Purification of Organic Solids

1
Purification of Organic Solids Identification
using Melting Point Determination Experiment
1 September 18 - 22, 2006
Experimental Objectives

• become familiar with common heating cooling
  methods (Technique 6)
• purify organic unknown using recrystallization
  (Technique 9)
• identify unknown by melting point solubility
  data (Technique 10)

2
Recrystallization

• crystalline material (solute) is dissolved in a
  hot solvent then returns to solid when cooled
• important to understand dissolving

• choice amount of solvent is critical

http//server1.fandm.edu/departments/chemistry/Van
20Arman20Virtual20Lab/Recrystallization/Recryst
allization3.html
3
Solubility

• solvent temperature - solubility increases with
  temperature
• solvent volume - amount of solute vs. volume of
  solvent
• solvent properties - solute/solvent interactions
  like dissolves like
• - polar H2O dissolves most (polar) inorganic
  compounds
• - nonpolar organic solvents dissolve most
  (nonpolar) organics

4
Recrystallization Solvents
www.chem.umd.edu/organiclabs/Chem232/01Recrystalli
zation/Lab01.ppt
5
7 Steps to Recrystallization

• find best solvent
• dissolve bulk sample in solvent
• decolorize (if necessary)
• remove insoluble impurities (if necessary)
• cool solute/solvent mixture
• collect crystals
• dry analyze crystals

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Step 1

• find best solvent
• small (test tube) size using a sand bath
• dissolves went hot not when cold
• limited to six initial solvent choices
• water
• ethanol
• ethyl acetate
• methylene chloride (dichloromethane)
• toluene
• hexane

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Steps 2 - 4

• dissolve bulk sample in solvent
• minimum amount of solvent to dissolved solute
  with heat
• this will be unique to solute solvent choice
• use boiling chips or sticks
• decolorize (if necessary)
• charcoal adheres to colored impurities
• remove insoluble impurities (if necessary)
• hot gravity filtration

8
Hot Filtration
9
Step 5

• cool solute/solvent mixture
• cool slowly to eliminate soluble impurities from
  crystallizing
• If crystals do not form try
• scratching with glass stir rod
• add a seed crystal
• evaporate some excess solvent

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Step 6

• collect crystals
• vacuum filtration using Buchner Funnel

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Step 7

• dry
• leave on vacuum filter
• leave crystals in funnel open in desk
• heat under a heat lamp take care
• Review Steps 2 6 video

http//server1.fandm.edu/departments/chemistry/Van
20Arman20Virtual20Lab/Recrystallization/recryst
allization1.mov
12
Percent Recovery

• evaluates recrystallization quantitatively
• not the same as a percent yield
• do not forget to record weight of unknown

13
Melting Points

• temperature where solid liquid phases in
  equilibrium
• characteristic physical property of solid
• the identity of an organic solid
• the purity of an organic solid
• most organic solids melt 25 - 250 oC

14
Melting Points

• Organic molecules within the solid are held
  together
• by intermolecular forces
• Hydrogen bonding 2 10 kcal/mol
• Dipole-dipole interactions 0.5 - 2 kcal/mol
• Van Der Waals forces (temporary dipole) lt 0.5
  kcal/mol
• compare to a intramolecular covalent bond 100
  kcal/mol

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Melting Points

• Consider the effects of intermolecular forces on
  the melting point of three organic compounds of
  similar mass

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Hydrogen Bonding
Benzoic acid melting point 122-3 oC
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Dipole-Dipole
Ethyl benzoate, melting point -23 oC
Dipole-dipole interactions are weaker. Ethyl
benzoate is heavier than benzoic acid, yet its
melting point is below room temperature, almost
150o below benzoic acid.
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Van Der Waals
Ethyl benzene, melting Point -95 oC
Temporary dipoles are the weakest form of
intermolecular force. Solids held together by
these forces have low melting points.
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Melting Point Determination

• The energy (heat) required to break these
  intermolecular interactions is the same for any
  two molecules within the solid
• pure organic compounds have a distinct melting
  point that can be used for identification
• melting point should also occur over a very
  narrow range of temperatures
• impurity in the solid disrupts intermolecular
  forces takes less energy to melt solid
  therefore lowering and broadening melting point
  range

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Pure Solid
The Van Der Waals forces that hold the solid
together are regular are the same from molecule
to molecule Pure naphthalene melts at 82-83oC
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Impure Solid
cyclohexane contaminated naphthalene, mp 62-69oC
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Melting Point Determination
Problem Suppose you are cleaning the chemical
stockroom and you encounter a reagent bottle
whose label has decomposed. You suspect that the
compound could be either benzoic acid (mp 122-3
oC) or succinimide (mp 123-5 oC). You take a
melting point, and sure enough the unknown melts
at 123 oC. How would you use what you have
learned to determine the true identity of the
contents of the bottle?
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Mixed Melting Point

• Solution
• obtain known samples of benzoic acid
  succinimide
• mix each of the known samples with a portion of
  the unknown
• determine the melting point for each mixed sample

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Thomas-Hoover

• silicone oil bath
• five samples simultaneously
• oil slow to respond to temperature changes so a
  slower, better melting point can be obtained
• silicone oil has a temp limit 200 C
• good choice for organics that melt between 20-200
  C

25
Mel-Temp

• uses a heated aluminum block
• three samples simultaneously
• aluminum is quick to respond to temperature
  changes higher temps can be achieved more
  quickly
• advantage in determining high melting points
• good choice for organics that melt over 200 C

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MP Sample Size
Use the minimum amount of sample slowest
heating rate to obtain the sharpest, most
accurate melting point.
Sample size samples placed in a mp capillary
tube. Use the minimum amount seen through the
magnifier (1-2 mm)
Heating Rate quickly go to 20 C below the
expected MP, then slow to 1-2 C per minute to
observe the correct MP
Melting Point Video
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Introduction

• Brief Paragraph (2-5 sentences)
• interesting background information
• goals/objective
• reaction (if applicable)
• proper reference
• Do not include
• explanation
• procedure

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Conclusion

• Brief Paragraph (2-3 sentences)
• statement summarizing discussion
• must include conclusion
• Typed, 12 point black font, double spaced
• grammar, spelling, etc.

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Notebook

• Chapter 3 in Mohrig
• table of contents (leave room)
• numbered pages (no pages torn out)
• Prelab
• title, date
• purpose including balanced reaction (if
  applicable)
• chemical data table
• do not write out the procedure (outline if
  helpful)
• Observations
• in class notes recorded in ink
• calculations
• For more information see the Notebook Handout

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Questions?

• Comments?