DUE DATES
• Today
– At beginning of lab -- Melting Pt. and Ref. Index Report
– At end of lab -- copy of laboratory notebook pages for
today's experiment (do not turn in your pre-lab notes or in-
class notes)
• Next Week
– Nothing. I will return M.pt./RI reports.
– There is a big reading assignment.
• Two Weeks
– At beginning of lab -- Crystallization Report
Just Ahead
Next Week
– We will meet in SUB 2, rooms 1&2 for a
long lecture (and quiz).
Today
• You can look at your M.pt./RI notebook copy, but you
cannot keep it. Please return it to me today.
– Make a note of errors you made on your own notebook copy. This is
part I of your report. The number at the top right are the points
that will be deducted from the whole report (100 pts.)
– There is a grading scheme for part I on my bench. Please look at it
so you will understand how it was graded.
• The quiz (and all future quizzes) are a maximum of 50
points.
– Notice that most of the quiz questions came directly from the
assigned reading.
– #5 was not graded.
In Lab Today
• Crystallization (recrystallization) of a solid
sample.
– Solid compounds synthesized in the Organic
laboratory usually need to be purified (recrystallized).
• Next week a melting point range will be
measured to ascertain the purity of the
isolated compound.
An Ideal Recrystallization Solvent Should:
• dissolve all of the solid when the solvent is hot
(boiling).
• dissolve none of the solid when the solvent is at
room temperature.
• have different solubilities for the solid and the
impurities.
• have a lower boiling point than the melting point
of the solid.
• have a fairly low boiling point
Steps in Recrystallization
1. Dissolve crude solid in minimum amount of hot solvent.
2. Let solution cool to room temperature, so that crystal
lattice reforms slowly.
3. Cool solution in an ice bath for further crystallization.
4. Vacuum filter the pure solid, leaving impurities in
solution. Some of the original substance will also
remain dissolved (cannot achieve 100% recovery).
5. Let purified solid air dry in a beaker to remove traces of
solvent.
Recrystallization
1. Add hot solvent until solid
dissolves: (use minimum
amount of hot solvent)
Recrystallization
2. Let solution cool slowly to room
temperature, so crystal lattice
can form.
3. Cool in ice bath to make
sure that the maximum amount of
solid has crystallized.
Recrystallization
4. Vacuum filter solid away
from impurities.
5. Let crystals air dry in a beaker
covered by a watch glass in your
drawer until next week.
Notes
• See Pavia, p. 683 for pictures of crystallization
steps and p. 688 for list of steps.
– Use hot water in a large beaker for heating methanol solvent and
solution.
• Add boiling stones for smooth boiling.
– If you forget to add them in the beginning, NEVER add stones to
a hot solution. Cool solution first.
• Get ice from the stock room to make an ice bath.
– Use the ice bath to cool solvents for washing crystals,
and to cool solutions below room temperature.
Calculating the approximate amount of
solvent to use at the boiling point:
• 4-Methoxybenzoic acid has the highest solubility
in the MeOH-soluble list at the b.pt., 5g/100ml.
• If your unknown sample weighs 2.5 g, it will
require
5g 2.5 g
100ml X ml
Notebook Keeping
• Quantities of all substances used.
• Keep a running total of solvent added.
• Estimate quantity if solvent has
evaporated during heating.
• Write your procedure as you do it.
• Please space your writing to make it
readable!