NON AQUEOUS SOLVENT
DEEPSHIKHA
DEPARTMENT OF CHEMISTRY
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, H2 SO4 , 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.
Physical Properties of Solvents and their Role in Chemical Reactions:
Every liquid cannot be used as a solvent in a chemical reaction.Here we compare the properties of some non-aqueous solvents with the properties of H20.
1. Melting point and boiling point: Most of the chemical reactions are carried out in the liquid phase. Hence the melting point and boiling point of a solvent indicates the range of temperature within which the solvent can be used. Melting points and boiling points(0C) of some solvents are given below
2. Dielectric constant (ɛ):
Dielectric constant ( ɛ) of a solvent determine ability of the solvent to dissolve polar and non-polar substances in it.The coulombic force (F) between a cation and an anion of an ionic compound is given by the expression:
F = q1q2 / ɛ(r1+r2 )2
In this expression q 1 and q2 are the charges on cation and anion respectively, r1 and r 2 are the radii of the two ions and ɛ is the dielectric constant of the solvent. The value of ɛ depends on the nature of the solvent in which the ionic compound is dissolved.
It may be clear from the above expression that if ɛ of a solvent is large, F would be small i.e. if ɛ is large, small amount of energy would be required to separate the ions and hence it would be easy for a solvent having a high value of ε to dissolve an ionic compound in it. For example, since anhydrous HF and H 2O have high values of ɛ, these are the best solvents for ionic compounds.
On the other hand, since liq.NH 3 and liq.SO2 has low values of ɛ , these solvents show smaller ability to dissolve ionic compounds especially those containing multi-charged ions. Thus carbonates, sulphates and phosphates which contain multi-charged ions are insoluble in liq.NH3 and liq.SO2 .
3. Dipole moment:
Greater is the polarity of the bond in a solvent molecule.greater is the charge separation and higher will be the value for dipole moment. Substances having high dipole moment values are good solvent for polar solutes. This is because of the fact that greater is the polarity of a solvent molecule, greater is the solvation energy released on dissolution of a solute. Dipole moment value of a solvent also gives an idea about the extent of association of the molecules of a liquid and hence its liquid range. Dipole moment values (in D) of H 2O, NH3 and SO2 are1.85, 1.47and1.61 respectively.
4. Viscosity:
Viscosity gives a measure of the fluidity of the solvent. Solvents like water, carbon tetrachloride have low viscosity and flow rapidly under ordinary temperature. In solvent of low viscosity, the operations such as precipitation, crystallization, filtration, etc. can be easily carried out without any difficulty. With increasing viscosity of a liquid, the difficulty of such operations increases. Solvents like anhydrous sulphuric acid have higher viscosities and this reduces their usefulness as solvent, Viscosity of H2O, NH3 and SO2 is 1.00, 0.241 and 0.009 respectively
5. Proton affinity:
It is applicable for protonic solvents only. It greatly affects the behaviour of a solute in a given solvent. NH 3 has greater proton affinity than H2O. Hence acetamide (CH3 CO NH2 ) which behaves as a very weak base in aqueous solution shows acidic properties in liq.NH3 .
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Types of Chemical Reactions Taking Place in Non-aqueous Solvent
1.Metathetical (Precipitation) Reactions
The reactions in which precipitate is formed by mixing two solutions of two compounds are called metathetical or precipitation reactions. Thus precipitation reactions are normally double decomposition. The formation of a precipitate in different solvents depends on the solubilities of the products in those solvents. For example the precipitate of AgCl isbtained by mixing BaCl2 and AgNO3 in aqueous medium.
BaCl2 + 2AgNO3 ---------> AgCl↓ + Ba(NO3 )2
H2O
In liquid ammonia the above reaction is reversed, i.e., in liq.NH3 ppt. of BaCl2 is obtained by combining AgCl with Ba (NO3 )2 .
2. Acid-Base Reactions We have already said that the ionic solvents are polar compounds and undergo self-ionisation, Self-ionisation of some important solvents is
An acid -base reaction in a non-aqueous solvent can be explained on the basis of the solvent system concept of acids and bases. According to this concept an acid is a substance which contains the solvent cation and a base is a substance which contains the solvent anion. For example since self-ionisation of liq.NH3 .
