Solvents other than water are called non-aqueous solvents.Most chemical reactions take place in solution, and water is the most commonly used and important solvent.But there are others besides watersolvent, such as liquid ammoniasulphuric acid、carbon tetrachloride, benzene and various non-metallic halides.According to statistics, more than 300 kinds of non-aqueous solvents have been studied.The commonly used nonaqueous solvents can be divided into proton solvents and proton transfer solvents.[1]Non aqueous solvents have properties that water does not have. Different reaction results can be obtained in non aqueous solvents than in water. Many reactions that cannot occur in waterchemical reaction, but can occur in non-aqueous solvents or in the opposite direction;Non aqueous solvents are of great significance in preparing anhydrous salts, preparing some special complexes with abnormal oxidation state, changing the speed of some reactions, improving the process and increasing the yield[2]
The application of nonaqueous solvents has opened up a new research path in the field of chemistry, and has developed into one of the important topics of modern chemistry. Most of the inorganic chemical reactions observed in the laboratory are mainly carried out in aqueous solvents. However, when other solvents are used to replace water, different reaction results can be obtained, and many chemical reactions can not occur in water,It can occur in other solvents or go in the opposite direction, which is not only a problem of great theoretical interest, but also of great significance in industrial and agricultural production. Not only that, organic chemists have long applied non-aqueous solvents to various organic synthesis work, and developed a large number of organic and inorganic non-aqueous solvents, HF, HthreeSOthreeInorganics such as F have been used in organic synthesis for a long time. Biochemists also often use some non-aqueous solvents as the medium for some biochemical reactions. Metallurgists cannot use the aqueous solution of electrolyzing salts to produce metals with high activity. Therefore, electrolysis of molten salts has been developed to produce active metals such as alkali metals, alkaline earth metals and aluminum,The high temperature melt system has developed as a large class of nonaqueous solvents. Analytical chemists use nonaqueous solvent analysis to determine those substances that cannot be determined in aqueous solutions. Nonaqueous solvents are used for inorganic synthesis, mainly to prepare those substances that cannot be prepared in aqueous solutions.In addition, extraction separation technology is also widely used in various nonaqueous solvents. Due to the increasingly widespread application of aqueous solvents, physical chemists have also been prompted to study the properties of solvents and solutions, and have also carried out the measurement and accumulation of related physical and chemical data to guide practice. Therefore, the chemical prospects of applying and studying nonaqueous solvents are very broad[2]
Classification of non-aqueous solvents
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Common nonaqueous solvents can be divided into acid solvents, alkaline solvents and aprotic transfer solvents:
(1) Acidic solvents, also known as proton sparse solvents, are significantly more acidic than water, such as formic acid, acetic acid, sulfuric acid, hydrofluoric acid, trichloroacetic acid, etc.
(2) Alkaline solvents, also known as protophilic solvents, are significantly more alkaline than water, such as ethylenediamine, butylamine, pyridine, liquid ammonia, etc.
(3) Non proton transfer solvents, which do not produce solvated protons, have highDielectric constantThey do not react with the added acid or base. Only when the second acid or base is added, can the balance of A1+B2 ←→ B1+A2 ① be established.Such as hydrocarbons and their halogen derivativesBromine trifluoride、sulfur dioxideDichlorosulfoxide, nitrobenzene, etc.
Note: ① "←→" means reversible symbol.
Common important non-aqueous solvents
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liquid ammonia
Liquid ammonia is much more protophilic than water, and has been studied more as a solvent, which is often regarded as the representative of alkaline solvent.
(1) Acid base reaction in liquid ammonia
The self dissociation reaction of liquid ammonia is:
So the strongest acid and base in the liquid ammonia system is NH+fourIons and NH-twoIon. Since ammonia is more alkaline than water, ammonia is a conjugate acid NH+fourCojugate H far more than waterthreeO+Its acidity is weak, while its conjugate base NH-twoIs much more than the conjugate alkali OH of water-Therefore, substances that are weakly acidic in water are strongly acidic in liquid ammonia. If acetic acid is weak in water, it is strong in liquid ammonia, and can be leveled by liquid ammonia solvent [3] to NH+4. The liquid ammonia solution of ammonium salt reacts with active metals, just as the aqueous solution of acid reacts with more active metals to produce hydrogen.
Alkalis in liquid ammonia can aminate potassium (KNHtwo）Is representative (similar to NaOH and KOH in water), because NaNHtwoThe solubility in liquid ammonia is relatively small, so KNH is often usedtwoReactions with metal ions often produce metal amino compounds, imino compounds or nitride precipitates, which are associated with the metal generated in waterhydroxideThe reaction of precipitation is similar.
