Non Aqueous Solvents

Solvents and Reactions in Non-Aqueous Solvents

1. Physical Properties of Solvents

Property Description Effect on Reaction

Physical State Solid, Liquid, or Gas at RT Determines handling conditions

Boiling Point (BP)         Temperature at which solvent boils Determines the reaction temperature range

Melting Point (MP) Freezing point of solvent Reaction at low temp feasibility

Polarity Measure of separation of charges Solubility of polar/non-polar solutes

Dielectric Constant (ε) Ability to reduce electrostatic forces Higher ε favours ionic solubility

Viscosity Resistance to flow Influence reaction kinetics

Conductivity         Ability to conduct ions Ionic reactions require conducting solvents

2. Types of Solvents and General Characteristics

(a) Polar Protic Solvents

• Capable of hydrogen bonding

• Solvate both cations and anions

• Examples: Water (H2O), Ammonia (NH3), Alcohols (ROH)

(b) Polar Aprotic Solvents

• Solvate only cations (poor H-bonding ability)

• Do not donate protons

• Examples: SO2, Acetone, Acetonitrile (CH3CN), DMSO

(c) Non-Polar Solvents

• No significant polarity

• Dissolve non-polar substances

• Examples: Benzene (C6H6), Carbon tetrachloride (CCl4)

(d) Acidic/Basic/Amphiprotic Solvents

• Solvents having a strong tendency to release protons (H+ ions) are called Acid Solvents. For Example, Liquid  HF, H2SO4, CH3COOH, etc.

• Solvents which possess a strong affinity for protons are called Basic Solvents. For Example Liquid NH3, pyridine, hydrazine, etc.

• Amphiprotic Solvents are those that neither have a strong tendency to gain nor a strong tendency to lose protons. For Example, water, methanol, ethanol, etc.

 (e)Ionising and Non-ionising Solvents

• Ionising Solvents are those which are capable of undergoing auto or self-ionisation. E.g. water,  Liquid ammonia, liquid sulfur dioxide etc.

•  Solvents which do not ionise at all are non-ionising solvents. Non-ionising solvents have low  Dielectric constant and are non-polar. e.g. hexane.

3.  Liquid Ammonia (NH₃)

Properties of Liquid Ammonia. 

•It is a protic solvent and finds application as a solvent to execute varied organic and inorganic reactions.

•Its freezing point (-77.5 oC) and boiling point (-33.5 oC) are much lower than water because of its relatively weak association through hydrogen bonding.

•It is a polar solvent molecule.

•It auto-ionises in the manner similar to water.

•However, the extent of autoionisation of liquid ammonia is much less than that of water. (Kw = 1.0 x 10-14, Kb = 1.9 x 10-33).

•It can conduct electricity only to a feeble extent.

• High Ionizing Power

• Auto-ionization:
  2NH3​⇌NH4+​+NH2−​

• The dielectric constant of liquid ammonia (22) is much smaller in comparison to water (78.4).

• Because of the low dielectric constant, ammonia possesses a weak ability to dissolve ionic compounds.

• Hence, liquid ammonia is a poor solvent for ionic substances.

• Low viscosity of liquid ammonia (0.254 centipoises at -33.5 oC) compared to water  (0.959 centipoises at 25 oC) is expected to promote greater ionic mobilities and thereby  compensate to some extent the effect of the comparatively lower dielectric constant

Reactions in Liquid NH₃

(i) Acid-Base Reactions

• Stronger acids donate a proton to NH3, forming NH4+

• Example:
  NH3​+HCl→NH4​Cl

(ii) Precipitation Reactions: Precipitation reactions normally involve double decomposition. Some of the important precipitation reactions in liquid ammonia are-

(a) In liquid ammonia, precipitation of potassium chloride is obtained by the reaction of AgCl and KNO3

(b) White precipitate of BaCl2 is produced when solutions of silver chloride and barium nitrate in liquid ammonia are brought together.

 (c) Bromides get precipitated when solutions of various metal nitrates and ammonium bromides in liquid ammonia are mixed together.