NH4Cl(acid) + KNH2 (base) ⇄ KCl(salt) + 2NH3 (solvent)
Acidic character of CH3COOH in H2O and liq. NH3 : When CH3COOH reacts with H2O and liq.NH3 (solvents), H2O and liq,NH3 both accept a proton from CH3COOH to produce H2O+ and NH4 + ions. Due to the production of these ions, CH 3COOH behaves as anacid in both the solvents’
CH3COOH + H2O ⇄ H2O+ + CH3CO0H
CH3COOH + NH3 ⇄ NH4 + + CH3CO0H
Solvation reaction is a general reaction in which a solute (a cation, an anion or a neutral molecule) reacts with one or more molecules of a solvent (e.g.H2O, liq.NH3 , liq.SO2 etc.) to form a product in which the solute and solvent species are attached to each other by a H-bond or by a coordinate bond. The product formed is called solvate. Solvate is an addition compound and hence is also called an adduct. The addition compound contains solvent of crystallisation. In the formation of a solvate, the solvent acts as a Lewis base while the solute species behaves as a Lewis acid. When the solvent used is water, the solvation reaction is called hydration and the addition compound formed is called hydrate.
Hydrate contains one or more molecules of water as water of crystallisation. Similarly when the solvent is liq. NH3, the reaction is called ammoniation and the addition compound formed is called ammoniate.
.Ammoniate contains one or more molecules of ammonia as ammonia of crystallisation.
4. Solvolytic Reactions: Solvolysis
Solvolytic reactions are the reactions in which the solvent molecules react with the solute molecule (salt) or ion in the way which consists of the following steps:
(a) The solvent molecule undergoes auto-ionisation (self-ionisation) to give solvent cations and solvent anions.
(b) The solute (salt) splits into solute cations and solute anions. The solute cations or solute anions interact with the solvent cations or solvent anions. Due to this interaction the concentration of the solvent cations or solvent anions is increased.When H 2O and NH3 are used as solvents in the solvolytic reactions, the reactions are called hydrolysis and ammonolysis (or ammonolysis reactions) respectively.
Liquid Ammonia (Liq. NH3 )
Solubility of Various Substances in Liq.NH3 :
A. Solubility of ionic compounds (inorganic salts)
We know that ammonia has a low value of its dielectric constant (= 22 at -34°C). This low value suggests that liq.NH 3 has a poor ability to dissolve ionic compounds. Ammonium salts (e.g. NH 4NO, NH4SCN, CH3COONH4 etc.) and most of the nitrites, nitrates, cyanide, thiocyanate, perchlorates, are soluble in liq.NH3 . Salts containing highly charged ions (e.g., oxides, sulphides, sulphates, phosphates and carbonates) are insoluble. Fluorides and chlorides (except Be2+ and Na+ chlorides) are practically insoluble, bromides are less soluble while iodides are freely soluble. Thus the solubility of the halides of a given metal increases in going from fluoride to iodide (MF < MBr <MI).
. Solubility of non-ionic compounds (organic compounds):
Halogen compounds, alcohols, ketones, esters, simple ethers, amines, phenol and its derivatives etc. are soluble. Alkanes are insoluble and alkenes and alkynes are slightly soluble. In this sense, liq.NH3 is a better solvent for non ionic and nonpolar compounds (organic compounds).
Solubility of non-metals:
The non-metals like S, P, I2 , Se etc. are soluble and they react with the solvent
. Solubility of alkali metals and alkaline earth metals:
All the alkali metals and alkaline earth metals (except Be) are soluble in liq.NH3 . The solubility of alkali metals in liq. NH3 increases as we pass from Li to Cs (Li < Na <K< Cs).
