Drying agents, 25

Drying a reagent, solvent, or product is a task that must be performed at some stage of nearly every reaction conducted in the organic chemistry laboratory. The techniques of drying solids and liquids are described in this and the following sections. 

Some commonly used drying agents and their properties are listed in Table 2.1. These desiccants function in one of two ways (1) the dr)dng agent interacts reversibly with water by the process of adsorption or absorption (Eq. 2.1), or (2) it reacts irreversibly with water by serving as an acid or a base. 

Drying Agent Acid-Base Properties Comments  

Na2S04 Neutral Inexpensive, high capacity relatively slow action and low efficiency good general preliminary drying agent preferred physical form is that of small granules, which may be easily separated from the dry solution by decantation or filtration. 

H2SO4 Acidic Good for alkyl halides and aliphatic hydrocarbons cannot be used with even such weak bases as alkenes and ethers high efficiency. 

In most cases purification of a solvent simply involves drying and distilling it, so the next sections are devoted to drying agents and methods used for drying common solvents, and the final section contains descriptions of typical continuous stills. Apart from water the only other commonly encountered contaminant is peroxidic material formed by aerial oxidation of ethereal solvents. Methods for dealing with these dangerous impurities are described in the section on the purification of diethyl ether. 

When the solvent is to be distilled after standing over a desiccant, the drying agent should be filtered off before distillation if it removes water reversibly, e.g. by hydrate formation (MgSOa, CaCh), or by absorption (molecular sieves). The solvent can be distilled without removal of the desiccant in cases where water removal is irreversible (CaH2, P2O5). 

The recommendations in this and the following section are largely based on the work of Burfield and Smithers et al. who have carried out quantitative studies on the efficiency of drying agents for a wide range of solvents. Their work has supplanted earlier studies many of which are of doubtful reliability. Other useful sources of information include Perrin and Armarego,2 and Riddick, Bunger, and Sakano.  

Neutral or basic alumina of activity grade I is an efficient drying agent for hydrocarbons, 5% w/v loading giving extremely dry solvents. It is also useful for the purification of chloroform and removal of peroxides. 

The commercially available anhydrous product is an inexpensive drying agent which is useful for amines and pyridines (30-50ppm after standing for 24h over 5% w/v). It is strongly basic and is ineffective for alcohols and dipolar aprotic solvents. 

Calcium sulfate (CaS04) Lithium aluminium hydride (IiAlH4) 

Hydrocarbons, alkyl halides, ethers, many esters 

Hydrocarbons, ethers, amines, esters, higher alcohols ( C4) 

Very high capacity very fast reactivated by heating Slow but efficient not suitable for compounds sensitive to strong base 

Not very efficient good for predrying not suitable for most nitrogen and oxygen compounds Not suitable for aldehydes and ketones 

---

After drying over quicklime, calcium oxide CaO, the ammonia is collected by upward delivery. (N.B. Both of the common drying agents, calcium chloride and concentrated sulphuric acid, combine with the gas.)... 

Sometimes used in place of potassium hy- droxide for amines, etc., when a strongly alkaline drying agent is to be avoided. 

A neutral drying agent, which, like sodium sulphate, can be used on most occasions. Its drying action is more rapid than that of sodium sulphate. 

The ethereal extracts are then united, dried with a suitable drying agent and filtered. The filtrate is then cautiously distilled, the ether being first distilled and finally the organic compound if volatile if the compound is solid, the crude residue is purified by recrystallisation. Very great care must be taken on all occasions when ether is distilled because of the risk of fire or of an explosion full experimental details for this operation are given, both on p. 8o (Preparation of Ether) and on p. 164 (Pre-... 

Drying agents may be divided broadly into (a) those which combine with water reversibly and (6) tho.se which react chemically with water by a non-revcrsible process giving rise to a new water-free compound. Sodium, calcium carbide and phosphorus peiitoxide belong to the latter class and wih be discussed in Section 11,39. 