 (ii) Ammonolysis Reactions

• Replacement of halide group by NH2 group

• Example:
  RCl+2NH3→RNH2+NH4Cl

(iii) Metal-Ammonia Solutions (Blue Solutions)

• Alkali metals dissolve in liquid NH3, forming solvated electrons

• Example:
  Na + (x)NH3​ → Na+ (NH3​)x​ + e−(NH3​)y​

• Characteristic Blue Color due to solvated electrons

• Used in reduction reactions

(iv) Oxidation Reactions

Oxidising action of various oxidising agents is weaker in liquid ammonia than in aqueous solutions.  For instance, HNO3 in liquid ammonia does not act as an oxidising agent.

 Similarly, KMnO4 in liquid ammonia acts as a very weak oxidising agent. It reduces the solution of potassium in liquid ammonia to K2MnO4 and finally to MnO.

 (v)Reduction Reactions

 Liquid ammonia serves as an excellent medium for reduction reactions involving inorganic  Species. For instance, alkali metals dissolve in liquid ammonia, giving blue-coloured solutions.  For instance, sodium metal in liquid ammonia reduces CuI to Cu.

 (vi) Solubility of Substances

•Poor solvent for ionic substances.

•Amongst inorganic compounds, nitrates, thiocyanates, perchlorates, and most of the cyanides are soluble in liquid ammonia.

•Oxides, hydroxides, carbonates, phosphates, sulphate, and most of the sulphides are insoluble.

•Most of the iodides are soluble and bromides are less soluble. Fluorides and chlorides (except•Be2+ and Na+ chlorides) are practically insoluble.

•Amongst organic compounds, halogen compounds, alcohols, ketones, esters, simple ethers,  phenol, and its derivatives are soluble.

•Aromatic hydrocarbons are sparingly soluble. Alkanes are insoluble, and alkenes alkynes are  Slightly soluble.

•Amongst the elements, metals like Mg, Al, Ca, La, etc. have low solubilities in liquid ammonia.

•Alkali and alkaline earth metals are highly soluble in liquid ammonia.

 Advantages of Liquid Ammonia:

•Dissolution of alkali metals in liquid ammonia without chemical reaction is the greatest advantage of using liquid ammonia as a solvent. The dissolved alkali metal can be recovered  From the solution by evaporation.

•The alkali metal solutions in liquid ammonia are strong reducing agents, even stronger than  hydrogen.

•It can be used in preparative chemistry,i.e., can be used to precipitate sulphides, halides,  sulphates and alcoholates.

 Limitations of Liquid Ammonia as a Solvent:

•Low temperature or high pressure is required while working with liquid ammonia. (liquid  for liquid ammonia = -33.5 oC to -77.5 oC.

•Hygroscopic in nature. Hence, all the reactions are to be carried out in sealed tubes.  Offensive odour.

4. Liquid Sulphur Dioxide (SO₂)

Properties of Liquid SO₂

• Under normal temperature and pressure, liquid sulphur dioxide is a gas

• The freezing point (-75.5 oC) and boiling point (-10.1 oC) of liquid sulphur dioxide, and hence can serve as a good solvent.

• Its dielectric constant is small (17.4 at -20 oC), which makes it a good solvent for covalent compounds but a poor solvent for ionic compounds.

• Weak Ionizing Power

• Auto-ionization:
  2SO2⇌SO2+ + SO3−

Reactions in Liquid SO₂

(i) Acid-Base Reactions

In sulphur dioxide, the process of neutralisation is observed to be similar to that found in the case of an aqueous solution. The process of neutralisation in liquid sulphur dioxide involves a combination of SO2+ and SO 2-   ions to form un-ionised SO2. Thus, all compounds containing or making available SO 2- ions in liquid sulphur dioxide are termed Bases, and those compounds containing or making SO2+ ions are termed Acids. E.g.

(i) Reaction of thionyl chloride and caesium sulphate gives a neutralisation reaction

(ii) Reaction between thionyl thiocyanate and potassium sulphite.

• Lewis acid-base type reactions

• Example:
  Cl2 + 2SO2→SOCl+  + SO3Cl−

(ii) Precipitation Reactions

• Used for gravimetric analysis

• Example:
  BaCl2  + SO32−    →  BaSO3↓  + 2Cl−

(iii) Redox Reactions

(i) Oxidation of Iodine

SO2  +  I2  + 2H2O →  2HI + H2SO4

(ii) Reduction of Manganese Dioxide

MnO2+SO2→MnSO4