Still solvent

Solvent stills (with continuous still collecting head) 

Large scale reactions and slow addition of reagents 

Reactions above room temperature using a condenser 

The figures (1) through (18) in this section have been adapted fi-om Advanced Practical Organic Chemistry , Blackie Academic and Professional, London. 

The most common and classical distillation set-up usually comprise of a distillation vessel, still-head, thermometer, double-surfaced condenser, receiver-adapter, and a collection vessel. 

There are two main types of solvent still that are commonly employed in organic research laboratories. One is the classical distillation set-up consisting of distillation pot, still-head, thermometer, condenser, receiver-adaptor, and collection vessel. This arrangement is described in more detail in Chapter 11, and is used for the distillation of solvents that are either required infrequently, or that can be stored without deterioration for long periods of time. The other is a continuous still set-up that consists of a distillation pot, collecting head, and condenser (see Fig. 5.1). 

This type of stiU arrangement is used for solvents that are required on a regular basis and the still system is usually left set up, although generally it is only turned on when the solvent is required - it is not recommended that any type of solvent still is left on unattended for prolonged periods of time.  

Continuous still systems typically have an upright arrangement that takes up less space than the conventional stiU set-up and a collecting essel that is positioned between the still-pot and the condenser. The apparatus is 

A typical design for the continuous still collecting head is outlined in Fig. 5.2, and can be simply constructed from a round-bottom flask, ground glass cone, 2-way tap, and 3-way tap. The 2-way tap allows the solvent to be withdrawn via syringe which is particularly convenient for anhydrous solvents. The 3-way tap allows the solvent to be collected, drawn off, or poured back into the distillation pot. Obviously the size of the still depends upon the quantity of solvent required, however the still head should always be smaller in capacity than the still pot, so as to avoid the possibility of the still boiling dry. 

The design of collecting heads can vary, for instance they can also be constructed using a conical flask instead of the round-bottom flask (Fig. 

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Distillation Aqueous or non-aqueous solutions high organic concentrations Recovered solvent still bottom liquids, sludge, and tars... 

An example of such a closed-loop system might include a closed solvent recovery system in which the dirty solvents are piped from the degreasing unit to a solvent still where the solvent is cleaned, and then piped back to the degreasing unit. 

Materials. Styrene (Aldrich) was purified by distillation from CaH2 before use. n-Butyllithium (Aldrich) was used without further purification. Tetrahydrofuran (Fisher Scientific) was purified in a solvent still by distillation from a sodium/benzophenone mixture. Toluene (Fisher Scientific) was used without further purification. Reagent grade methylene chloride (Baker Chemical Co.) was dried on 5A° molecular sieves. Reagent grade triethy-lamine (Baker) was dried over KOH. Methanesulfonyl chloride (Aldrich, 98%) was used without further purification. 

Along with methods to evaluate different pharmaceutical processes and unit operations, several methods have also been developed to evaluate commonly used solvents in the pharmaceutical industry. Solvents still account for a majority of the mass utilization in any pharmaceutical process. Therefore, various methods have been developed which focus on measuring the greenness of solvents, locating possible alternatives and reducing the overall amount of solvent used in any given process. Some of these methods use a combination of physical property data, LCA... 

Although several methods are used to reduce or ehminate solvent consumption within a pharmaceutical process, solvents are often used in excess in order to carry out reachons in a dilute environment because of solubihty and product selectivity issues [2]. As solvents still have a great influence on the quality of the final products, it can be very difficult to find suitable replacements [43]. It is therefore desirable to find solvents for a process that can be easily recovered, separated, and purified for reuse. Spent solvents that are not recovered must be disposed of as wastes, which can be quite costly and add to the environmental burden. 

Extractive Distillation Recovery of Isoprene. A typical flowsketch and material balance of distillation and solvent recovery towers for extracting isoprene from a mixture of cracked products with aqueous acetonitrile appears in Figure 13.26. A description of the flowsheet of a complete plant is given in Example 2.10. In spite of the fact that several trays for washing by reflux are provided, some volatilization of solvent still occurs so that the complete plant... 

