As we know most of the chemical reactions requires solution phase for them to carry out, hence for this we need different kinds of solvents. In the earlier days of development of chemistry, the reactions have been carried out only in one solvent”water”. Because of different physical and chemical properties of water such as
,Long liquid range(0-100 ℃),High dielectric constant (78.5) ,Specific heat, heat of vapourisation,Dipole moment etc
,is used as universal solvent.
Beside these properties, water is non-toxic, non-poisonous, neutral, and colourless and this makes it as an excellent and useful solvent for a large variety of substances.
But nowadays we have many more solvents available which can behave as good solvent like water.
These solvents other than water which are used for carrying out chemical reactions are called non-aqueous solvents. For example:
Benzene, carbon tetrachloride, acetone, ether etc are organic solvents and Liquid ammonia, sulphuric acid, liquid HF, liquid sulphur dioxide etc are inorganic solvents.
Physical properties of solvent
a)Melting & Boiling point (Liq. Range)
Water has excellent liquid range( 0 to 100℃) at ordinary temp. And pressure. Acids can also act as solvent at ordinary temp while liq.NH3 (-77.7 to -33.4℃) and liq. SO2(-75.5 to -10.2℃) can act as solvent only at low temp.
b)Heat of Fusion & heat of vapourisation:
helps to know the nature and strength of forces which hold the molecule of the solvent together in solid or liquid state.
A high heat of vapourisation of liquid indicates that the intermolecular forces in it are strong.
∆Hvap/Tboiling pt. = Trouton Constant
The ratio of the heat of vapourisation expressed in joules to the boiling point(K) is a constant known as Trouton Constant.
•High value of Trouton constant indicates associated molecules.
*Molar heat of fusion for water (6.0kJ/mol) and ammonia(5.65kj/mol) are almost same .this means the forces which hold molecules together in H2O and NH3 are of same magnitude.
in solvent of high dielectric constants ,the ionic substances will dissolve readily .
•With decrease in dielectric constant, the solubility of ionic compounds decreases.
•NH3 and SO2 with small value of dielectric const. Values show decreased tendency to dissolve ionic compounds.
Greater is the Polarity between bonds in the molecules ,larger is the dipole moment.
And with Increase in dipole moment ,tendency to dissolve ionic substance increases.
it is the measure of fluidity of the solvent.
In solvents of low viscosity the operations such as precipitation , crystallisation ,filtration etc can be easily carried out without any difficulty.
|Types of non aqueous solvents|
|Acidic ,Basic and Amphiprotic solvents||Protonic and Non Protonic Solvents||Ionising & Non ionising Solvents||Coordinating & non-Coordinating Solvents|
•The Solvent which have strong tendancy to donate protons are acidic in nature and are called acidic Solvents. Example H2SO4,HF,CH3COOH.
•Basic solvent: Solvents which have strong tendency to accept protons are basic in nature and are called basic or protophillic solvents. Eamples: NH3,Py,Hydrazine(NH2–NH2), ethylenediamine.
•Solvents from which protons (H+ ions) can be derived are called Protonic or protic solvents. Example: H2O ,HF,H2SO4,HCl.
•The Solvents do not contains ionizable hydrogen atom are called non-Protonic Solvents.Example: SO2, N2O4, C6H6,CHCl3, CCl4.
|Solvents which are capable of undergoing self ionisation are called ionisation solvents. Example: H2O , NH3,HF ,SO2 ,N2O4 ,ClF3,BrF3,POCl3 etc.
Solvents which do not ionise at all are non-ionising solvents.Example: Benzene, CCl4, hydrocarbons.
|the solvents which are capable of coordinating with the metal ion or anions of the solute are coordinating Solvents. Example: NH3,SO2 ,nitro methane etc.
Solvents which are not capable of coordinating with the metal ions or anions of the solute are non- coordinating Solvents ,Example: CCl4, saturated hydrocarbons.
It is highly used as non-aqueous solvent.
Comparison of properties of liq.NH3 & H2O
•Liquid Range: the melting and boiling points of liq.NH3 are lower then those of water,its liquid range is -77 to -33℃ and its difficult to handle.
•Hydrogen Bonding: liq.NH3 also show hydrogen bonding but NH3 molecules are less strongly associated bcoz N is less electronegative than Oxygen. Therefore freezing point & boiling point of ammonia are less.
Dielectric Constant: dielectric constant of ammonia (22 at -33℃) is lower as compared to water (78.5 at 25℃). Therefore liq. NH3 is a poor Solvent of ionic Subatances.
However,ammonia has lower viscosity(0.254cp at -33℃) than water (0.959cp at 25℃).this might be expected to permote greater ionic mobilities and thereby compensate to some extent for the effect of dielectric constant.
Self ionisation: the extent of auto ionisation of liq.NH3 is less than that of water.this is clear from its low specific Conductance and lower ionic product.
