CHE-240 Unit 1 Structure and Stereochemistry of Alkanes .ppt

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CHE-240 Unit 1 Structure and Stereochemistry of Alkanes .ppt Powered By Docstoc
					           CHE-240
            Unit 1
Structure and Stereochemistry
          of Alkanes

        CHAPTER ONE

           Terrence P. Sherlock
         Burlington County College
                    2004
        Bond Formation
• Ionic bonding: electrons are transferred.
• Covalent bonding: electron pair is shared.




                                      =>

                   Chapter 1               2
       Lewis Structures
• Bonding electrons
• Nonbonding electrons or lone pairs




    Satisfy the octet rule!                =>
                  Chapter 1            3
Multiple Bonding




                   =>
       Chapter 1    4
           Dipole Moment
• Amount of electrical charge x bond length.
• Charge separation shown by electrostatic
  potential map (EPM).
• Red indicates a partially negative region and
  blue indicates a partially positive region.



                                                =>
                     Chapter 1              5
 Electronegativity and
    Bond Polarity
Greater DEN means greater polarity




                                     =>
                 Chapter 1           6
Calculating Formal Charge
• For each atom in a valid Lewis
  structure:
• Count the number of valence electrons
• Subtract all its nonbonding electrons
• Subtract half of its bonding electrons



                                       =>
                  Chapter 1                7
    Ionic Structures




X
                       =>
           Chapter 1    8
         Resonance
• Only electrons can be moved (usually
  lone pairs or pi electrons).
• Nuclei positions and bond angles
  remain the same.
• The number of unpaired electrons
  remains the same.
• Resonance causes a delocalization of
  electrical charge.

                Chapter 1
                             Example=>   9
     Resonance Example



• The real structure is a resonance hybrid.
• All the bond lengths are the same.
• Each oxygen has a -1/3 electrical charge.

                                       =>
                  Chapter 1             10
 Major Resonance Form
• has as many octets as possible.
• has as many bonds as possible.
• has the negative charge on the most
  electronegative atom.
• has as little charge separation as
  possible.

                               Example=>
                  Chapter 1                11
Major Contributor?




  major                   minor,
                       carbon does
                      not have octet.

                                         =>
          Chapter 1                     12
     Chemical Formulas
• Full structural formula
  (no lone pairs shown)

• Line-angle formula

• Condensed structural
  formula                          • CH3COOH
• Molecular formula                • C 2H 4O 2
• Empirical formula                • CH2O
                                                 =>
                       Chapter 1                      13
Arrhenius Acids and Bases
•   Acids dissociate in water to give H3O+ ions.
•   Bases dissociate in water to give OH- ions.
•   Kw = [H3O+ ][OH- ] = 1.0 x 10-14 at 24°C
•   pH = -log [H3O+ ]
•   Strong acids and bases are 100%
    dissociated.



                                            =>
                       Chapter 1                   14
        BrØnsted-Lowry
        Acids and Bases
• Acids can donate a proton.
• Bases can accept a proton.
• Conjugate acid-base pairs.


 acid       base               conjugate   conjugate
                                 base        acid

                                                       =>
                   Chapter 1                       15
     Acid and Base Strength
•   Acid dissociation constant, Ka
•   Base dissociation constant, Kb
•   For conjugate pairs, (Ka)(Kb) = Kw
•   Spontaneous acid-base reactions
    proceed from stronger to weaker.


      pKa 4.74   pKb 3.36         pKb 9.26   pKa 10.64

                                                =>
                      Chapter 1                  16
Determining Relative Acidity

 • Electronegativity
 • Size
 • Resonance stabilization of conjugate
   base

                                          =>


                   Chapter 1               17
       Electronegativity
As the bond to H becomes more
 polarized, H becomes more positive
 and the bond is easier to break.




                                           =>
                 Chapter 1            18
                Size
• As size increases, the H is more loosely
  held and the bond is easier to break.
• A larger size also stabilizes the anion.




                                         =>
                   Chapter 1             19
             Resonance
• Delocalization of the negative charge on the
  conjugate base will stabilize the anion, so the
  substance is a stronger acid.
• More resonance structures usually mean
  greater stabilization.




                                              =>
                      Chapter 1                 20
   Lewis Acids and Bases
• Acids accept electron pairs = electrophile
• Bases donate electron pairs = nucleophile

nucleophile   electrophile




                                              =>
                     Chapter 1           21
              POWER POINT IMAGES FROM
           “ORGANIC CHEMISTRY, 5TH EDITION”
                      L.G. WADE
    ALL MATERIALS USED WITH PERMISSION OF AUTHOR

PRESENTATION ADAPTED FOR BURLINGTON COUNTY COLLEGE
             ORGANIC CHEMISTRY COURSE
                        BY:
 ANNALICIA POEHLER STEFANIE LAYMAN    CALY MARTIN




                       Chapter 1                     22

				
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