Solvent

• A solvent is a substance that dissolve a solute, resulting in solution.

• It maybe a liquid, a gas, a solid or a supercritical fluid.

• Solvents find various applications in chemical, pharmaceutical, oil and gas industries,

including in chemical synthesis and purification process.

• Common uses of solvents are in dry cleaning (e.g. tetrachloroethylene), as paint thinners

(e.g., toluene), in spot removers (e.g. hexane), in detergents (e.g. citrus terpene) etc.

• The most common solvent used but living things is water. All the ions and proteins in a cell

are dissolved in water within a cell. On account of its high dielectric constant, it is capable of

reducing forces of electrostatic attraction binding the charged ions in electrolytes in the solid

state. Thus, salts and other electrolytes get dissociated into ions when they dissolved in water.

Classification of Solvents:

The various solvents are generally classified as follows-

1. Protonic and Aprotic solvents:

Solvents from which protons (i.e., H+ ions) can be derived are called as Protonic Solvents.

e.g. H2O, liquid NH3, HF etc.

Solvents from which protons cannot be ordinarily derived are called as Aprotonic Solvents.

e.g. CCl4, C6H6, acetonitrile, etc.

2. Acid Solvents, Basic Solvents and Amphiprotic Solvents:

Solvents which have a strong tendency to give protons are called as Acid Solvents. e.g.

Liquid HF, H2SO4, CH3COOH, etc.

Solvents which have a strong affinity for protons are called as Basic Solvents. E.g. Liquid

NH3, pyridine, hydrazine, etc.

Amphiprotic Solvents are those which neither have a strong tendency to gain nor a strong

tendency to lose protons. E.g. water, methanol, ethanol, etc.

3. 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.

Liquid Ammonia:

• Liquid ammonia is one of the most extensively studied non-aqueous solvent.

• It is a protonic solvent and its water like make it useful solvent for carrying out several types

of organic and inorganic reactions.

• The freezing point (-77.5 oC) and boiling point (-33.5 oC) of liquid ammonia are lower than

water because of less strongly association of hydrogen bonding in liquid ammonia.

• It has a pyramidal structure which make it polar.

• Autoionisation is 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)

• Ammonia can conduct electricity only in feeble extent.

• The dielectric constant of liquid ammonia (22) is much smaller that of water (78.5). The low

dielectric constant results in a generally decreased ability of liquid ammonia to dissolve

ionic compounds. Therefore, liquid ammonia is a poor solvent for ionic substances.

• Low viscosity of liquid ammonia (0.254 centipoise at -33.5 oC) compared to water

(0.959 centipoise at 25 oC) is expected to promote greater ionic mobilities and thereby

compensate to some extent the effect of the comparatively lower dielectric constant.

Chemical Reactions in Liquid Ammonia

(1) Precipitation Reactions:

Precipitation reactions normally involve double decomposition. Some of the important

precipitation reaction 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 solution of various metal nitrates and ammonium

bromides in liquid ammonia are mixed together.

2. Acid-Base Reactions in Liquid Ammonia:

In liquid ammonia, the process of neutrilisation is observed similar to found in case of aqueous

solution.. The process of neutrilisation in liquid ammonia involves combination of NH4+ and

NH2- ions to form un-ionised NH3. For instance, the neutrilisation of ammonium chloride with

potassium amide may be written as

Thus, NH4Cl may be regarded as a strong acid and KNH2 as strong base in liquid ammonia.

They are termed as “Ammono Acids” and “Ammono Bases”

(Neutrilisation)

Reactions of Ammono Acids:

Ammono acid is a substance which in liquid ammonia furnishes NH4+.

(a) Replacement of protons by reactive metal: The solutions of ammonium salts in liquid

ammonia react with alkali and other metal to give hydrogen.

(b) Protoysis: Certain compounds like urea, acetamide, sulphamide, etc. which are incapable

of donating protons to water, can readily undergo protolysis in liquid ammonia, i.e., they

can protons to NH3 in liquid ammonia. Hence, organic amides act as acids in liquid NH3.

Reactions of Ammono Bases:

Ammono Base is a substance which in liquid ammonia furnishes NH2-.

(a) Amides (-NH2), imides (=NH) and nitrides (≡N) behave as bases in liquid ammonia.

(b) Since alkaline earth amides and lithium and sodium amides have very low solubility,

potassium amide is much more soluble, is usually employed as ammono base. Thus,

salts of metals in liquid ammonia are precipitated as amides, imides and nitrides.

3. Amphoterism:

A compound that dissolves both in acids and bases is said to be amphoteric in character. In

liquid ammonia, zinc amide shows amphoteric behaviour.

4. Ammonation and Formation of Ammoniates:

Ammonation is a type of solvation reaction in which liquid ammonia is used as solvent and

in which one or more ammonia molecules are attached to a solute species (a cation, a anion

or a neutral molecule) by a chemical bond. The products of ammonation are called as

Ammoniates.

