Molecular Geometry and Bonding Theories

Two Simple Theories of Covalent Bonding Valence Shell Electron Pair Repulsion Theory VSEPR R. J. Gillespie - 1950’s Valence Bond Theory Hybridized orbitals L. Pauling - 1930’s & 40’s 2 2

Stereochemistry Study of the 3 dimensional shapes of molecules TWO MODELS VSEPR Theory Valence Bond Theory Some questions to examine: Why are we interested in shapes? What role does molecular shape play in life? How do we determine molecular shapes? How do we predict molecular shapes? 3 3

Determining Molecular Structure Draw the Lewis dot structure identify central atom Count # of regions of high electron density on central atom VSEPR tells the geometry around central atom 4 4

Determining Molecular Structure Identify lone pair effect on ideal molecular geometry Repeat procedure for more than one central atom Determine polarity from entire molecular geometry electronegativity differences 5 5

VSEPR Theory regions of high electron density around the central atom go as far apart as possible to minimize repulsions five basic shapes based on # of regions of high electron density several modifications of these five basic shapes will also be examined 6 6

VSEPR Theory Two regions of high electron density 7 7

VSEPR Theory Three regions of high electron density 8 8

VSEPR Theory Four regions of high electron density 9 9

VSEPR Theory Five regions of high electron density 10 10

VSEPR Theory Six regions of high electron density 11 11

VSEPR Theory electronic geometry (electron domain) molecular geometry determined by the locations of regions of high electron density around the central atom(s) molecular geometry determined by the arrangement of atoms around the central atom(s) electron pairs are not used in the molecular geometry determination 12 12

VSEPR Theory CH4 - methane electronic geometry molecular geometry tetrahedral molecular geometry bond angles = 109.5o 12 12

VSEPR Theory H2O - water electronic geometry molecular geometry tetrahedral molecular geometry bent or angular bond angle = 104.50 13 13

VSEPR Theory lone pairs of electrons (unshared pairs) require more volume than shared pairs there is an ordering of repulsions of electrons around central atom 14 14

VSEPR Theory lone pair to lone pair repulsion is strongest lone pair to bonding pair repulsion is intermediate bonding pair to bonding pair repulsion is weakest mnemonic for repulsion strengths lp/lp > lp/bp > bp/bp lone pair to lone pair repulsion is why bond angles in water are less than 109.50 14 14

Valence Bond Theory covalent bonds are formed by overlap of atomic orbitals atomic orbitals on the central atom can mix and exchange their character - hybridization common hybrids pink flowers, mules, corn, grass 15 15

Valence Bond Theory hybridized orbitals describe same shapes as VSEPR Name of orbital Shape of orbital sp3 tetrahedral sp2 trigonal planar sp linear sp3d trigonal bipyramidal sp3d2 octahedral 15 15

Hybrid Orbitals Electronic Structures Lewis Formulas 1s 2s 2p Be ­¯ ­¯ Cl [Ne] ­¯ ­¯ ­¯ ­ 16 16

Hybrid Orbitals Dot Formula Electronic Geometry 17 17

Hybrid Orbitals VSEPR Polarity 17 17

Hybrid Orbitals VSEPR Polarity 17 17

Hybrid Orbitals Molecular Geometry same as electronic geometry symmetrical & nonpolar 18 18

Hybrid Orbitals Valence Bond Theory (Hybridization) 1s 2s 2p 1s sp hyb 2p Be ­¯ ­¯ Þ­¯ ­ ­ 3s 3p Cl [Ne] ­¯ ­¯ ­¯ ­ 19 19

Hybrid Orbitals Linear

Hybrid Orbitals examples all are trigonal planar, nonpolar molecules BF3, BCl3 all are trigonal planar, nonpolar molecules 20 20

Hybrid Orbitals Electronic Structures Lewis Formulas 1s 2s 2p Cl [Ne] ­¯ ­¯ ­¯ ­ 20 20