Chemical Reactions
Metathetical (Precipitation) Reactions
Examples of some precipitation reactions taking place in liq. NH 3 are given below
1.Chlorides are precipitated:
2NH4CI(Ammono acid) + Sr(NO3 )2 → SrCl2 +2NH4N03
Ba(NO3 )2 + 2AgCl → BaCl2 ↓ + 2AgNO3
2.Bromides and iodides are precipitated:
Sr(NO)2 + 2NH4Br (Ammono acid) → SrBr2 ↓ +2NH4N03
Zn(NO)2 + 2NH4 I (Ammono acid) → ZnI2 ↓ + 2NH4N03 -
3.Sulphides are precipitated:
(NH4 )2S(ammono acid) + 2AgNO3_ Ag2S↓+2NH4NO3
(NH4 )2S(ammono acid) + M(NO3 )2 (M=Ba,Cu,Cd) →MS↓ + 2NH4NO3
Barium alcoholate can be precipitated:
K(OC2H5 ) + Ba(NO3 )2 → Ba(OC2H5 )2↓ + 2KNO3
Acid-base Neutralisation reaction(salt formation reactions)
NH4Cl → NH4 + + Cl-
KNH2 → K+ + NH2 -
On adding:
NH4Cl + KNH2 → K+ + Cl- + NH4 + + NH2 -
NH4 + (solvent cation) + NH2 - (solvent anion) → 2NH3
Above equation indicates that neutralisation reaction in liq.NH3can also be defined as the combination of solvent cation(NH4 + ) and solvent anion(NH2 - ) to form the un-ionised solvent(liq.NH3 ).
Other examples in liq.NH3 are:
Ammono acid Ammono base Ammono salt Solvent
NH4X (Cl,NO3 ) + NaNH2 → NaX + 2NH3
2NH4X (Cl,I) + PbNH → PbX2 + 2NH3
3NH4X (Cl,I) + BiN → BiX3 + 4NH3
2CH3COONH4 + Zn(NH2 )2 → Zn(CH3COO)2 +4NH3
Amphoteric Behaviour of Zn(NH2 )2 in Liq.NH3 :
Zn (NH2 ) 2 dissolves in ammono acid as well as in ammono base in liq. NH3 and hence is said to show amphoteric character in liq. NH3
Zn(NH2 )2 + 2NH4CI ( Ammono acid) → ZnCl2 (Salt) + 4NH3
Zn(NH2 )2 + 2NaNH2 ( Ammono base) → Na2 [Zn(NH2 )] + 2NH3
Above reactions can be compared with the following reactions in aqueous solution. In these reactions Zn(OH) 2 shows amphoteric character since it dissolves in HCL (acid) as well as in NaOH (base).
Zn(OH)2 + 2HCI → ZnCl2 + 2H2O
Zn(OH)2 + 2NaOH → Na2 [ZnO2 ] + 2H2O
. Ammonation Reaction : Formation of Ammoniates (Solvation Reactions - Formation of Solvates):
Solvation reaction in which liq, NH3 is used as solvent is called ammonation reaction and the solvate formed is called ammoniate. In the formation of ammoniate liq. NH3 (solvent) acts as a Lewis base and the solute behaves as a Lewis acid. The formation of some ammoniates in liq. NH 3 has been shown below. These ammoniates may be 1 : 2 or 1 : 1 adducts. Solute (Lewis acid) Solvent (Lewis base) Ammoniates
Solute (Lewis acid) Solvent (Lewis base) Ammoniates
SO3 + 2NH3 → SO3 .2NH(1:2 adduct)
SiF4 + 2NH3 → SiF4 .2NH3 (1:2 adduct
BF3 + NH3 → BF3 .NH3 (1:1 adduct)
The formation of NH4 + by action of liq.NH3 on H 2O is also an example of ammoniation.More examples of ammoniates formed by liq. NH 3 with inorganic salts are MgI2 . 6NH3 , Nil2 . 6NH3 , Nal. 4NH3 etc. In the formation of these ammoniates, NH 3 molecules (solvent molecules) arecoordinated to the metal ions of the inorganic salt (solute).
