Solubility

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Solubility

• Experiment 2

2
Required Reading Before Class

• Experiment 2, Parts A-D
• Techniques 7.10, 7.11, 10, 11.1 (4th ed.)
• Techniques 3.9, 3.10, 5.1 (3rd ed.)

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Solubility

• Solubilitythe extent to which a substance is
  soluble in a solvent
• Like dissolves like
• (polar polar nonpolar nonpolar)
• Examples (solubility in water at 25C)
• Cholesterol   0.002 mg/mL
• Caffeine   22 mg/mL
• Citric acid   620 mg/mL

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Types of Intermolecular Attractions

• van der Waals Forcesforce of attraction between
  NONPOLAR molecules
• Dipole-Dipole Forcesforce of attraction between
  POLAR molecules
• Hydrogen Bondingspecial type of dipole-dipole
  interaction

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Guidelines for Predicting Solubility

• All hydrocarbons are NONPOLAR.
• Hydrocarbons with longer chains are more nonpolar
  than ones with shorter chains
• Ex Octane vs. Pentane
• Hydrocarbons w/ p bonds (ex. Benzene) are
  slightly more polar than straight chained
  hydrocarbons b/c of a greater of van der Waals
  interactions.
• Compounds w/ oxygen or nitrogen are POLAR.
• The most polar compounds are ones that form
  HYDROGEN BONDS. (NH or OH)

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More Guidelines

• There are differing degrees of solubility, based
  on
• Shape of the molecule
• Size of the carbon chain
• The molecules ability to form hydrogen bonds

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Solubility of Liquids

• Misciblesolute and solvent mix homogeneously in
  all proportions
• Ex Ethanol in Water
• Immisciblesolute and solvent do not mix they
  form two separate phases, or layers
• Ex Hexane in Water

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Ionic Solutions

• In an ionic solution, the solute ionizes and
  dissociates, becoming water-soluble in most cases
• Many covalent organic molecules are not very
  soluble in water even if they are somewhat polar
• Ex Benzoic Acid
• The most common way organic compounds become
  ionic is via acid-base reactions
• Ex When and organic acid such as benzoic acid
  is reacted with NaOH, it is converted to an ionic
  substance (sodium salt), The ionic substance is
  now much more soluble in water
• Likewise with organic bases such as benzocaine,
  the lone pair on the nitrogen atom becomes bonded
  to a proton when mixed with HCl, creating an
  ionic substance

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Procedural Notes

• Part A Solubility of Solid Compounds
• Questions to consider
• How does the polarity of the solute influence the
  solubility in each of these solvents?
• Predict the solubility of malonic acid in water. 
  Notice that the acid has two very polar carboxyl
  groups in the compound. Would you expect this
  compound to be soluble in the non-polar hexane
  solvent?
• Do the same for both benzophenone and biphenyl

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Procedural Notes

• Part A Solubility of Solid Compounds
• This section is especially time consuming!
• Be sure to approximate the amounts of solids you
  are using in Part A.  You dont need to be exact
  on the masses.
• Fill in your data table according to the
  instructions (Soluble, Insoluble, Partially
  Soluble)
• If you use abbreviations, make a key indicating
  what each one stands for.

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Procedural Notes

• Part B Solubility of Different Alcohols
• Things to consider
• All three of these alcohols have a polar hydroxyl
  group.  However, the rest of the molecule has a
  nonpolar hydrocarbon chain.
• How does the length of the carbon chain influence
  the solubility of the alcohol in water? What
  about in hexane?
• Fill in your data table according to the
  instructions (Soluble, Insoluble, Partially
  Soluble)

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Procedural Notes

• Part C Miscible or Immiscible Pairs
• Questions to consider
• Examine the structures of each pair of compounds
  mixed together.
• How do differences in polarity affect solubility?
• How do intermolecular attractions influence the
  solubility of a solute in a particular solvent?
• Fill in your data table according to the
  instructions (Miscible, Immiscible)

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Procedural Notes

• Part D Solubility of Org. Acids Bases
• Things to consider
• What is the important component in 1.0 M NaOH?
  What about in 1.0 M HCl?
• What principle explains the solubility of a
  substance in aqueous acid or aqueous base?
• Remember, to reform an organic acid, you must
  make the mixture acidic, and to reform an organic
  base, you must make the mixture basic.