What is the definition of a reflux? What happens to the solvent during a reflux

Question

What is the definition of a reflux? What happens to the solvent during a reflux experiment? What are three characteristics of a "good" solvent to use for refluxing? You are asked to reflux a solution for one hour. At what point does the reflux time start? What is the temperature of the solution during the reflux? Should you report a melting point as a single number or a range? How are the high and low endpoints of the range determined experimentally during the melting point experiment? What is the relationship between the purity of a compound and its melting point range? Briefly describe how a rotary evaporator works. What is likely to happen if cooling water is not run through the condenser when solvent is vaporized using a rotary evaporator? That is, where is the solvent likely to end up?

Explanation / Answer

2)
The highest temperature attainable in an open vessel is limited by the boiling point of the reaction solvent. Thus, for reactions that require high temperature, it is often necessary to boil the reaction for extended periods, a technique that requires special provisions. To keep the reaction from boiling dry, the contents are placed in a round-bottom flask, to which is attached a West condenser cooled by tap water; the reaction flask is then warmed by means of a heating mantle. As the solvent vapour rises into the condenser it is cooled and converted back to liquid form, at which point it flows back into the reaction vessel. This technique is known as reflux.

3)
I. The solvent should not decompose during the course of reaction
II. A good solvent should be able to easily solvate the reactants
III. A good solvent should not react with the reactants.

4)
Melting point of a compound should be reported in range (which does not exceed 0-5°C), the difference between the temperature at which the collapse of the crystals is first observed and the temperature at which the sample becomes completely liquid. The presence of impurities will usually show a variation which is so noticeable, usually produces a marked increase in the melting point range and causes the commencement of melting to occur at a temperature lower than the melting point of the pure substance. The melting point is therefore a valuable criterion of purity for an organic compound. A sharp melting point is usually indicative of the high purity of a substance.

5)
A rotary evaporator is a specially designed instrument for the evaporation of solvent (single-stage or straight distillation) under vacuum. The evaporator consists of a heating bath with a rotating flask, in which the liquid is distributed as a thin film over the hot wall surfaces and can evaporate easily. The evaporation rate is regulated by the heating bath temperature, the size of flask, the pressure of distillation and the speed of rotation. With a vacuum rotary evaporator you can carry out single-stage distillation runs quickly and gently. The evaporation capacity of a rotary evaporator is about 4 times greater than that of a conventional, static distillation apparatus. Heat transmission in the heating bath as well as inside the flask is greatly improved by rotation of the evaporating flask. Rotation greatly enlarges the active surface inside the flask, hastening the evaporation. With the liquid remaining at any one point of the flask wall for a short time only, it is subject to minimum stress during the distillation (no overheating, no incrustation). Bumping and foaming are greatly reduced by the rotation.

If cooling water is not run through the condenser, the solvent vapour will directly reach the vacuum pump.

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