K[NH3]=[NH4+][NH2–]= 1.9×10-23 ,at -50℃
K[H2O]= [H3O+][OH–] = 1×10-14 ,at 25℃
|AMMONIA as better solvent than water|
i)Liq. NH3 is a very poor Conductor of electricity.
ii) Specific heat of ammonia is more than that of water.
iii) Liq.NH3 has low viscosity which greatly increases the mobility of the ions.
iv)Liq.NH3 has high critical temp and pressure.
v) it is less associated than water and therefore it is better solvent for organic compounds.
|Reactions of Liq.Ammonia|
Substance that produces Ammonium ions(NH4+) in liq.NH3 behave as acid while those produces amide ions(NH2–) behave as base.
NH4Cl → NH4+ + Cl– (Acidic Character)
KNH2 → K+ + NH2– (basic character)
some compounds like urea,acetamide ,sulphamide etc which are incapable of donating protons to water can readily donate proton to NH3 in liquid ammonia .hence these compounds act as acid in liq.NH3.
NH4Cl(acid) + KNH2(base) → KCl + 2NH3
NH4+ + NH2– → 2NH3
Zn(NH2)2 shows Amphoteric behaviour in liq.NH3
Zn(NH2)2 (base) +2NH4Cl (acid) → ZnCl2 + 4NH3
Zn(NH2)2(acid) + 2KNH2(base) → K2Zn(NH2)4
Ammono Acid: is a Substance which furnishes NH4+ ion in liq.NH3 .Ammonium halides ,nitrate or thiocynate are very soluble in liq. NH3 to give NH4+ ion.
Ammono base: Ammono base is a Substance which in liq.NH3 solution gives NH2– ion .Example ,KNH2.
Precipitation reaction involves double decomposition ,due to difference in the Solubilities of various substances in water and liq.NH3 .
•Liq.NH3 is better in precipitation reaction than water.
In Aqueous medium, KCl + AgNO3 → AgCl↓ + KNO3
In liq.NH3 ,
i) 2AgCl + Ba(NO3)2 → BaCl2↓ + 2AgNO3
¡¡) NH4Cl + LiNO3 → LiCl↓ + NH4NO3
the solvolysis reaction of liq.NH3 are called Ammonolysis reactions.
NaH + NH3 → NaNH2 + H2
HgCl2 → Hg(NH2)Cl + HCl
In case of organic halides which undergo slow Ammonolysis at the boiling point of liquid ammonia ,mixture of primary ,secondary, tertiary amines are formed.
RX + 2NH3 → RNH2 +NH4X
RX + RNH2 + NH3 → R2NH + NH4X
RX + R2NH + NH3 → R3N + NH4X
Where X=Cl ,Br or I
Complex formation reactions:
these rxns are similar to the reactions occurring in the aqueous system.example:
CuCl2 + 4NH3 → [Cu(NH3)4]Cl2
AgCl + 2NH3 → [Ag(NH3)2]Cl
Formation of ammoniates
Just as hydrates ,ammonia also foem ammoniates with many inorganic salts.
BHF3 + NH3 → BF3.NH3
MgI2 +6NH3 → MgI2.6NH3
In liquid Ammonia , the strong oxidising agent show weaker oxidising character than in water solution.
For example ,HNO3 does not act as oxidising agent in liq. NH3 ,On the other hand reducing agents show increased Reactivity.
Example : CuI + Na → Cu + NaI
KMnO4 + K → K2MnO4
•Here liq.NH3 reduces CuI ,KMnO4 to Cu & K2MnO4 respectively.
|Solution of alkali metals in liquid Ammonia|
•these are blue in colour and colour is independent of the metal dissolved.
•When concentration of metal increased ,the colour changes to bronze and shows metallic appearance.
•these solutions are good conductor and show strong reducing properties.
•These Solutions are paramagnetic and the paramagnetic character decreases with increasing concentration.
Na ⇋ Na+ + e–
Na+ + xNH3 → [Na(NH3)x]+ ,ammoniated cation
e– + yNH3 → [e(NH3)y]– , ammoniated electron
Na + liq.NH3 → [Na+(NH3)x]+ + [e(NH3)y]–
•Ammoniated electrons are responsible for blue colour of the solution. the electrical conductivity is also due to the ammoniated cations and ammoniated electrons.
•As Concentration of Solution increased ,ammoniated Cations & electrons associate ,Resulting in a decreased Conductivity.
•Strong Reducing character of these solutions is due to the presence of readily available electrons.
Advantages of liq.NH3
¡) dissolution of alkali metal without chemical reaction is one of the greatest advantage of using liq.NH3.
¡¡) Alkali metal solutions are strong reducing agent even stronger than hydrogen.
¡¡¡) the tendancy of Solvolysis is less in liq.NH3 Than water.
iv) Ammonium salt dissolved in liq.NH3 find application in preparative chemistry. they can be used to precipitate sulphides ,halides, sulphates etc.
Disadvantage of Liq.NH3
¡) liquid range of liq.NH3 is very low (-77℃ to -33℃). Therefore a low temp and high pressure is required.
