Polarity and Intermolecular Forces

Report
Comparing Acid Strengths by
Comparing Structures
 Look at the stability of the conjugate
base. The more stable the conjugate
base, the stronger its acid.





Electronegativity
Size/polarizability
Resonance Stabilization
Induction
Hybrid orbital containing electrons
Which H is more acidic?
Where does the equilibrium lie?
O
O
+
OH
Cl
Cl Cl
O
O
O
-
O-
+
Cl
Cl Cl
OH
Does an Acid-Base Reaction
Occur? Write the products.
(CH3)3N + NH2- 
Bond Polarity - Part I
 A bond is polar when the charge is not
equally shared between the two atoms.
 The more electronegative atom will
have a partial negative charge (δ-).
The arrow
shows the
dipole
moment.
Here we
show
partial
charges.
Bond Polarity - Part II
 A polar bond has a dipole moment μ:
μ(in debyes) = 4.8 δ d
 δ is the charge at either end of the
dipole
 d is the bond length in angstroms
(charge separation) (1Å=10-10m)
dipole
moment, μ
bond length, d
Bond Polarity - Part II
μ(in D) = 4.8 δ d
 The dipole moment μ gives a
quantitative measure of the polarity
of a bond.
C=O (2.4D) is more polar than C-O (0.86 D)
Bond Dipole Moments from
Wade, 7th ed (Table 2-1)
C-O
0.86D
C-N
0.22D
C-F
1.51D
C=O
2.4D
C≡N
3.6D
C - Cl
1.56D
H-O
1.53D
H-N
1.31D
C - Br
1.48D
H-C
0.3D
C-I
1.29D
Bond Polarity - Part II
μ(in D) = 4.8 δ d
 Knowing μ and d allows the charge
separation δ to be calculated.
 C=O has a dipole moment of 2.4D and a
bond length of 1.21Å.
δ = 2.4/(4.8x1.21)= 0.41
 C-O has a dipole moment of 0.86D and a
bond length of 1.43Å.
δ = 0.86/(4.8x1.43)= 0.13
Molecular Polarity
 The polarity (or dipole moment) of a
molecule is the vector sum of the dipole
moment for each bond in the molecule.
 A molecule with a significant dipole moment is
polar.
 A molecule with little or no dipole moment is
considered nonpolar.
Molecular Polarity
 The dipole moment of a molecule
can be measured.
 The dipole moments of the individual
bonds can then be estimated.
 Lone pairs contribute to the dipole
moments.
Intermolecular Forces
 arise from the charged nature of
the subatomic particles (electrons
and protons).
 are responsible for the
cohesiveness of materials.
 are what determine physical
properties of pure substances such
as melting point, boiling point,
vapor pressure, and solubility.
Intermolecular Forces
 Substances that are gases at room
temperature have weak intermolecular
forces.
 Substances that are condensed
(liquids or solids) at room temperature
have much stronger intermolecular
forces.
 If intermolecular forces did not exist,
all substances would be gases, even at
extremely low temperatures.
Intermolecular Forces
 Dipole-dipole
 generally attractive
 Hydrogen bonding
 a special category of very strong dipole-dipole
force that involves the attraction between an
electropositive H atom and nonbonding
electrons on an electronegative atom (usually
N, O, F, or Cl)
 London dispersion force
 instantaneous dipole-induced dipole
 increases with increasing surface area of the
molecule
 present in all molecules
Intermolecular Forces
 Which will have the higher boiling point?
or
Intermolecular Forces
 Why does CCl4 have the higher boiling
point?
chloroform, CHCl3 (μ = 1.0D)
or
carbon tetrachloride, CCl4 (μ = 0)
bp CHCl3 = 62°C
bp CCl4 = 77°C
Intermolecular Forces and
Solubility
 “Like dissolves like.”
 Polar substances dissolve in polar
solvents.
 Nonpolar substances dissolve in
nonpolar solvents.
 The other pairings (polar
substance/nonpolar solvent and
nonpolar substance / polar solvent) will
not dissolve.
Intermolecular Forces and
Solubility
 For one substance to dissolve in another,
there must be an attraction similar in
magnitude to the forces holding the solvent
together.
 In water, H bonding holds the molecules of
water together pretty tightly.
 For a substance to dissolve in water, there
must be an attraction between the substance
and water that is close in magnitude to those H
bonds.
 Ions, alcohols, and ethers all dissolve in
water…can you show why?
Intermolecular Forces and
Solubility
 Carbon tetrachloride does NOT
dissolve in water.
 Water is held together by H bonds, a
strong intermolecular interaction.
 Carbon tetrachloride is nonpolar.
 The only force of attraction between
CCl4 and H2O is dispersion, and that is
not strong enough to push apart the Hbonded water molecules.
Intermolecular Forces
 Which are soluble in water and why?
Phosphatidyl choline – a lipid
found in cell membranes
http://www.agen.ufl.edu/~chyn/age2062/lect/lect_06/4_18.GIF
Intermolecular Forces and
the Cell Membrane
http://www.youtube.com/watch?v=UL
R79TiUj80&feature=related

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