Technique C

1

• Technique C
• Melting Point
• Exercise F.1
• Recrystallization of an
• Impure Solid

2
Purpose

• Learn how to measure the melting point of organic
  solids.
• Learn how to recrystallize an organic solid.

3
Melting Point

• Physical property
• Help identify an unknown
• Characterize reaction product
• Indicator of a compounds purity
• Melting point range
• temperature at which 1st drop of liquid appears
  to the temperature at which all solid has been
  converted to liquid
• 1 2 C for pure compounds
• ALWAYS report as a range

4
Theory of Melting Point

• Effect of impurities
• Less energy needed to disrupt crystal lattice
  when impurities are present.
• Generally results in a lower melting point with a
  broader range.

Pure solid Tight crystal lattice
Impurities disrupt crystal lattice
5
Melting Point Measurement

• Load sample into a capillary tube
• 2-3 mm of material in tube is sufficient
• Too much material will broaden range may
  increase melting point
• Put capillary tube into Mel-Temp
• Control heating rate
• Heat rapidly until 10-15C from expected m.p.
• Reduce rate to 1-2C/minute
• Refer to heating rate curves in lab

6
Melting Point Measurement

• Data Collection
• Record identity of compounds being tested
• Record melting point range
• Put used capillary tubes in broken glass box
• Never re-melt a sample (inaccurate)

7
Recrystallization

• Method for purifying solid organic compounds
• Used in all organic labs to purify reaction
  products that are solids.

8
Steps in Recrystallization

• Dissolve crude solid in minimum amount of hot
  solvent (saturated solution)
• Let solution cool to room temperature, so that
  crystal lattice reforms slowly.
• Cool solution in an ice bath to minimize
  solubility (maximize recovery).
• Suction filter the pure solid, impurities stay in
  solution.
• Let solid air dry to remove traces of solvent.
• Weigh pure crystals.
• Determine melting point of pure crystals.

9
Solubility Curves
For most compounds solubility increases with
temperature.
Reference www.fofweb.com/Subscription/Science/Ph
ysical/P0812.jpg
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Solvent Characteristics

• Dissolve all of the compound when the solvent is
  hot
• Dissolve very little or none of the compound when
  the solvent is at room temperature
• Have different solubilities for the compound and
  the impurities
• Have a lower boiling point than the melting point
  of the compound
• Have a relatively low boiling point
• Be inexpensive, non-toxic, non-reactive, and
  odorless

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Dissolve Impure Solid
Add hot solvent until solid dissolves (use
minimum amount of hot solvent)
Solid crystal with impurities in it
12
Recrystallize Solid
Let solvent cool slowly to room temperature, so
crystal lattice can reform. Cool in ice bath
to make sure that the maximum amount of solid has
crystallized. (filtrate gets cold!)
13
Isolate Pure Crystals
Suction filter pure crystals to separate from
solvent and impurities.
14
(No Transcript)
15
Pour a small amount of the hot solvent into the
flask containing the solid (or use a Pastuer
pipet to transfer solvent)
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Swirl the flask to dissolve the solid.
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Place the flask on the hot plate to keep the
solution hot.
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If the solid is still not dissolved, add a small
amount more of the hot solvent and swirl again.
Keep solution hot.
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When all of the solid has dissolved, let the
solution cool. Do not disturb it.
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As the solution cools crystals will form in the
flask.
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When the solution has cooled to room temperature,
put it in an ice bath to finish the
crystallization process.
22
Suction Filtration Apparatus.
23
Pre-lab Preparation

• Read Technique C Melting Point
• Not calibrating the Mel-Temp thermometer (p. 24)
• Not doing any of the C exercises (p. 26-27)
• Read Technique F Recrystallization Filtration
• Intro (34), theory (34), solvents (35-38), How to
  do a Microscale Recrystallization (39-40),
  Important tips (42-43), Intro to filtration (43),
  Microscale suction filtration (43-44), Important
  tips (45)
• Also Exercise F.1
• This is where your procedures will come from

24
Pre-lab Preparation

• Prepare Your Notebook before class
• Header info (Title, name, date)
• Purpose (2-3 sentence summary of what you are
  going to do)
• Reference (book title, authors, edition, pages)
• Table of reagents (acetanilide and benzoic acid)
• Formula, structure, MW, MP, water solubility,
  hazards
• Use CRC, Merck, online catalogs and/or MSDS
• Procedures
• Put the planned steps in the left column leave
  plenty of space between steps
• Most of the procedures will come from Exercise
  F.1
• Incorporate the procedural changes at an
  appropriate place
• see next slide

25
Procedural Changes

• There will be known compounds for you to practice
  melting points with. Do at least one of these.
  (This does not need to be in the table of
  reagents.)
• After you get the impure unknown solid from your
  instructor, prepare two MP capillary tubes so you
  can get a melting point of the crude compound
  sometime during the lab period. (The reason you
  need two is in case the first one gets messed
  up.)
• After you filter the unknown crystals, put them
  on a watch glass and set them on the side shelf
  to dry overnight. Make sure your name is on the
  watch glass.
• Sometime before your next lab, come in and weigh
  the dry crystals and get a melting point. (Record
  this in your notebook and on the datasheet you
  will turn in.)

26
In Lab

• Follow your procedures to find the MP range of
  one of the known compounds recrystallize your
  unknown solid and get a MP range
• Known compound things to consider
• Name of known compound(s) used along with
  observed MP range and literature MP (on bottle)
• Unknown compound things to consider
• MP range of the impure, unknown solid before
  recrystallization
• Unknown number
• Weight of impure unknown you started with
• Approximate volume of H2O
• Weight and MP range of the air-dried,
  recrystallized unknown

27
After Lab

• Calculations Section
• Calculate the error in the MP of the known.
• Calculate the recovery of the unknown.
• Calculate the error in the melting point of the
  unknown based on what compound you think it is.
• Results Section
• Identify unknown solid based upon melting point.
• Draw the structure of the compound.

28
Example

• Calculate error of melting points
• A single temperature literature value represents
  the upper temperature of the experimental melting
  point range.
• Example
• Literature value 122.4ºC
• Experimental data 119 - 121ºC

29
Example

• Calculate Percent Recovery