¡¡)liq.NH3 has offensive odour & hygroscopic in nature.therefore reactions are to be carried out in sealed tubes.
|Anhydrous Sulphuric Acid|
Boiling pt. Of anhydrous sulphuric acid is 300℃ and its dielectric constant is 110. The viscosity of anhydrous H2SO4 is about 25 times higher than that of water. Therefore ,solutes dissolve very slowly in sulphuric acid and also difficult to crystallise from H2SO4.
anhydrous H2SO4 autoionises to give H3SO4+ and H2SO4– ions.
2H2SO4 ⇋ H3SO4+ + HSO4–
•Therefore any species producing HSO4– ion will act as base and any chemical species producing H3SO4+ ion acts as acid in H2SO4.
•KHSO4 acts as a base in anhydrous sulphuric acid.
KHSO4 ⇋ K+ + HSO4–
•Species that are basic in water are also basic in H2SO4.
OH– + 2H2SO4 → 2HSO4– + H3O+
NH3 + H2SO4 → NH4+ + HSO4–
•water behaves as base in anhydrous sulphuric acid.
H2O + H2SO4 → HSO4– + H3O+
•Species Containing lone pair of electrons easily accept a proton from H2SO4 and acts as bases.thus amides such as urea ,which acts as acid in liquid NH3 ,neutral in water and behave as bases in anhydrous H2SO4
H2N(CO)NH2 + H2SO4 → HSO4– + [H2N(CO)NH3+]
•most of acids behave as base in anhydrous sulphuric acid.even very strong acids like perchloric acid behave as weak acids in Anhydrous H2SO4.
HClO4 + H2SO4 → H3SO4+ + ClO4–
H2SO4 acts as strong dehydrating agent and extract water even from chemical compounds .
C11H22O11 +H2SO4→ 12C (black ) + 11H2O
Super Acids :
acids which bechave as strong acid solvents are termed as super acids .
Example,[HB(HSO4)4] ,whose solution can be prepared in sulphuric acid ,behaves as exceptionally strong acid.but it is not been isolated as such and only its solution can be prepared.
Other super Acids in H2SO4 solution are H2Sn(HSO4)4 and H2Pb(HSO4)4.
|Liquid Sulphuric Dioxide|
Liq.SO2 is a non-Protonic solvent ,bcoz it does not contain any hydrogen atom.
|Liq. Range (-75.5 to -10.2℃)|
|viscosity 0.428cp at -10℃|
|dipole moment 1.61|
|specific conductance 4×10-8|
its dielectric constant is small (15.6 at 0℃) .Therefore its a good solvent for Covalent compounds.
Structure of SO2
AutoIonisation of SO2
2SO2 ⇋ SO2+( thionyl ion ) + SO32- (Sulphite ion)
Reaction of Liq.SO2
•Acid base Reaction:
Compound which gives SO2+ is acidic and substances which gives SO32- act as base.
Example , Acidic :- SOCl2(thionyl Chloride) ⇋ SO2+ + 2Cl–
Basic :- CsSO3 ⇋ 2Cs + SO32-
SOCl2 (Acid) + Cs2SO3 (base) → 2CsCl + 2SO2
Solvolytic rxns in liqSO2 are not as common as they are in other Protonic solvents such as NH3 ,H2O.
PCl5 + SO2(liq) → POCl3 + SOCl3
2CH3COONH4 +2SO2 → (NH4)2SO3 + (CH3COO)2SO
(CH3COO)2SO ⇋ SO2 + (CH3CO2)O
A large number of precipitation reactions can be carried out in liquid Sulphur dioxide due to Selective Solubilities of Compounds.
Example:- SbCl3 + 3LiI → SbI3↓ +3LiCl
AlCl3 + 3NaI → 2NaCl↓ + AlI3
2KI + SOCl2 → 2KCl↓ + SOI2
Complex Formation Reactions
2AlCl3 + 3SO32- ⇋ Al2(SO3)3↓ +6Cl–
Al2(SO3)3 + 3SO32- ⇋ 2[Al(SO3)3]3- ,( soluble complex ion)
Similarly , SbCl3 + 3KCl + liq SO2 → K3[SbCl6]
SbCl5 +NOCl + liq.SO2 → NO[SbCl6]
Organic Reactions :
a large no. Of organic compounds are soluble in liq.SO2 ,bcoz it is relatively inert solvent.
i) Sulphonation : C6H6 + SO3 → C6H5SO3H
SO2 of Crystallisation (Salvation Reaction)
These are formed with the highly soluble salts and are analogous to hydrates.
Examples: NaI.4SO2 ,LiI.2SO2 ,AlCl3.SO2 ,BaI2.4SO2.
Redox reaction :
liq. SO2 does not show any marked reducing or oxidising tendency.it simply act as a medium for certain redox reaction.
Example , 6KI + 3SbCl6 +liqSO2→ 2K3[SbCl6] + SbCl6 +3I2
Some special feature of liq.SO2 as a solvent
•it has low boiling point and this allows its easy evaporation. ie ,removal from the reaction mixture.
•Liquid SO2 is an inert Solvent.
•it has unusual solvating powers for both covalent and electrovalent compounds.
•it is used as dilutent for the superAcid medium (CHSO3F-SbF5) which helps in reducing its viscosity without affecting its acidity.