Boiling point determination: technique

Weigh the product and calculate the percentage yield of the ester. Determine its boiling point (bp 142°C) using a microscale boiling-point determination (Technique 13, Section 13.2). See the end of Experiment 14B for a spectral analysis. 

It is noteworthy that the use of the earliest analytical techniques for the systematic study of essential oils, such as SG, relative density, optical activity, and refractive index or melting, congealing, and boiling points determinations, is generally applied for the assessment of pure compounds and may be extended to evaluate essential oils composed of a major compound. Classical methods cannot be used as stand alone methods and need to be combined with modern analytical techniques, especially GC, for the assessment of essential oil genuineness. 

Boiling Point Determine the boiling point of the carvone you were assigned. Use the microboiling-point technique (Technique 13, Section 13.2). The boiling points for both carvones are 230°C at atmospheric pressure. Compare your result to that of someone using the other carvone. 

For the boiling point determination, we prefer the Semimicroscale Direct Method described in Technique 13, Section 13.2. The best way to perform this method is to use a digital thermometer with a stainless steel probe (see Technique 13, Section 13.4 and Figure 13.7). 

Boiling-point determination and distillation are two techniques in which an accurate temperature reading may be obtained most easily with a partial immersion thermometer. A common immersion length for this type of thermometer is 76 mm. This length works well for these two techniques because the hot vapors are likely to surround the bottom of the thermometer up to a point fairly close to the immersion line. If a total immersion thermometer is used in these applications, a stem correction, which is described later, must be used to obtain an accurate temperature reading. 

External monitoring of the temperature has the disadvantage that the exact temperature at which liquid distills is never known. In many cases, this does not matter or is unavoidable, and the boiling point of the distilled liquid can be checked later by performing a microboiling-point determination (Technique 13, Section 13.2). 

Because the volume of distillate is small, it is somewhat difficult to measure the distillation temperature (boiling point) accurately at the microscale level. An approximate method is to determine the temperature of the heat source at the point when distillation occurs. This temperature is about 20 °C or so above the distillation temperature, so it is necessary to make the corresponding correction in the boiling point recorded for the distillate. Alternatively, a thermometer can be inserted through the condenser attached to the stillhead so the thermometer bulb is in the lower neck of the stillhead. A more accurate measurement of the boiling point of the distillate is possible using the technique of microscale boiling point determination (Sec. 2.8). 

Place the sample to be anal) ed (1 mg or less) in a 100-jxL conical vial and add a few drops of a solvent to dissolve the sample. Use a capillary miaopipet to apply a small fraction of the solution from the vial to the plate (Fig. 5.35). (These pipets are prepared by the same technique used for constructing the capillary insert for ultramicro boiling-point determinations, see Chapter 4.) Apply the sample to the adsorbent side of the TLC plate by gently touching the tip of the filled capillary to the plate. Remove the tip from the plate before the dot of solvent grows to much more than a few millimeters in diameter. If it turns out that you need to apply more sample, let the dot of solvent evaporate and then reapply more sample to exactly the same spot. 

The specific surface area is usually determined by the BET technique discussed in Section 6.2.2. For the most reliable BET measurements the adsorbate gas molecules should be small, approximately spherical, inert (to avoid chemisorption), and easy to handle at the temperature in question. For economy, nitrogen is the most common choice with measurements usually made at 77 °K, the normal boiling point of liquid nitrogen. Krypton is another material that is frequently employed. 

Colligative properties are dependent on the number of particles present and are thus related to M . M values are independent of molecular size and are highly sensitive to small molecules present in the mixture. Values of are determined by Raoult s techniques, which are dependent on colligative properties such as ebulliometry (boiling point elevation), cryometry (freezing point depression), osmometry, and end-group analysis. 

Measurements of the common physical constants such as boiling point or refractive index are not sufficiently sensitive to determine the trace amounts of impurities in question. Besides the common spectroscopic methods, techniques like gas chromatography (GC), high-pressure liquid chromatography (HPLC), or thin-layer chromatography (TLC) are useful. The surest criterion for the absence of interfering foreign compounds lies in the polymerization itself the purification is repeated until test polymerizations on the course of the reaction under standard conditions are reproducible (conversion-time curve, viscosity number of the polymers). 

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