1. Boiling-Point Determination of an Unknown
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Dichloromethane is an irritant and is toxic.
Toluene is a flammable liquid and is toxic.
n-Hexane is a flammable liquid and an irritant.
Methylcyclohexane is a flammable liquid and an irritant.
2-Propanol is a flammable liquid and an irritant.
All flammable materials must be kept away from microburner flames.
Mel-Temp machines can be very hot when in use.
Handle thermometers carefully.
Prepare several “glass bells,” as described on pages 59–60 of this manual, by sealing one end of the
short, pre-cut (0.4 mm, i.d.) capillary tubes issued to you by your instructor. Then follow the ultramicro
boiling-point procedure to determine the boiling point of your unknown. Compare the observed value to
the literature values for possible unknowns (dichloromethane, n-hexane, methylcyclohexane, 2-propanol or
toluene). Run the standards side-by-side. Make a tentative assignment based upon the boiling-point data.
2. Simple Distillation of α-Pinene
α-Pinene is a monoterpene that is found in many volatile oils. Oil of turpentine contains
approximately 60% of α-pinene and 30% of the regioisomeric alkene, β-pinene. α-Pinene possesses a
trisubstituted C-C double bond and is a bicyclo [3.1.1] alkene.
α-Pinene is a flammable irritant.
Sand baths can be very hot when in use.
Handle thermometers carefully.
Weigh out a 1.0-g sample of impure α-pinene. Add the sample to a 3-mL conical vial equipped
with a spin vane. Then assemble a Hickman still for microscale simple distillation, as discussed in the
section, “Microscale Simple Distillation with a Hickman Still”, on pages 60–61 of this manual. Position
the conical vial deep in the sand bath and cover both the vial and sand bath with a piece of aluminum foil to
obtain efficient heat transfer.
Turn the heat control of the hotplate to an intermediate setting and leave it there until boiling
commences. Observe and record the temperature of the still-head thermometer when the first few drops of
distillate collect in the collar of the Hickman still-head and at intervals thereafter until the distillation is
judged complete. Adjust the heat during the distillation to maintain a uniform distillation rate. This may
require a gradual increase in the amount of heat supplied (even when the temperature at the still-head
thermometer bulb is relatively constant), because the residue in the conical vial is becoming gradually
enriched with the less-volatile components. Whenever the Hickman still-head fills with distillate, remove
some of the distilled material with a long Pasteur pipet and place this sample into a labeled, tared
(preweighed), collection vial. Continue the distillation until it is judged complete. Remember that the
distillation must be discontinued before the residue in the conical vial boils completely dry, to avoid the
danger of superheating and possibly cracking the glass. When the distillation is complete, turn off the
heater and allow the apparatus to cool before disassembling and cleaning the equipment.
Percent Recovery and Characterization
Weigh the distilled material in the collection vial. Then calculate the mass recovery, based upon the
initial mass of impure α-pinene.
Read Appendix V which describes the use of an Abbé refractometer. Then use an Abbé
refractometer to observe the refractive index of the distilled material. Correct the refractive index reading to
the literature conditions. (The correction factor is 0.00045/oC, the refractive index decreasing as the
temperature rises.) Compare this value and the observed boiling-point range to the corresponding literature
values for α-pinene.
Submit the remainder of the distilled α-pinene in a properly labeled, sealed vial with the report.
1 Water freed from all dissolved air has been superheated to 137 oC at 1 atmosphere before
2 The comparison being made between compounds of approximately the same molecular weight.
3 The packing may consist of any of a wide variety of inert materials with extensive surface area:
glass beads or helices, metal helices, carborundum or porcelain stones, etc.
4 Such a column is said to have several hundred theoretical plates. The equivalent of a single
simple distillation for the first small portion of distillate is called a theoretical plate. Thus,
a column which will effect a separation equivalent to ten single distillations is said to have
ten theoretical plates.
The efficiency of a column is measured by its height equivalent to a theoretical plate (HETP),
i.e., the column length required to provide one theoretical plate. The HETP of a given column
is obtained by dividing the length of the column by the number of theoretical plates.
For example, a certain wrapped Vigreux column that is 40 cm in length has 4 theoretical plates.
Its HETP is therefore 10 cm. Many types of packed and externally-heated columns are
considerably more efficient, having HETP values of less than 1 cm.
A column with four theoretical plates will provide good separation of liquids whose boiling
points differ by 40 oC or more. A column with 100 theoretical plates can separate liquids
whose boiling points differ by only 2 oC.