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Polar Covalent Bonds Acids and Bases

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					2. Polar Covalent Bonds:
    Acids and Bases
        Topics to Discuss…

Electronegativity
Dipole Moments
Formal Charges
Resonance
Brønsted–Lowry Acids and Bases
Lewis Acids and Bases
Predicting Reactions using pKa
Drawing Chemical Structures
       Polar Covalent Bonds

 Q: What are polar covalent bonds?
Polar Covalent Bonds vs. Ionic Bonds

 Q: How do polar covalent bonds
 differ from ionic bonds?



 Q: How do they differ from nonpolar
 covalent bonding?
 Bond Polarity and Electronegativity

 Q: What is electronegativity (EN)?

 Q: Why is EN important?

 Q: How do we know which atoms are
 electronegative?

EN of C = 2.5
The Periodic Table and
  Electronegativity
 Bond Polarity and Inductive Effect

 Nonpolar Covalent Bonds

 Polar Covalent Bonds

 Ionic Bonds

 Q: What is the INDUCTIVE EFFECT
          Polar Covalent Bonds:
          Bond Dipole Moments
   Q: What is a bond dipole moment?

 Q: How do we represent the bond
    dipole moment?


                     +   -
                     H    F
       Polar Covalent Bonds:
   Net Molecular Dipole Moments
 Q: What factors affect the dipole
 moment?



 Q: How do we know if an entire
 molecule is polar?
         Polar Covalent Bonds:
     Net Molecular Dipole Moments
 All dipole arrows are like mini “tug of wars”.
 The direction of the net molecular dipole moment is
  the same as the direction in which the molecule
  would move if all of the imaginary tug of wars were
  going on.
 If all dipole arrows “cancel out”, then the molecule is
  NONPOLAR.

                          Cl            Cl
          direction                                 dipoles cancel out
          of dipole       C             C         Net molecular dipole = 0
                      H        H   Cl        Cl
                          H             Cl
    Dipole Moments in Water and
              Ammonia
Large dipole moments
  EN of O and N > H
  Both O and N have lone-pair electrons
   oriented away from all nuclei
              Example 1:
    Indicating Direction of Polarity
   Q: Use electronegativities to
    predict the direction of the dipole
    moments:

                 C-O
           Example 1:
 Indicating Direction of Polarity
 Q: Use electronegativities to
 predict the direction of the dipole
 moments:

              N-Cl
              Example 2:
     Predicting Direction of Dipole
                Moment
   Q: Predict whether each molecule
    has a net molecular dipole moment.
    If so, draw the direction.

               CHCl3
           Example 2:
  Predicting Direction of Dipole
             Moment
 Q: Predict whether each molecule
 has a net molecular dipole moment.
 If so, draw the direction.

            CH2=CH2
         Formal Charges
 Q: What is Formal Charge?



 Q: Why do we need to know
 Formal Charges?
     Calculating Formal Charges
Q: How do we
calculate Formal
Charge?
   Calculating Formal Charges

 Things to remember…
              Example 3:
      Calculating Formal Charges
   Q: Consider the nitrogen atom in the
    compound below and determine if it
    has a formal charge:

                  NH4
            Resonance
 Q: What is Resonance?
  How to think about Resonance
Drawings are great for showing…


 Electrons are not …


The clouds of electron density can
 spread…
             Resonance

 Q: Why is resonance important?
        Resonance Hybrids

 Q: What is a resonance hybrid?
  5 Rules for Resonance Forms

1.
2.
3.
4.
5.
Curved Arrows and Resonance Forms

We can imagine that…

A curved arrow shows…
     Drawing Resonance Forms

Any three-atom grouping…
Different Atoms in Resonance Forms
 Can involve…

 The types may…

 The “enolate” derived from acetone is a good
  illustration
               Example 4:
      Drawing Resonance Structures
   Q: Draw the important resonance
    forms for CO32-
       REMEMBER: YOU CAN MOVE ELECTRONS, BUT NOT
       ATOMS!
           Example 4:
  Drawing Resonance Structures
 Q: Draw the important resonance
 forms for H2C=CH-CH2+:
          Acids and Bases:
    The Brønsted–Lowry Definition

The idea that acids are solutions
 containing…


 Brønsted–Lowry theory
    Brønsted Acids and Bases

Brønsted acid



Brønsted base


       HA         H+   +   A-
      acid              conjugate
                          base
    Conjugate Acids and Bases

Conjugate acid

 Conjugate base



       HA    +   H2O   H3O+ +   A-
      acid                   conjugate
                               base
      Quantitative Measures of
           Acid Strength
The equilibrium constant (Ke) …

Stronger acids have…

Note that brackets [ ] indicate…
Ka – the Acidity Constant
     Acid and Base Strength

The “ability” of a Brønsted acid to…



The strength of the acid is…



Water is used as a common base for…
  pKa – the Acid Strength Scale

pKa = -log Ka
Predicting Acid–Base Reactions from
             pKa Values
  Example 5: Using pKa to predict
        Acid-Base reactions
 Q: According to the pKa data,
 predict whether the acid-base
 reaction will take place:
Organic Acids and Organic Bases

The reaction patterns of organic
 compounds often…



The transfer of a proton from…
              Organic Acids
 Examples:
              Organic Bases
 Examples:
           Acids and Bases:
          The Lewis Definition
 Lewis acid

 Lewis base

 Brønsted acids are not Lewis acids because…
 Lewis Acids and the Curved Arrow
             Formalism
 Examples:



Organic compounds that undergo
 addition reactions…

The combination of a Lewis acid and a
 Lewis base can…
  Using Curved Arrows:
Lewis Acid-Base Reactions
              Lewis Bases

 Examples:
    Drawing Chemical Structures:
       Condensed Structures
 Condensed structures
    Drawing Chemical Structures:
        Skeletal Structures
 Skeletal Structures




             H

     H C C C H

         C       C H
     H       C               CH3 CH
                                     2   CH3
             H
                       H3C   C     CH2

                             CH3
               Example 6:
           Drawing Structures
 Q: Draw a complete structural
  formula, condensed formulas, and
  skeletal structures for three
  compounds of formula: C3H8O
         Molecular Models

 Q: Do we REALLY need
 molecular models for this
 course?


                              Framework




                             Space-filling
             Summary…

Electronegativity
Dipole Moments
Formal Charges
Resonance
Brønsted–Lowry Acids and Bases
Lewis Acids and Bases
Predicting Reactions using pKa
Drawing Chemical Structures

				
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