B.Sc. SY, SEM- III Paper VI ,Sec-B�Unit-V [B]: Non-Aqueous Solvents.

Introduction: Most of the chemical reactions familiar to us take place in aqueous medium, since water due to high value of its dielectric constant has ability to dissolve number of substances.

• Up to 1892 chemists were used water as a solvent but after 1892 chemists started the chemical reactions other than water i.e. in non- aqueous solvents like alcohol, ether and ketones etc as electrolytic solvents for inorganic substances.
• Ni the year 1897 a chemist Cady proved that,ammonia is agood solvent . In the beginning of 20^th century chemist Walden utilize sulphur dioxide(SO2) as a another solvent for chemical reactions. Thus, later on anhydrous HF and liquid di-nitrogen tetra oxide(N2O4) like non-aqueous solvents are discovered.

• Classification of Solvents:
• Solvents have been classified in a number of ways depending on their physical and chemical properties.
• First classification:
• This type of classification is based on the proton-donating and proton accepting ability of solvent.
• This classification gives the following three types of solvents.
• Protic or Protonic Solvents: These solvents either lose or gain protons or can show both the tendencies, these solvents are of following three sub-types:

a) Acidic (proto-genic) solvents:

• These solvents can lose protons readily
• Examples are- HF,HCOOH, H₂SO_4, HCN, C₆H₅COOH,CH₃COOH.

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b) Basic (proto-philic) solvents:

• These solvents can accept protons.
• Examples are- NH_3,N₂H_4, NH₂OH, and amines.

c) Amphi-protic or Amhoteric solvents:

• These solvents show dual character, i.e., they can lose as well as accept protons, depending upon the nature of reacting species.
• Amphoteric solvents undergo auto or self ionization in which a proton transfer between two similar neutral molecules and a cation-anion pair of the solvent is obtained.

• For e.g.,
• Acid Base Acid Base

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H₂O + H₂O H₃O⁺ + OH^-

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• NH₃ + NH₃ NH₄⁺ + NH₂^-

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• CH₃CooH + CH₃COOH CH₃COOH₂⁺ + CH₃COO^-

2) Non-protic or non- protonic or aprotic solvents:

• These can can neither lose nor gain the protons.
• Examples are- C₆H_6, CHCl₃, CH₂Cl_2, CCl₄ etc.

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• Second classification:
• This type of classification is based on polar and non-polar nature of the solvents.
• This classification gives following two types of solvents.

1) Ionising, ionic or polar solvents:

• Ionizing solvents are those solvents which are capable to undergo self or auto ionization.

i) Examples :

• H₂O, NH_3, HF, SO₂

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• ii) They have high polarity and high dielectric constants.

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• iii) They dissolve ionic compounds and initiate ionic reactions. iv) They can undergo auto-ionization.

Acid Base Acid Base

HF + HF HF₂⁺ + F^-

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SO₂ + SO₂ SO²⁺ + SO₃^2-

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^V) Polar solvents tend to associate due to dipole- dipole interactions.

vi) The association is more effective in case of protonic solvents ( solvents those contain hydrogen ion).

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2) Non-ionizing solvents:

• Those solvents which do not undergo self or auto ionization at all are called as non-ionizing solvents.
• Examples: CCl₄ and C₆H_6.
• have low dipole moment and dielectric constant.
• They dissolve covalent compounds and cannot initiate ionic reactions.

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• Third classification :
• Water is aqueous solvents while others for e.g. NH₃, SO₂, HF, C₆H₆, CHCl₃ etc are called non-aqueous solvents.

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• Water as a universal solvent:
• A solvent is defined as a substance which has the power of dissolving other substances.
• Water is called as excellent or universal solvent due to following reasons.

i) It has high dielectric constant.

ii) Water has a long liquid range (0^o to 100⁰c) hence, water is liquid at ordinary temperature.

iii) It is most easily available and can be easily purified.

iv) It is neutral, odourless, non-toxic, and non-poisonous.

v) It can be handled safely.

• Due to all these characteristics, water serves as the most useful solvent.

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• Reactions in liquid ammonia as solvent:
• Liquid ammonia, is a protonic solvent, since it gives a proton (H+) on ionization.
• It is highly useful solvent thus number of chemical reactions are carried out in liquid ammonia are as follows:

1) Auto- ionization of liq. Ammonia:

• The auto-ionization of liq. Ammonia is:

NH₃ H⁺ + NH₂^-

H⁺ + NH₃ NH₄⁺

NH₃ + NH₃ NH₄⁺ + NH₂^-

(Ammonium ion) (Amide ion)

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• Thus, the auto-ionization of liq. Ammonia gives NH₄⁺ ions will act as an acid, while that produces NH₂^- ions will behave as a base in liq. NH_3.
• The compounds which gives NH₄⁺ ions in liq.NH₃ are called ammono acids,

while those which give NH₂^- in this solvent are called ammono bases.