Hybrid Orbitals Dot Formula Electronic Geometry 21 21

Hybrid Orbitals VSEPR Polarity 21 21

Hybrid Orbitals Molecular Geometry 22 22

Hybrid Orbitals Valence Bond Theory (Hybridization) 1s 2s 2p 1s sp2 hybrid B ­¯ ­¯ ­ Þ ­¯ ­ ­ ­ 5s 5p Cl [Ne] ­¯ ­¯ ­¯ ­ 23 23

Hybrid Orbitals Trigonal Planar

Hybrid Orbitals examples all are tetrahedral, nonpolar molecules CH4, CF4, CCl4, SiH4, SiF4 all are tetrahedral, nonpolar molecules as long as they have the same 4 substituents 24 24

Hybrid Orbitals 2s 2p C [He] ­¯ ­ ­ Electronic Structures Lewis Formulas 2s 2p C [He] ­¯ ­ ­ 24 24

Hybrid Orbitals 2s 2p C [He] ­¯ ­ ­ 1s H ­ Electronic Structures Lewis Formulas 2s 2p C [He] ­¯ ­ ­ 1s H ­ 24 24

Hybrid Orbitals Dot Formula Electronic Geometry 25 25

Hybrid Orbitals VSEPR Polarity 25 25

Hybrid Orbitals Molecular Geometry 26 26

Hybrid Orbitals Valence Bond 2s 2p four sp3 hybrid orbitals C [He] ­¯ ­ ­ Þ C [He] ­ ­ ­ ­ 1s H ­ 27 27

Hybrid Orbitals Tetrahedron

Hybrid Orbitals Examples PF5, AsF5, PCl5, etc. All are trigonal bipyramidal, nonpolar molecules. 41

Hybrid Orbitals 4s 4p As [Ar] 3d10 ­¯ ­ ­ ­ 2s 2p F [He] ­¯ ­¯ ­¯ ­ Electronic Structures Lewis Formulas 4s 4p As [Ar] 3d10 ­¯ ­ ­ ­ 2s 2p F [He] ­¯ ­¯ ­¯ ­ 42

Hybrid Orbitals Dot Formula Electronic Geometry 43

Hybrid Orbitals VSEPR Polarity 44

Hybrid Orbitals VSEPR Polarity 44

Hybrid Orbitals Molecular Geometry 45

Hybrid Orbitals Valence Bond (Hybridization) 4s 4p 4d As [Ar] 3d10 ­¯ ­ ­ ­ ___ ___ ___ ___ ___ ß five sp3 d hybrids ­ ­ ­ ­ ­ 45

Hybrid Orbitals Trigonal Bipyramidal Molecules Valence Bond (Hybridization) 45

Hybrid Orbitals Trigonal Bipyramid Molecules Valence Bond (Hybridization)

Variations of Trigonal Bipyramidal Shape If lone pairs are incorporated into the trigonal bipyramidal structure, there are three possible new shapes. One lone pair - seesaw shape Two lone pairs - T-shape Three lone pairs - linear

Hybrid Orbitals Example These are octahedral and nonpolar molecules. SF6, SeF6, SCl6, etc. These are octahedral and nonpolar molecules. if all 6 substituents are the same 46

Hybrid Orbitals Se [Ar] 3d10 ­¯ ­¯ ­ ­ 2s 2p F [He] ­¯ ­¯ ­¯ ­ Electronic Structures Lewis Formulas 4s 4p Se [Ar] 3d10 ­¯ ­¯ ­ ­ 2s 2p F [He] ­¯ ­¯ ­¯ ­ 47

Hybrid Orbitals Dot Formula Electronic Geometry 48

Hybrid Orbitals VSEPR Polarity 49

Hybrid Orbitals VSEPR Polarity 49

Hybrid Orbitals Molecular Geometry 50

Hybrid Orbitals Octahedral Molecules Valence Bond (Hybridization) 4s 4p 4d Se [Ar] 3d10 ­¯ ­¯ ­ ­ __ __ __ __ __ ß six sp3 d2 hybrids ­ ­ ­ ­ ­ ­