5. Ammonolysis:

Ammonolysis in liquid ammonia is similar to hydrolysis. In ammonolysis, the concentration

of either NH4+ or NH2

- ions increases due to interaction of cations or anions of a salt with

NH4+ or NH2

- ions furnished by autoionisation of NH3.

In ammonolysis, the atom or ion from the compound undergoing ammonolysis is replaced

by -NH2, =NH or ≡N group. For example, in excess of liquid ammonia, the ammonolysis

of TiCl4 takes place in steps as

6. Complex Formation Reaction:

Several complex formation reaction in liquid ammonia are known. For example, zinc nitrate

with potassium amide gives a soluble amide complex.

Similarly, many metal amides, imides and nitrides dissolve in a solution of potassium amide

in liquid ammonia forming soluble amide complexes.

7. 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.

8. Oxidation Reactions:

Oxidising action of various oxidising agents is weaker in liquid ammonia than 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 agents. It is reduced a

solution of potassium in liquid ammonia to K2MnO4 and finally to MnO.

Why the solutions of alkali metals in Liquid ammonia is blue colored?

The ability to dissolve the alkali metals is one of the striking features of liquid ammonia as a

solvent. The alkali metal solutions in liquid ammonia are blue in colour. The blue color is

characterized by-

(i) Blue colour is independent of the metal involved.

(ii) Density of solution is similar to the pure liquid ammonia.

(iii) Conductivity is in the range of conductivity of electrolytes dissolved in liquid ammonia.

(iv) Paramagnetic in character which indicates the presence of unpaired electrons.

The blue color of alkali metal solutions in liquid ammonia is due to the presence of

ammoniated cation as well as ammoniated electros

Very dilute solution of liquid ammonia are metastable and when catalyzed they undergo

decomposition.

Solubility of substances in liquid ammonia:

• 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, sulphates and most of the sulphides are insoluble.

• Most of the iodides are soluble and bromides are less soluble. Flourides 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 hydrocarbon are sparingly soluble. Alkanes are insoluble and alkenes, alkynes are

slightly soluble.

• Amongst the elements, metal 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 a sealed tubes.

• Offensive odour.

Liquid sulphur dioxide:

• Liquid ammonia is a non-aqueous solvent with good ease of handling and low cost.

• It is a non-protonic solvent and it is a useful solvent for carrying out several types of reactions.

• Under normal temperature and pessure, 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 make it a good solvent for covalent

compound but poor solvent for ionic compound.

• Autoionisation is similar to water

Chemical Reactions in Liquid Suphur Dioxide:

1. Acid-Base Reactions or Neutrilisation Reactions:

In sulphur dioxide, the process of neutrilisation is observed similar to found in case of

aqueous solution. The process of neutrilisation in liquid sulphur dioxide involves

combination of SO2+ and SO32- ions to form un-ionised SO2. Thus, all compounds

containing or making available SO32- ions in liquid sulphur dioxide are termed as Bases

and those compounds containing or making SO2+ ions are termed as Acids. E.g.

(i) Reaction of thionyl chloride and cesium sulphate gives a neutrilisation reaction

(ii) Reaction between thionyl thicyanate and potassium sulphite.

2. Solvolytic Reaction:

In liquid sulphur dioxide, solvolysis is seen in limited number of salts. Some of the examples

are-

(i) Ammonium acetate is solvolysed in liquid sulphur dioxide.

(ii) Binary halides such as PCl5, UCl6 etc. undergo solvolysis in liquid sulphur dioxide.

3. Precipitation Reaction:

A large number of precipitation reaction can be carried out in liquid sulphur dioxide due to

specific solubility relationship.e.g.

(i) Reaction between thionyl chloride and silver acetate.

(ii) Reaction of potassium iodide and thionyl chloride is an example of precipitation reaction

in liquid sulphur dioxide.

(iii) Reaction between lead flouride and lithiuim sulphate.

4. Complex Formation Reaction:

A large number of complex formation reaction in liquid sulphur dioxide are known. For

instance, the solubility of iodine in liquid SO2 is greatly increased by the addition of

potassium or rubidium iodide. This is due to the formation of complex KI3 and RbI3.

Similarly, , the increase in the solubility of cadmium iodide and mercuric iodide in liquid

SO2 is attributed to the formation of complexes.

5. Amphoteric Behaviour:

There are number of salts show amphoteric behaviour in liquid sulphur dioxide. For instance,

reaction of AlCl3 and tetramethyl sulphite shows a amphoteric behaviour

From the soluble complex, Al2(SO3)3 can be reprecipitated by adding the acid, SOCl2.

6. Redox Reaction:

Liquid sulphur dioxide does not have any strong oxidising or reducing properties. It serves

only as a medium for redox reactions. For instance, liquid sulphur dioxide can not reduce

iodine. However, a sulphite in liquid sulphur dioxide reduces iodine to iodide.

KI is oxidised to free iodine by SbCl5 in liquid sulphur dioxide.

Reference Book: Principles of Inorganic Chemistry, B.R. Puri, L.R. Sharma and

K.C. Kalia, Milestone Publishers & Distributors (2013-14)