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Dichloromethane is an irritant and is toxic.

62

Experimental

1. Boiling-Point Determination of an Unknown

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Safety

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.

Procedure

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

Introduction

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.

Safety

α-Pinene is a flammable irritant.

Sand baths can be very hot when in use.

Handle thermometers carefully.

 

 

63

Procedure

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.

 

 

64

Notes

1 Water freed from all dissolved air has been superheated to 137 oC at 1 atmosphere before

it boiled.

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.

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