Even for a simple reaction, involving just one reactant species and one product species, such as a keto-enol tautomerism or a cis-trans isomerization, Eq. (2.21) for a given solvent is complicated enough, not to speak of a comparison between several solvents. Still, in specific cases it is possible to unravel the solvent effects of cavity formation, if the solute species have different volumes, polarity/polarizability if the solute species differ in their dipole moments or polarizabilities, and solvent Lewis acidity and basicity if the solutes differ in their electron pair and hydrogen bond acceptance abilities. Thus, the enol form has a greater ability than the keto form to accept an electron pair from the solvent to form a hydrogen bond with it, but the keto form may have a larger dipole moment to interact with a polarizable solvent. 

Transfer the solution to a 500 mL round-bottomed flask and remove the solvent on a rotary evaporator with a vacuum pump. To the dry residue add 100 mL of dry toluene and evaporate the solvent, this operation should be repeated twice. Caution The vacuum pump should be protected by an efficient trap containing liquid nitrogen. Caution The collected solvent still contains oxalyl chloride which should be destroyed before discharge. 

Two basic types of solvent still are found in the lab. One is for distillation of solvents for routine use and the other is for distillation of ultra-dry solvents for carrying out reactions under dry conditions. 

If solvents are purchased in bulk (drams) they will normally need to be redistilled even for routine laboratory use. Large stills (about 5 litres) will normally be required for this purpose. Typical solvents requiring such stills are petroleum ether, ethyl acetate, dichloromethane and other solvents according to particular needs. There are many problems associated with routine distillation of all lab solvents stills take up valuable space large stills are hazardous the process is time consuming redistilled solvents are not always available when needed there is always a considerable volume of solvent wasted and disposing of these residues costs money. We have found that it is generally more efficient and cost effective to purchase bottled solvents that are sufficiently pure for routine use without distillation. We... 

Substitution controlled by an activating group yields a mixture of ortho and para isomers nevertheless, we must often make use of such reactions, as in the examples just shown. It is usually possible to obtain the pure para isomer from tht mixture by fractional crystallization. As the more symmetrical isomer, it is tKe less soluble (Sec. 12.3), and crystallizes while the solvent still retarris the soluble... 

The radical anion derived from a ketone is called a ketyl. The dark blue, ster-ically and electronically stabilized benzophenone ketyl is widely used in solvent stills as a deoxygenating agent. 

It will be observed that equation (36.11) is used as the basis of the familiar procedure for the determination of molecular weights by the freezing point method. It is obviously strictly applicable only to very dilute solutions at appreciable concentrations the approximations made in its derivation are no longer justifiable. These are as follows first, that AH/ is independent of temperature second, that ToT is equal to To third, that In ni may be replaced by — n and fourth, that Nj may be set equal to nj/ni. The two latter approximations are avoided in equation (36.8), and this ves somewhat better results than does (36.11) in solutions of moderate concentration, provided the solvent still obeys il oult s law. 

The Hansen method is very valnable. It has fonnd widespread use particularly in the paints and coatings indnstry, where the choice of solvents to meet economical, ecological, and safety constraints is of critical importance. It can explain cases in which polymer and solvent solubility parameters are almost perfectly matched, yet the polymer will not dissolve. The Hansen method can also predict cases where two nonsolvents can be mixed to form a solvent. Still, the method is approximate, it lacks the generality of a Ml thermodynamic model for assessing miscibility, and it requires some experimental measnrements. The determination of R is typically based on visnal observation of solubility (or not) of 0.5 g polymer in 5 cm solvent at room temperature. Given the concentration and the temperature dependence of phase boundaries, such a determination may seem a bit arbitrary. Still the method works out pretty well in practice, probably because the liquid-liquid boundaries for most polymer-solvent systems are fairly flat. ... 

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