To appreciate the action of a drying agent of class (a), let us imagine some anhydrous copper sulphate in an evacuated vessel provided with a pressure gauge, and water is allowed to enter slowly the temperature is assumed constant at 25°. The results may be best expressed by means of a vapour pressure - composition diagram (Fig. 7, 20, 1). The initial system is represented by the point A the pressure will rise along AB until the monohydrate CuS04,H20 commences to form at B. 

We may now understand the nature of the change which occurs when an anhydrous salt, say copper sulphate, is shaken with a wet organic solvent, such as benzene, at about 25°. The water will first combine to form the monohydrate in accordance with equation (i), and, provided suflScient anhydrous copper sulphate is employed, the effective concentration of water in the solvent is reduced to a value equivalent to about 1 mm. of ordinary water vapour. The complete removal of water is impossible indeed, the equilibrium vapour pressures of the least hydrated tem may be taken as a rough measure of the relative efficiencies of such drying agents. If the water present is more than sufficient to convert the anhydrous copper sulphate into the monohydrate, then reaction (i) will be followed by reaction (ii), i.e., the trihydrate will be formed the water vapour then remaining will be equivalent to about 6 mm. of ordinary water vapour. Thus the monohydrate is far less effective than the anhydrous compound for the removal of water. 

Magnesium sulphate is a useful drying agent as can be seen from the following table of equUibrium vapour pressures of the various possible tems at 25° ... 

Another valuable drying agent of general application is anhydrous calcium sulphate, marketed under the name of Drierite. It is rapid in its action, but has only a limited drying capacity because it forms the hydrate 2CaS04,H20 and thus combines with only 6-6 per cent, of its weight of water. The vapour pressure of the system ... 

It follows that liquids of high boiling point should not be distilled from drying agent systems which have appreciable vapour pressures. An extreme case of this action is the dehydration of oxalic acid dihydrate by distillation over toluene or over carbon tetrachloride. 

The theory of the action of drying agents has been considered in Section 1,20. We are now concerned with the practical methods for the removal of water from organic solids and liquids and from solutions of... 

Liquids or solutions of organic substances in organic solvents are usually dried by direct contact with a solid inorganic drying agent. The... 

A list of the common drying agents with their practical limitations and their important applications follows. 

Aluminium oxide. The commercial material, activated alumina, is made from aluminium hydroxide it will absorb 15-20 per cent, of its weight of water, can be re-activated by heating at 175° for about seven hours, and does not appreciably deteriorate with repeated use. Its main application is as a drying agent for desiccators. 

Absorbent cotton (cotton wool). This material is an excellent drying agent for use in the so-called calcium chloride tubes, i.e., drying tubes, placed at the top of dropping funnels, reflux condensers, etc., to exclude moisture. It is more convenient than calcium chloride, and should preferably be dried in an oven at 100° before use. 

The common drying agents that are suitable for various classes of organic compounds are listed in Table II, 39. 

Calcium chloride cannot be used to dry the ethereal solution because it combines with aniline (and other amines) to form molecular compounds. The best drying agent is sodium or potassium hydroxide (pellet form). 

You see that vacuum adapter stuck to the top of the condenser in fig. 7a Well, a closer look at it in fig. 7b will show that it has some drying agent sandwiched between two cotton balls and the nipple (tee heel) sealed with plastic wrap or foil. The drying agent can be either a commercial product called Drierite or calcium chloride. This attachment is placed on top of a condenser when refluxing solutions that have no water in them and must remain that way during the time they are refluxed. All this is to prevent moisture in the outside air from coming into contact with the cold surface of the of the inside walls of the condenser. This will surely happen and the condensed outside-air water will drip down into the reaction flask and ruin the experiment. This is not so much a... 

Stage one - the reaction happens in the form of a reflux with a dry set-up. You will need a 2000ml reaction flask, a condenser and a drying tube packed with calcium chloride or other drying agent. The reaction needs to be stirred in a big way, so before we add the ingredients make sure you have a clean stirrer bar ready, I prefer the eggy shape bars as they tend to be less noisy. 

---