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					Molecular Geometry and Hybrid Orbitals


                             H-C-H bond angle 90o

          +
                        Molecular Geometry
Why Should I Care About Molecular Geometry?
Molecular geometry (shape) influences...

                               H   H   H                        CH3

•Physical properties:   H3C    C   C   C   CH3            H3C   C     CH3

                               H   H   H                        CH3


                               Pentane (bp 36oC)      2,2-Dimethylpropane (bp 10oC)


•Chemical properties: (CH3)3C-Br + HO-             (CH3)2C=CH2 + Br-
                              H3C-Br + HO-           CH3OH + Br-


•Biological properties: Enzyme specificity
                     Molecular Geometry
How Do We Determine Molecular Shape?
•Empirically (spectroscopy, etc)
•Predict via theory

                H

Example:    H   C    O   H    CH3OH Lewis structure implies planarity
                H             But other geometries can be conceived
                              So what is the real structure of this molecule?

•Bonds vibrating, bending, etc.
•By ‘geometry’ we usually mean ‘most common’ geometry
•Most common geometry = geometry with lowest energy
  What Controls Molecular Geometry?
           Most common geometry = geometry with lowest energy


Lowest energy results from...
...minimizing electron repulsion (nonbonded interactions; van der Waals repulsions)
...resonance, conjugation, and aromaticity

How does the molecule minimize electron repulsion?
•Moving electron clouds (electron groups) away from each other
•Electron groups = Atoms
                   Groups of atoms
                   Lone pairs
•Valence Shell Electron Pair Repulsion theory (VSEPR)
Molecular Geometry Survey
Best geometry = largest distance between electron groups

Composition Geometries                                    Example
   AB         Linear              A       B                   H        H



    AB2         Linear        B       A       B           O        C       O


          Bond angle                                              180o

                                      A                            C
                 Bent         B               B           O                O




                                                                  180o
                                  B       A           B   H        B           H
    AB3        T-shape                                                     90o
                                          B                        H
                                                  B                        H

            Trigonal planar       B       A                   H        B   120o
                                                  B                        H
         Molecular Geometry Survey
        Best geometry = largest distance between electron groups

       Composition Geometries                                               Example
                                             B                                  H
                                                                                    180o
            AB4     Square planar     B      A    B                         H   C    H

                                             B                                  H   90o
                       Receding back         B                      B           H
                                                                                     109.5o
                      Tetrahedral            A
                                                  B
                                                                    A   B       C
                                                                                     H
                                         B                 B
                                                  B                     B H          H

                        Projecting out

                                                      Cl
                                                               Cl
             AB5 Trigonal bipyramid          Cl       P
                                                               Cl
                                                      Cl


Explore and verify molecular geometry examples with molecular models
                              Bond Angles
               Best geometry = largest distance between electron groups
Are bond angles always equal?
•Bond angles controlled by magnitude of electron group repulsions

                                                                H
       H                                                                H
                                                   H       H        C
           H                                   N
       C                C
                                          H                             H    C
   H                                               H                C
           H
                                                            H
                                                                        H

   Electron groups equal size             H < LP                   H < CH3
        All H-C-H 109.5o                H-N-H 107o              H-C-H < 109.5o
 Electron group repulsions equal              Electron group repulsions unequal
        Bond angles equal                            Bond angles unequal

Which electron groups have greatest repulsion? (Which electron groups are largest?)

       Approximate size ranking: H, F < lone pair, Cl, Br, I < all groups of atoms
                            Bond Angles
In general...
•Two electron groups = linear
•Three electron groups = trigonal planar or distorted trigonal planar
•Four electron groups = tetrahedron or distorted tetrahedron


Warning! Atomic geometry may also be influenced by...
...resonance (covered in Chem 30A)


               } Not a consideration in Chem 30A
...conjugation
...aromaticity

                                   Four attachments
                                   Trigonal planar due to resonance
Example:               N   H

                       H
            Molecular Geometry and Orbitals
Covalent bonds formed by overlap of orbitals

 •Example: Two H 1s orbitals (spheres) overlap to form H-H sigma bond (s bond):




                   +

            Hydrogen atoms                     Hydrogen molecule
       Molecular Geometry and Orbitals
What orbitals are used for organic molecules?   Example: Methane (CH4)
H (1s) + C (2s, 2px, 2py, 2pz)  C-H bonds

                 orthogonal

                                                          H-C-H bond angle 90o

                   +




             H 1s + C 2px + C 2pz  wrong H-C-H bond angle!

              Therefore this orbital combination incorrect
                         Verify with your model kit
              Molecular Geometry and Orbitals
Solution (Linus Pauling, 1931)
•Combine s, px, py, and pz orbitals to form correct # of bonds with correct geometry
•Combinations = hybrid orbitals


Hybridization scheme for atom with four electron groups
•CH4 geometry = tetrahedral; need four bonds and four hybrid orbitals
•Orbital conservation: four hybrid orbitals come from four atomic orbitals
•s + px + py + pz  sp3 + sp3 + sp3 + sp3       •C sp3 + H 1s  C-H s bond

                                                                109.5o
                                                            H
                                                                  H
                                                            C
                                                        H
                                                                 H

                                                       sp3 carbon
           Molecular Geometry and Orbitals
                                                                         H             H

Hybridization scheme for atom with three electron groups     Example:        C    C

                                                                         H             H
•Carbon = trigonal planar; need three bonds and three hybrid orbitals
•Three hybrid orbitals come from three atomic orbitals:

                         s + px + py + pz  sp2 + sp2 + sp2 + pz

                           Mixed    Not used

           Top view without pz      Top view with pz       Side view

                             sp2                                                 pz
          120o                pz                                                 sp2

                                                                   90o
             Molecular Geometry and Orbitals
                                 H           H
                                                 C sp2 + H 1s     C-H s bond
     Combine orbitals to build       C   C       C sp2 + C sp2    C-C s bond
                                 H           H    C pz + C pz     C-C p bond


                      Side view                    Top view
                     C-H s bond

                                                                 p bond not shown
C-C p bond


                     C-C s bond                                  p bond included


                    Explore and verify with molecular models
              Molecular Geometry and Orbitals
   Hybridization scheme for atom with two electron groups      Example:   H   C   C   H


   •Carbon = linear; need two bonds and two hybrid orbitals
   •Two hybrid orbitals come from two atomic orbitals:

                              s + px + py + pz  sp + sp + py + pz

                              Mixed Not used


Top view without pz        Top view with pz    Side view without py   Side view with py

                      sp
                      py
                      pz
              Molecular Geometry and Orbitals
                                                   C sp + H 1s        C-H s bond
                                                   C sp + C sp        C-C s bond
     Combine orbitals to build   H   C   C   H
                                                   C py + C py        C-C p bond
                                                   C pz + C pz        C-C p bond

                      Side view                          Top view
               py-py p bond not shown            pz-pz p bond not shown
                      C-H s bond



C-C p bond


                      C-C s bond
                      Explore and verify with molecular models
               Molecular Geometry and Orbitals
A few final notes...
•Other double bonds (C=O, C=N) similar orbital arrangements to C=C

•Other triple bonds (CN) similar orbital arrangements to CC

•C, O, N almost always hybridized in organic molecules


  Four electron groups        Three electron groups      Two electron groups
          sp3                          sp2                       sp
          H                                                     H
                                     H
      H    C    O      H                 C   O              H   C    C   N

          H                          H                          H

				
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posted:3/31/2012
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