do so by combining the wave functions for the individual atomic orbitals. • We say that the orbitals “overlap.” • The region of space defined by this combination of orbitals is the molecular orbital.

15. Sigma Bonds • Head-on overlap of atomic orbitals • Electron density is a symmetrical cylinder around the bond axis Atomic orbital combinations that give s bonds:

16. Pi Bonds • Side-on overlap of atomic orbitals • Electron density is above and below a nodal plane on the internuclear axis Atomic orbital combinations that give p bonds:

17. HOW ARE THE OBSERVED BOND ANGLES ACHIEVED? HYBRIDIZATION “Vision is the art of seeing things invisible.” Jonathan Swift

18. WHY DOESN’T THE ATOMIC ORBITAL APPROACH WORK ? After bonding (overlap) we get a totally new solution for the new molecule. During bonding …. new orbitals form that are more suitable for making bonds. These orbitals are for the atom - we can’t expect that they are suitable for the molecule. 2s 2px,2py,2pz sp,sp 2py,2pz s, p, p ,n atomic orbitals hybrid atomic orbitals molecular orbitals overlap LCAO HYPOTHETICAL BONDING PROCESS NOTE. Formally LCAO theory and Molecular Orbital theory are two completely different approaches. You do not need to use hybid orbitals to derive the molecular orbitals, combinations of any type of function will do. Nevertheless, the abstraction presented above is quite useful, as we will see quite soon.

19. FORMATION OF TETRAHEDRAL HYBRID ORBITALS New orbitals point to the corners of a tetrahedron. 4 pair sp3(1) 2p FILLED VALENCE SHELL 109o28’ 2s O hybridization occurs when orbitals are full and have finished bonding sp3(3) sp3(4) sp3(2) (1) (2) (3) (4) tetrahedral geometry sp3 hybrid orbitals (cartoon)

20. FORMATION OF SP3 HYBRID ORBITALS (1) (2) (3) (4) X X sp3 hybridized atom These orbital shapes are cartoons - actual shapes are shown on the next slide. FORMATION OF SP3 HYBRID ORBITALS 2p 2pz 2s 2s 2px 2py unhybridized atom [animation]

21. SP3 HYBRID ORBITAL … and its cartoon ( cross section ) The hybrid orbital has more density in the bonding lobe than a p orbital and forms stronger bonds. sp3 The shape shown is calculated from quantum theory. To avoid confusion the back lobe is omitted from the cartoons, already shown, and the front lobe is elongated to show its direction. Courtesy of Professor George Gerhold omitted

22. ORIGIN OF THE SP3 DESIGNATION add together, divide in four hybridization 2s 2p (1) (2) (3) (4) sp3 hybrid orbitals each new orbital is 1/4 s + 3/4 p (25% s, 75% p) S1P3 = SP3 ( 1+3 ) = 4 parts total

23. sp3 hybrid orbital - + x HYBRIDIZATION ORIGIN OF THE SP3 ORBITAL SHAPE 2s orbital 2p orbital + - + x RECALL: signs are mathematical coordinates, not electronic charge [animation] HYBRIDIZATION

24. Ikatan pada • Alkana sp3 • Alkena sp2 • Alkuna sp

25. . . . . C Carbon has 4 valence electrons, 2s22p2 Carbon can form single, double or triple bonds sp, sp2 and sp3 hybrid orbitals. Let’s do sp3 first.

26. 2p sp3 hybridize 2s

28. H H H C C H H H

29. H H C C H H 2p 2p hybridize 2s sp2 Multiple Bonds and hybridization Ethylene Each carbon is hybridized sp2 . The hydrogens are 1s. One of the double bonds is sp2 - sp2. The other one is p - p.

30. C C Note that a double bond consists of a s and a p type bond

31. H H C C H H

32. C C H H 2p 2p hybridize sp 2s What about acetylene? Each carbon atom is sp hybridized. The hydrogens are unhybridized, 1s orbitals. Note that a triple bond consists of a s and 2p bonds. The two p bonds use unhybridized p orbitals.

33. H H C C

34. COMPARISON OF SPx HYBRID ORBITALS bigger “tail” more “p” character sp3 sp2 sp more “s” character more electron density in the bonding lobe

35. BOND STRENGTHS - MULTIPLE BONDS CC bond bond bond energy molecule bond type length per mole measured Kcal (KJ) C-C sp3-sp3 1.54 Å 88 (368) CH3- CH3 C=C sp2-sp2 1.34 Å 145 (607) CH2=CH2 and p - p C=C sp - sp 1.21 Å 198 (828) HC=CH = = and two p-p increasing s-character

36. Bond Energy

37. Molecular Distortions: VSEPR Revisited • Four situations: 1) electron pair repulsion 2) effect of electronegative atoms 3) double bond and electronegativity 4) steric repulsion

38. Electron Pair Repulsion H .. : angle becomes larger larger repulsion C N .. .. .. .. H H H : H : repulsion smaller H H angle becomes smaller symmetrical molecule all repulsions are equal not all pairs are equivalent perfect tetrahedral the unshared pairs repel more strongly than the bonded pairs all angles 109o28’

39. Effect of double bond and electronegativity 121.5o 122o 117o 116o 121.5o 117o 123o 124.5o 114o 111o 116o 122o 125o 126o 110o 108o

40. MOLECULES WITH PI BONDS Most pi bonds have a bond energy of 50 - 60 Kcal / mole ( 210 - 250 Kj / mole ) When the total energy of a multiple bond is given, you must subtract the energy of the pi bonds to obtain the sigma bond energy. C-C = 95 Kcal/mole C=C 145 Kcal/mole thus: ( 145 - 50 = 95 ) both bonds TOTAL BOND ENERGY

41. MOLECULES WITH UNSHARED PAIRS Non Bonded Electrons (unshared pairs) do not significantly change their energy in going from an atom to a bonded molecule

42. H H H H 2p Metanol sp3 sp3 2s hybridization sp3 hybrids C O

43. H H sp2 2p 2p 2s sp2 hybridization 2p used for pbond sp2 used for sbonds C O

44. H sp sp 2p 2p 2s hybridization 2p sp hybrids C N

45. H C H H H H H C C C H H end view sp2 sp sp2 allena molecule has a twist in the center

46. C N H H C N H H H ASSEMBLY METHOD H H : Start with the Lewis Diagram C N H H H Determine the geometry of each atom C = 4 pair = tetrahedral VSEPR N = 4 pair = tetrahedral C = sp3 N = sp3 Use the correct hybid in each case Assemble the molecule from the hybrids. ..

47. Sample Problems • Predict the hybridization, geometry, and bond angle for each atom in the following molecules: • Caution! You must start with a good Lewis structure! • NH2NH2 • CH3-CC-CHO