• Examples of ammono acids:

i) NH₄X X^- + NH4⁺

(X= Cl,Br,I)

ii) H₂N.CO.NH₂ + NH₃ H₂N.CO.NH^- + NH₄⁺

(Urea)

iii) CH₃CONH₂ + NH₃ CH₃CONH^- + NH₄⁺ (Acetamide)

iv) CH₃COOH + NH₃ CH₃COO^- + NH₄⁺

( Acetic acid)

v) H₂N.SO₂. OH + 2NH₃ H^- N.SO₂.O^- + 2NH₄⁺

(Sulphamic/Dibasic acid)

• It may be noted that sulphamic acid produces two ions, this acid behaves as a dibasic in liq.NH_3. .

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• Examples of ammono bases:

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• i) KNH₂ liq. NH₃ K⁺ + NH₂^-

ii) PbNH₂ liq.NH₃ Pb²⁺ + NH₂^-

iii) BiN liq.NH₃ Bi³⁺ + N^3-

2) Acid-base/ Neutralization/ salt formation Reactions:

• In liq.ammonia acid-base reaction in which a compound containing NH₄⁺ ion (ammono acid) combines with a compound containing NH₂^- ion(ammono base) to form the salt.
• Examples:
• Ammono acid Ammono base Salt Solvent

i) NH₄Cl + KNH₂ liq.NH3 KCL + 2NH₃

ii) NH₄X + NaNH₂ liq.NH3 NaX + 2NH₃

(X= Cl, NO₃)

iii) 2NH₄X + PbNH liq.NH3 PbX₂ + 3NH₃

(X=Cl,I)

iv) 3NH₄X + BiN liq.NH3 BiX₃ + 4NH₃

(X=Cl,I)

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3) Precipitation Reactions:

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3) Precipitation Reactions:

• Because of difference in solubilities of various substances in liq. ammonia and water, large number of reactions which do not occur in water but reported in liq.ammonia.
• Examples: Reactions of Ammono Acids :

i) KI + NH₄Cl liq.NH3 KCl + NH₄I

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ii) Sr(NO₃)₂ + 2NH₄Br liq.NH3 SrBr₂ + 2NH₄NO₃

strontium nitrate

iii) Zn(NO₃)₂ + 2NH₄I liq.NH3 ZnI₂ + 2NH₄NO₃

Zinc nitrate

iv) Ba(NO₃)₂ + 2AgCl liq.NH3 BaCl₂ + 2AgNO₃

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vi) (NH₄)₂S + 2AgNO₃ liq.NH3 Ag₂S + 2NH₄NO₃

vii) (NH₄)₂S + Cu(NO)₃ liq.NH3 CuS + 2NH₄NO₃

viii) (NH₄)₂S + Cd(NO)₃ liq.NH3 CdS + 2NH₄NO₃

ix) (NH₄)₂S + 2ZnNO₃ liq.NH3 Zn₂S + 2NH₄NO₃

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• Reactions of Ammono Bases:

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i) KNH₂ + AgNO₃ liq.NH3 KNO₂ + AgNH₂ (Amide)

ii) KNH₂ + PbI₂ liq.NH3 KI + 2HI + Pb(NH) (imide)

iii) KNH₂ + BiI₃ liq.NH3 KI + 2HI + BiN (Nitride)

4) Ammonolysis/ ammonolytic Reactions:

• Definition: Any chemical reaction in which a molecule is split up into two parts reacting with ammonia or an amine. OR
• Those reactions involving ammonia are also known as ammonolysis reactions.
• These reactions are also termed as solvolysis or solvolytic reactions.
• Examples:

i) SiCl₄ + 8NH₃ Si(NH₂)₄ + 4NH₄⁺ + 4Cl^-

( Silicon tetramide)

ii) HgCl₂ + 2NH₃ Hg(NH₂)Cl + NH₄⁺ + Cl^-

(Mercuric amidochloride)

iii) Hg₂Cl₂ + 2NH₃ Hg(NH₂)Cl + Hg + NH₄⁺ + Cl^-

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iv) ^{AlCl3 + 2NH3 AlCl2(NH2) + NH4+ + Cl-}

^{(Aluminium chloroamide)}

v) BX₃ + 6NH₃ B(NH₂)₃ + 3NH₄⁺ + 3X^-

(Boron amide)

vi) 2PbI₂ + 6NH₃ PbI(NH₂) + Pb(NH₂)₂ + 3NH₄⁺ + 3I^-

(lead iodoamide)

vii) TiCl₄ + NH₃ TiCl₃(NH₂) + HCl

( Titanium tricholroamide)

viii) NaH + NH₃ NaNH₂ + H₂

(sodium amide)

ix) Na₂O + 2NH₃ 2NaNH₂ + H₂O

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5) Ammoniation Reactions:

• Ammoniation is a type of solvation reaction in which liq. ammonia is used as a solvent and in which one or more ammonia molecules are attached to a solute species (a cation, an anion or a neutral molecule) by a chemical bond.
• In which the solute and solvent species are attached to each other by a H- bond or by a coordinate bond.
• The products of solvation are called solvates and in the case of ammoniation they are known as ammoniates.
• Eexamples: The formation of typical ammoniates are shown below
• SO₃ + 2NH₃ SO₃.2NH₃ 1:2 adduct
• SiF + 2NH₃ SiF.2NH₃ 1:2 adduct
• BF₃ + NH₃ BF₃.NH₃ 1:1 adduct