FROM LEWIS DIAGRAMS TO MOLECULAR SHAPE VSEPR THEORY
H. H. H. H. HH IKATAN KOVALEN Tumpang tindih orbital Pembentukan Ikatan Atom yang terpisah atoms move closer ( Model tumpang tindih orbital)
QUESTION ….. LET’S TRY IT FOR H 2 O Can we predict the shapes of molecules simply by combining the atomic orbitals available on each atom? HO H :..
O y x z 2s. 2p. oxygen = [He]2s 2 2p 4 OXYGENORBITALS [ cartoon ] 2p Orbital saling tegak lurus (90 o ) 2p 2 2p 1 2p 1
O y x z.... 2s 2p OXYGENORBITALS oxygen = [He]2s 2 2p 4 unpaired
O y x z H... H... Combining atomic orbitals to form H 2 O. Incorrectly predicts a 90 o angle. 2s 2p 1s oxygen = [He]2s 2 2p 4 hydrogen = 1s 1
H H O 105 o The actual H-O-H angle in water (measured by electron diffraction) is 105 o EXPERIMENTAL RESULT This is not very good agreement with the atomic orbital model!
VSEPR Theory Valence Shell Electron Pair Repulsion Based on the simple idea that groups of electrons repel each other Predicts molecular shapes quite well
A better result is predicted by VSEPR theory consider the completed valence shell to be a spherical volume around the nucleus try to minimize repulsions by maximizing the distance between all pairs of electrons electron pairs (4 pair) repel each other in the final solution, they should all be equidistant valence shell nucleus TETRAHEDRAL
V ALENCE S HELL E LECTRON P AIR R EPULSION VSEPR THEORY 4 pairtetrahedral109 o 28’sp 3 (pyrimidal, angular ) 3 pairtrigonal planar120 o sp 2 2 pairlinear180 o sp pairsgeometryangles hybridization For most molecules, these predictions are correct to within a few degrees ( 5 o ). 6 pair octahedral 90 o d 2 sp 3 5 pair trigonal bipyramid 120 o, 90 o dsp3 O R G A N I C
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.
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.
Sigma Bonds Head-on overlap of atomic orbitals Electron density is a symmetrical cylinder around the bond axis Atomic orbital combinations that give bonds:
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 bonds:
HYBRIDIZATION HOW ARE THE OBSERVED BOND ANGLES ACHIEVED? “Vision is the art of seeing things invisible.” Jonathan Swift
atomic orbitals hybrid atomic orbitals molecular orbitals HYPOTHETICAL BONDING PROCESS 2s 2px,2py,2pz sp,sp 2py,2pz , , ,n These orbitals are for the atom - we can’t expect that they are suitable for the molecule. WHY DOESN’T THE ATOMIC ORBITAL APPROACH WORK ? During bonding …. new orbitals form that are more suitable for making bonds. After bonding (overlap) we get a totally new solution for the new molecule. overlap 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. LCAO
O tetrahedral geometry 2s 2p sp 3 hybrid orbitals hybridization 109 o 28’ (cartoon) FORMATION OF TETRAHEDRAL HYBRID ORBITALS 4 pair sp 3 (1) (1)(2)(3)(4) sp 3 (3) sp 3 (4) sp 3 (2) New orbitals point to the corners of a tetrahedron. FILLED VALENCE SHELL occurs when orbitals are full and have finished bonding
X unhybridized atom 2s 2p z 2p y 2p x 2s 2p FORMATION OF SP 3 HYBRID ORBITALS X sp 3 hybridized atom (1)(2)(3)(4) FORMATION OF SP 3 HYBRID ORBITALS These orbital shapes are cartoons - actual shapes are shown on the next slide. [animation]
sp 3 SP 3 HYBRID ORBITAL The hybrid orbital has more density in the bonding lobe than a p orbital and forms stronger bonds. ( cross section ) 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. omitted Courtesy of Professor George Gerhold … and its cartoon
2s2p add together, divide in four sp 3 hybrid orbitals (1)(2)(3)(4) each new orbital is 1/4 s + 3/4 p (25% s, 75% p) S 1 P 3 = SP 3 ( 1+3 ) = 4 parts total ORIGIN OF THE SP 3 DESIGNATION hybridization
+ + - 2s orbital 2p orbital x HYBRIDIZATION x + - sp 3 hybrid orbital HYBRIDIZATION signs are mathematical coordinates, not electronic charge RECALL: ORIGIN OF THE SP 3 ORBITAL SHAPE [animation]
What about acetylene? C H H Each carbon atom is sp hybridized. The hydrogens are unhybridized, 1s orbitals. 2p 2s hybridize 2p sp Note that a triple bond consists of a and 2 bonds. The two bb onds use unhybridized p orbitals.
COMPARISON OF SP x HYBRID ORBITALS more “p” character more “s” character sp 3 sp 2 sp bigger “tail” more electron density in the bonding lobe
BOND STRENGTHS - MULTIPLE BONDS CC bond bond bond energy molecule bond type length per mole measured Kcal (KJ) C-C sp 3 -sp 3 1.54 Å 88 (368) CH 3 - CH 3 C=C sp 2 -sp 2 1.34 Å 145 (607) CH 2 =CH 2 C=C sp - sp 1.21 Å 198 (828) HC=CH == increasing s-character and p - p and two p-p
Molecular Distortions: VSEPR Revisited Molecular Distortions: VSEPR Revisited Four situations: 1) electron pair repulsion 2) effect of electronegative atoms 3) double bond and electronegativity 4) steric repulsion
C H H H H : :.. symmetrical molecule all repulsions are equal perfect tetrahedral all angles 109 o 28’ N H H :.. H angle becomes larger repulsion smaller angle becomes smaller not all pairs are equivalent the unshared pairs repel more strongly than the bonded pairs Electron Pair Repulsion
Effect of double bond and electronegativity 117 o 121.5 o 123 o 125 o 117 o 114 o 110 o 116 o 122 o 116 o 124.5 o 126 o 111 o 108 o 122 o
Most pi bonds have a bond energy of 50 - 60 Kcal / mole ( 210 - 250 Kj / mole ) MOLECULES WITH PI BONDS 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 145 Kcal/mole C-C = 95 Kcal/mole ( 145 - 50 = 95 ) both bonds thus: TOTAL BOND ENERGY
Non Bonded Electrons (unshared pairs) do not significantly change their energy in going from an atom to a bonded molecule MOLECULES WITH UNSHARED PAIRS
O C H HH H 2s 2p sp 3 hybrids hybridization sp 3 Metanol
2s 2p sp 2 hybridization 2p CO HH used for bond used for bonds sp 2
CCC H H H H allena sp 2 sp C H H HH end view molecule has a twist in the center
Start with the Lewis Diagram Determine the geometry of each atom Use the correct hybid in each case Assemble the molecule from the hybrids. C N H H H H H : C = 4 pair = tetrahedral N = 4 pair = tetrahedral C = sp 3 N = sp 3 VSEPR CNCN HHHHH ASSEMBLY METHOD..
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! – NH 2 NH 2 – CH 3 -C C-CHO