(2) Liquid ammonia solution of metals and its reaction
The specialty of liquid ammonia as a solvent is that it can dissolve alkali metals, alkaline earth metals, aluminum and some rare earth and other active metals, and the resulting solutions are all blue. Diluting these solutions has the same absorption spectrum and has nothing to do with the dissolved metals. The ammonia solutions of these metals also have good conductivity. At high concentrations, the solutions are bronze,Its conductivity is close to that of metals. These phenomena have aroused great interest of chemists and a lot of research has been carried out.It is generally believed that:
The first is that in dilute solution, metal atoms are basically dissociated into solvated cations and solvated electrons;
Second, with the increase of metal concentration, ammoniated metal ions tend to combine with ammoniated electrons, forming Mtwo、MthreeThe percentage of unpaired electrons in the solution decreases, and the molar susceptibility of the solution decreases accordingly
Third, in concentrated solution, ammoniated metal ions combine with ammoniated electrons. Like molten metal, its density is lower than that of dilute solution, and its conductivity and magnetic susceptibility are close to that of pure metal, so it is called "diluted metal". In liquid ammonia solution of some metals, there are two immiscible equilibrium liquid phases within a certain temperature and concentration range. The heavier liquid phase is blue, and the concentration of metal is smaller;The lighter liquid phase is bronze, and the concentration of metals is high. This interesting fact has been seen in the liquid ammonia solutions of sodium, potassium, calcium, strontium, barium and other metals. Fourth, the liquid ammonia solutions of all metals are not very stable, and are placed for a long time or in catalysts such as Pt, Fe, FetwoOthreeIt will decompose when it exists.
In a word, the chemistry of liquid ammonia is similar to that of water, and the reaction types in water can be basically seen in liquid ammonia. However, the alkalinity of liquid ammonia is stronger than that of water, and its dielectric constant is smaller than that of water, which makes it different from water. The solution of active metals in liquid ammonia can be used for many reduction reactions, which is the special advantage of liquid ammonia as a solvent.
acetic acid
acetic acid
Acetic acid is a good proton sparse solvent. From the structural formula of acetic acid, its molecule has no symmetric structure, but it is strange that the dipole moment of the compound with asymmetric molecular structure is measured to be equal to zero, which can be seen as the result of two acetic acid molecules forming a dimer with symmetric structure through hydrogen bonding,Thus, the dipole of two molecules is offset. Some solvents can cause the dimer to dissociate and form corresponding solvates, in which the interaction between solute and solvent is mainly the hydrogen bond. Due to this feature and the small dielectric constant, the electrolyte dissolved in acetic acid is not easy to form completely dissociated ions, but easier to form ion pairs.
The distinguishing effect of acetic acid on different acids [8] is much stronger than that of waterHClOfour、HBr、HCl、HNOthreeThe strength of acetic acid in water shows the same strong acid, but there is a significant difference in acid strength in acetic acid, which is caused by the fact that the protophilic property of acetic acid is much weaker than that of water. The leveling effect of acetic acid on alkali is obviously much stronger than that of water, so substances that are strongly alkaline in water will be leveled to Ac in acetic acid-Ions.
Acetic acid is a good solvent for organic compounds, and is widely used as a reaction medium that requires stronger acidity than water but cannot occur in water.[2]
Application progress
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(1) Change the solvent so that some reactions that cannot occur in water can occur to the opposite direction
For example, tin tetraiodide (SnIfour）And urea sodium salt (HtwoN - CO - NHNa) can be hydrolyzed immediately in water, so it can not be prepared and separated in water. However, the former can be prepared by using anhydrous acetic acid or carbon disulfide as solvent and directly reacting iodine and metal tin;The latter can be prepared by the reaction of urea and sodium aminate with liquid ammonia as solvent. The two reactions are:
(2) Preparation of anhydrous salt with non-aqueous solvent
Anhydrous chlorides are often used in molten salt electrolysis to produce active metals. Satisfactory anhydrous chlorides can often be obtained by using thionyl chloride (SOCl2) as the solvent, because water can be removed due to the following reactions:
The preparation of anhydrous nitrates is more difficult. Except that the nitrates of alkali metals and silver are amorphous, almost all nitrates have crystal water. The nitrates of transition metals can hardly be dehydrated by heating to obtain anhydrous salts. It is convenient to prepare some nitrates by using non-aqueous solvents.
(3) Preparation of Some Special Complexes with Abnormal Oxidation State by Non aqueous Solvent
For example, in liquid ammonia, the strong reducibility of metal sodium or potassium can be used to generate the following reactions:
(4) Changing the solvent can change the speed of some reactions, such as reaction:
stayDioxaneIf it is carried out in benzene, the speed can be increased by 80 times, 500 times in acetone and 2800 times in nitrobenzene.[2]
