Comparing acid strengths by comparing structures
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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.

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Comparing Acid Strengths by Comparing Structures

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Comparing acid strengths by comparing structures

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


Comparing acid strengths by comparing structures

Which H is more acidic?


Comparing acid strengths by comparing structures

Where does the equilibrium lie?


Comparing acid strengths by comparing structures

Does an Acid-Base Reaction Occur? Write the products.

(CH3)3N + NH2-


Bond polarity part i

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

Bond Polarity - Part II

  • A polar bond has a dipole moment μ:

    μ(in debyes) = 4.8 δd

    • 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 ii1

Bond Polarity - Part II

μ(in D) = 4.8 δd

  • 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 7 th ed table 2 1

Bond Dipole Moments from Wade, 7thed (Table 2-1)


Bond polarity part ii2

Bond Polarity - Part II

μ(in D) = 4.8 δd

  • 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

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 polarity1

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.


Comparing acid strengths by comparing structures

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 asmelting point, boiling point, vapor pressure, and solubility.


Comparing acid strengths by comparing structures

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

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 forces1

Intermolecular Forces

  • Which will have the higher boiling point?

or


Intermolecular forces2

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

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 solubility1

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 solubility2

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 H-bonded water molecules.


Intermolecular forces3

Intermolecular Forces

  • Which are soluble in water and why?


Comparing acid strengths by comparing structures

Phosphatidyl choline – a lipid found in cell membranes

http://www.agen.ufl.edu/~chyn/age2062/lect/lect_06/4_18.GIF


Comparing acid strengths by comparing structures

Intermolecular Forces and the Cell Membrane

http://www.youtube.com/watch?v=ULR79TiUj80&feature=related


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