Hybrid Orbitals Octahedral Molecules Valence Bond (Hybridization)

AB6- No Lone Pairs - Octahedral Molecules

Variations of Octahedral Shape If lone pairs are incorporated into the octahedral structure, there are two possible new shapes. One lone pair - square pyramidal Two lone pairs - square planar

Compounds Containing Double Bonds Ethene or ethylene is the simplest double bond containing organic compound. Lewis Dot Formula for ethene C2H4 N = 2(8) + 4(2) = 24 A = 2(4) + 4(1) = 12 S = 12 51

Compounds Containing Double Bonds Lewis Dot Formula for ethene C2H4 51

Compounds Containing Double Bonds VSEPR suggests C atoms at center of two equilateral triangles H H C C H H 52

Compounds Containing Double Bonds Valence Bond Theory sp2 hybrids for the C atoms 1 electron remains in an unhybridized p orbital 2s 2p three sp2 hybrids 2p C ­¯ ­ ­ Þ ­ ­ ­ ­ 53

Compounds Containing Double Bonds C atoms are sp2 hybridized with single electrons in each of the lobes Top view

Compounds Containing Double Bonds The unhybridized 2p orbital on the C atom lies perpendicular to the planar sp2 lobes. Side view 54

Compounds Containing Double Bonds C=C double bond results from head-on overlap of two sp2 hybrid orbitals 55

Compounds Containing Double Bonds The portion of the double bond formed from the head-on overlap of the sp2 hybrids is designated as a s bond. 56

Compounds Containing Double Bonds The other portion of the double bond, resulting from the side-on overlap of the p orbitals, is designated as a p bond.

Compounds Containing Double Bonds The final result is a bond that looks like this.

Compounds Containing Triple Bonds Ethyne or acetylene is the simplest triple bond containing organic compound. Lewis Dot Formula for ethyne C2H2 N = 2(8) + 2(2) = 20 A = 2(4) + 2(1) =10 S = 10 51

Compounds Containing Triple Bonds Lewis Dot Formula for ethyne C2H2 51

Compounds Containing Triple Bonds VSEPR suggests C and H atoms are 180o apart. H C C H 52

Compounds Containing Triple Bonds Valence Bond Theory sp hybrids for the C atoms 2 electrons remain in unhybridized p orbitals 53

Compounds Containing Triple Bonds Valence Bond Theory sp hybrids for the C atoms 2 electrons remain in unhybridized p orbitals 2s 2p two sp hybrids 2p C ­¯ ­ ­ Þ ­ ­ ­ ­ 53

Compounds Containing Triple Bonds triple bond results from head-on overlap of two sp hybrid orbitals 55

Compounds Containing Triple Bonds Note that this gives us one s bond and two p bonds. 56

Compounds Containing Triple Bonds The final result is a bond that looks like this.

Summary of Electronic & Molecular Geometries

Synthesis Question 1 The basic shapes that we have discussed are present in essentially all molecules. Shown below is the chemical structure of vitamin B6 phosphate. What is the shape and hybridization of each of the indicated atoms in vitamin B6 phosphate?

Synthesis Question 1

Group Question 1 Shown below is the structure of penicillin-G. What is the shape and hybridization of each of the indicated atoms in penicillin-G?

Synthesis Question 2 As we all know, in the wintertime we are more likely to get shocked when we walk across carpet and touch the door knob. Here is another wintertime experiment to perform. Turn on a water faucet until you have a continuous but small stream of water coming from the faucet. Brush your hair vigorously then hold the brush near the stream of water. You will notice that the stream bends towards the brush. Why does the water bend?

Synthesis Question 2 Since water is a highly polar molecule, it is attracted by the electromagnetic field generated by the hair brush. This causes the stream to bend.

Group Question 2 On a recent “infomercial” it was claimed that placing a small horseshoe magnet over the fuel intake line to your car’s carburetor would increase fuel mileage by 50%. The reason given for the mileage increase was that “the magnet aligned the molecules causing them to burn more efficiently.” Will this work? Should you buy this product?