Polar Covalent Bonds; Acids and Bases

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					Polar Covalent Bonds;
   Acids and Bases

Bond Moments and Dipole Moments
         Formal Charge
           Resonance
    Bronsted-Lowry Acid/Base
        Lewis Acid/Base
Pauling Electronegativity Scale
     Electronegativity Trends
Ability to Attract the Electrons in a Covalent Bond
Electrostatic Potential Map
       Cholormethane
Opposite polarity in CH3Li
Methanol
Dipole Moment (m) is sum of
    the Bond Moments
Nonpolar Compounds
Bond Moments Cancel Out
             NCl3 and BCl3
    sp3                          2
                                 sp
 pyramidal                   trigonal plan
                             Cl

Cl N                         B
Cl      Cl            Cl             Cl

N-Cl bond moments          m=0
do not cancelN
Nitromethane
      Nitromethane has 2 Formal
              Charges
Formal Charge = [Group #] - [# bonds] - [# non-bonding elect


                     H            O
    CH3NO2                            O = 6-1-6 =-1
                         C    N
                    H
                    H             O
              N = 5-4-0 =+1
  Bonding Pattens for C, N, and O
   sp3                 sp2                       sp
tetrahedral geometry   trigonal planar                linear

                                         C
   C                   C
                                             C



    N                  N                 N




    O                  O
Common Cationic, Neutral and
     Anionic Forms
   Both Resonance Structures
Contribute to the Actual Structure
                     CH3NO 2


  H           O                 H         O

 H
      C   N                    H C    N
 H            O                H          O

          2 Equivalent Resonance Structures
Dipole Moment reflects Both
   Resonance Structures
H           O                  H        O

H
    C   N                     H C   N
H           O                  H        O

                H           O 
                    C   N
                H
                H           O 
                Resonance Hybrid
        Curved Arrow Formalism
             Shows flow of electrons


H            O                 H          O

H
    C    N                    H C     N
H             O                H          O

        Arrows depict electron pairs moving
          Resonance Rules
• Cannot break single (sigma) bonds
• Only electrons move, not atoms
               3 possibilities:
  – Lone pair of e- to adjacent bond position
     • Forms p bond
   p bond to adjacent atom
   p bond to adjacent bond position
Resonance Stabilization of Ions
        Pos. charge is “delocalized”
H           H                           H           H
    C   C                                   C   C
                H                                       H
H           C                           H           C
            H                                       H
                    H   H
                      C C 
                             H
                    H     C
                             H
                     resonance hybrid
Two Nonequivalent Resonance
 Structures in Formaldehyde
Resonance in Acetate Anion
Resonance in   [CH2NH2]+
   Definitions of Acids/Bases
                                +
                             3O
      Arrhenius acid - forms H in H2O
                               -
                                 in
      Arrhenius base - forms OH H2O
                                    +
    Bronsted-Lowry acid - donates a H (proton)
                                    +
    Bronsted-Lowry base - accepts a H (proton)

Lewis acid - accepts an electron pair to form a new bond
Lewis base - donates an electron pair to form a new bond
       Dissociation in H2O
   Arrhenius Acid forms H3O+
Bronsted-Lowry Acid donates a H+
          Resonance Stabilization
H        O
                                              H     O
H C      C        H
                         + NaHCO3
              O                               H C   C
                                                        O
 H
                                               H
             pKa 4.7
                                    H2CO3 +                 Na
             O 
    H
                    Na                        H     O
    H C     C
                 O                          H C   C
     H                                                  O
                                               H
     resonance hybrid
Measuring Acid Strength
          Ka
Acid Strength defined by pKa
HCl + H2O             H3O + Cl

        [H3O ][Cl ]
  Keq =
        [HCl][H2O]
                      [H3O ][Cl ]      7
  Ka = K [H2O] =
        eq                          = 10
                       [HCl]

     pKa = -log(K ) = -7
                 a
      Stronger Acid Controls
           Equilibrium

     HCl + H O
             2       H3O + Cl
     acid  base    conjugate conjugate
                     acid    base
pKa = -7                -1.7
    stronger           weaker
Reaction Described with Arrows


                              H
                  O
 H   Cl   +                   O       +   Cl
              H       H
                          H       H
Equilibrium Reactions
Acids Can React with Water
The Effect of Resonance on pKa
   Identify the Acid and Base

            O                   O
CH 3OH +   OCOH     CH 3O + HOCOH
  Equilibrium Favors Reactants
                  O                         O
      CH3OH   +   OCOH       CH3O + HOCOH

      acid            base     conj. base       conj. ac
pKa    15.5                                         6.5
      Draw the Conjugate
        Acid and Base

CH 3CH 2CH 2OH   +   H2SO4
          Propanol is a Base

                                        H
CH3CH2CH2OH   +   H2SO4   CH3CH2CH2OH
   base           acid
                               + HSO4
      Draw the Conjugate
        Acid and Base

CH 3CH 2CH 2OH + Na   NH 2
         Propanol is an Acid


CH3CH2CH2O   H + Na NH2     CH3CH2CH2O   Na
     acid            base       c.b.
                              + NH   3
                                 c.a.
Some Acid-Base Reactions
     Carboxylic Acids can be
    Separated from non-acidic
          Compounds O
   O
  OCCH3 O                                OCCH3 O
                                               C
          C       H                                 O Na
              O
                        + Na OH
                                                   + H2O
 acid                         base   conj. base   conj. acid
Aspirin                                     O
                                         OCCH3 O Na
                  HCl                          C
                                                 O
       Lewis Acids and Bases

  B      H                   B    H
base     acid                    complex
arrow always goes from the base to the acid
always originate your arrow at the e- donor
     Lewis Acid and Base Reactions
      H
                                  H           H
      B       +        N         H
                  H                   B   N
 H        H                H                  H
                   H              H         H
 Lewis Acid       Lewis Base     acid-base complex



e- pair acceptor e- pair donor
Lewis Base = Nucleophile
Lewis Acid = Electrophile
Draw the Acid-Base Complexes

 CH OCH3 + FeCl3
   3

     +
 CH 3    + H 2O

 HOC(CH 3)3 + AlCl 3

				
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posted:12/11/2011
language:English
pages:47