Ammonolysis or Ammonolytic Reaction :(Solvolysis or Solvolytic Reactions)
The solvolytic reactions taking place in aqueous medium (H 2O) and liq. NH3 are called hydrolysis and ammonolysis (or ammonolytic reactions) respectively. In hydrolysis H 2O is used as a solvent while in ammonolysis liq. NH3 is used as a solvent.
Ammonolysis of SnCl4 , SiCl4 and SO2CI2 :
In the ammonolysis of SnCl4 and SO2CI2 (solute) the concentration of NH4 + (solvent cations) is increased as shown below:
SnCl4 (Solute) + 8 NH3 (Solvent) → Sn(NH2 )4 + 4Cl- + 4NH4 +
Ammonolysis of alkali metal hydrides and oxides:
The ammonolysis of alkali metal hydrides and oxides gives alkali metal amides (MNH2 ). In these reactions the concentration of NH2 - ions (solvent anions) is increased..
NaH (Solute) + NH3 (Solvent) → NaNH2 + H2
Na2O (Solute) + 2NH3 (Solvent) → 2NaNH2 + H2O
Ammonolysis of TiCl4 :
Ammonolysis of TiCl4 (salt) in presence of excess of liq. NH3 produces Ti(NH2 )4 . In each step Cl-atom of the salt undergoing ammonolysis is replaced by -NH2 group.
TiCl4 + NH3 → TiCl3 (NH2 ) + HCI
TiCl3 (NH2 ) + NH3 → TiCl2 (NH2 )2 + HCl
TiCl2 (NH2 )2 + NH3 → TiCl(NH2 )3 + HCl
TiCl(NH2 )3 + NH3 → Ti(NH2 )4 + HCl
On adding: TiCl4 + 4NH3 → Ti(NH2 )4 + 4HCI .
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Ammonolysis of alkyl and aryl halides:
The ammonolysis of alkyl halides RX(R = alkyl group. X = Cl, Br, I) takes place slowly at the boiling point of lig.NH3 . In this reaction mixture of primary (RNH2 ) secondary (R2NH) and tertiary (R3N) amines is obtained.
. Complex Formation Reactions:
a. Many metal salts (e.g. Zn(NO3 )2 , AICi3 , etc.) react with excess of KNH2 solution in liq NH3 to form soluble amido complexes. For example:
i. Zn2+ + 4NH2 - (excess) → [Zn(NH2 )4 ]
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Redox reactions in which alkali metals in liq. NH 3 acts as reducing agent
Liq.NH3 can dissolve the alkali metals in it. The solution thus obtained contains ammoniated electrons. These electrons reduce various substances. Thus alkali metals in liq. NH 3 are able to reduce a variety of substances and are themselves oxidised to some metal salt.
Sodium in liq.NH3 reduces ammonium salt to H2
2Na (Na=0) + 2NH4Br (H=1) → 2NaBr (Na=+1) + H2 (H=0)
ii. Potassium in liq. NH3 reduces nitrous oxide (N2O) to N2
2K (K=0) + NH3 + N2O → KNH2 (K=+1) + KOH + N2 (N=0
Advantages of Using Liquid Ammonia as a Solvent:
1. All the alkali metals,without reacting with liq. NH3 is soluble in this solvent. The dissolved alkali metals can be recovered by evaporating the alkali metal-liq.NH3 solution.
2. The alkali metal-liq NH 3 solution contains ammoniated electrons and hence these solutions act as strong reducing agents.
3. The study of precipitation reactions taking place in liq.NH3 these reactions can be used to precipitate metallic halides, sulphides, alcoholates, amide, imide and nitrides.
Disadvantages of Using Liquid Ammonia as a Solvent :
1. Low temperature or high pressure is necessary while working with liq.NH3 .This is because of the fact that the liquid range for liq.NH3 is from -34.0°C to -77.70C
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2. Liq. NH3 is hygroscopic in nature and hence all the reaction must be carried out in a sealed tube.
3. Liq, NH 3 has an offensive odours and hence the use of liq.NH3 as a solvent and as a reaction medium requires special technique. Liquid Sulphur Dioxide (Liq.SO2 )