2. Background Information
Melting point: temperature at which solid is in equilibrium with its
liquid
Melting point range: span of temperatures from the point at which
the crystals first begin to liquefy to the point at which the entire
sample is liquid
Pure compounds usually melt over a narrow temperature range of 1-
2°C
Presence of soluble impurity almost always decreases and
broadens the melting point range
3. Melting Point of Mixture (X and Y)
Eutectic point
Eutectic temperature: lowest possible melting point for a mixture of
2 compounds
Eutectic composition: composition at which both solid X and solid
Y are in equilibrium with the liquid mixture (at eutectic temperature)
4. Taking a Melting Point: Packing
Pack the capillary tube by pressing the The solid should fill the tube to a
open end gently into a sample of the depth of ~2 mm. Tap the bottom of
compound to be analyzed. the capillary on a hard surface so
that the crystals pack down into the
(Capillary tubes break easily!)
bottom of the tube.
5. Melting Point Apparatus: Mel-Temp
Pack the crystals at the bottom of the Watch through eyepiece and record
tube and place the tube in the slot temperature at which the crystals
behind the eye-piece on the Mel-Temp. first begin to liquefy and record
Turn on the Mel-Temp and turn the temperature at which the entire
knob to adjust heating rate (not too fast sample is liquid (mp range).
or too slow).
6. Experiment: Part I - Practice Melting Range
Use only benzoic acid (mp = 122-123°C).
Use proper size of sample (not more than 2 mm in capillary) just
enough to see!!!
Do not move the Mel-Temp to your bench or remove the probe.
Increase Temperature by 10°C/min until 10-15°C below the
known mp range, then reduce rate to 1-2°C/min.
Allow mp apparatus to cool down naturally after experiment (no
wet paper towels or ice – may use air)
7. Experiment: Part III - Identifying Your Unknown
Record the code of your unknown.
Take 2 melting points (2 different samples):
1st: taken quickly to approximate range (~10°C/min)
2nd: taken slower (1-2°C/min) when 10°C below mp range approximated
from 1st run
Choose the known compound that could match the mp of your unknown
(table 1 - p. 39).
Mix known vs unknown on a watch glass (1:1 mixture) thoroughly by
grinding with a spatula.
Take mp of the mixture:
>If mp is the same as mp of unknown, unknown is identified.
>If mp of mixture is different from mp of unknown, repeat process with
another compound.
8. Part III - Identifying Your Unknown
Unknown mp = 133-135°C
⇒ Benzoin (mp = 135-137°C) or trans-Cinnamic Acid (mp = 133-134°C)
Mix Unknown and Benzoin
mp = 135-137°C mp <<133°C
⇒ Unknown is Benzoin ⇒ Unknown is not Benzoin
Mix Unknown and trans-Cinnamic Acid
mp <<133°C mp = 133-134°C
⇒ Unknown is not trans-Cinnamic Acid ⇒ Unknown is trans-Cinnamic
Acid
9. Waste Handling
Don’t discard capillaries in the trash. Put them in the collection
beaker labeled “3221 Melting Capillaries”. No glass in the trash!!
Any unused unknown is to be returned to the TA in the original
sample vial (properly capped).
11. Definition
Recrystallization: purification process used to remove
impurities from organic compounds which are solid at room
temperature.
Principle:
Solubility of a compound in a solvent increases with temperature.
As the solution cools, crystals form and grow.
Molecules in a crystal have a greater affinity for molecules of the
same kind than for impurities.
12. Choosing a Recrystallizing Solvent
The compound being purified must be insoluble in solvent at
room temp but soluble in boiling solvent.
Solvent’s boiling point must be lower than the compound’s
melting point.
An abundant quantity of crystals must be recoverable from the
cool solvent.
A B C Note:
Room Not Not 1. Be careful that your
Soluble
Temperature Soluble Soluble compound does not melt before
Solvent’s the solvent boils. Why? –
Soluble Soluble Soluble
Boiling Point Oiling Out
Not Not 2. Make sure that you record
Cold Solvent Soluble
Soluble Soluble your observations in a chart
Mass like the one shown here.
NA 0.85 g 0.63 g
Recovered
13. Step 1 – Choosing the Right Solvent for Acetanilide
Water Ethanol Acetone Petroleum
Ether
(polar) (polar) (polar)
Record whether the acetanilide is soluble or not at room temperature.
Heat the solution(s) to boiling (in a hot water bath). Record whether the
acetanilide is soluble or not in each hot solvent.
Let them cool at room temperature first, and then in an ice water bath.
Record your observations – Does crystal form?.
Choose the appropriate solvent by
consulting with your TA and go to the next step.
14. Step 2 – Dissolving the Compound
Dissolve the compound in a MINIMUM volume of boiling solvent.
After all of the compound dissolves, add 5% excess to prevent
premature crystallization. (For example: if 10 mL of boiling solvent is
required to just dissolve a compound, 0.5 mL will be added)
1- Heat some solvent to 3- Swirl the flask
boiling. Place the solid
to be recrystallized in an
Erlenmeyer flask.
2- Add small amount (1 4- Place the flask on the
mL) of the hot solvent hot plate to keep the
into the flask. solution warm
(Use a Pasteur pipet)
5- Repeat steps 2 to 4 until you have dissolved
the compound, and then add the 5% excess.
15. Step 3 - Decolorizing
Hot & Ready!
Decolorizing the solution
Add a spatula tip of activated carbon after the solution
is removed from the heat and has stopped boiling !!!
Stir in the carbon and then allow it to sit briefly.
Hot Filtration!!!
The hot solution is filtered (in this step) by gravity
filtration through a stemless funnel containing a fluted
filter paper. Keep the paper and funnel warm to avoid
crystals forming during filtration.
Be quick!!! at the hot filtration step to avoid premature
crystallization in the filter.
16. Step 4 – Forming Crystals
Recrystallizing Pure Compound
Slowly cool the solution at room temperature before placing in an ice-
water bath to maximize crystal formation.
TIP: If crystals do not form, induce crystallization yourself:
- scratching the inside of the flask with a glass stirring rod
- or concentrating by evaporation (last resort!).
Prepare ice cold solvent for washing step!
17. Step 5 – Collecting, Washing, and Drying the Crystals
Vacuum filtration: Pre-weigh your filter paper and record mass!
18. Step 6 – Recovery Analysis (Next Week)
Sandwich your crystals between two filter
papers. Let your crystals dry for 1 week
in your lab drawer.
Next Week:
Measure and record the mass and
melting point.
Submit your crystals to the TA in a
labeled vial (your name, compound’s
name, mass of compound, and MP).
mass of recrystallized compound, g
% Recovery = ————————————————— x 100
mass of crude compound, g
