Non-aqueous solvents - This article includes: Introduction to non-aqueous solvent Why we use non-aqueous PDF

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This article includes:
Introduction to non-aqueous solvent
Why we use non-aqueous solvents
Their properties
Chemical reactions that take place in n0n aqueous solvents
Their applications...

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NON-AQUEOUS SOLVENTS CONTENTS: 1. Introduction to solvents 2. Classification of solvents i.

First classification a) Protic or protonic solvents (acidic, basic and amphoteric) b) Nonprotic or aprotic solvents Second classification a) Ionizing or polar solvents b) Non ionizing or non- polar solvents Third classification a) Aqueous solvent (water) b) Non aqueous solvents

ii.

iii.

3. Non- aqueous solvents introduction 4. Importance 5. Physical properties i. ii. iii. iv. v.

Dipole moment Di-electric constant Electric conductance Density Viscosity

6. Chemical properties (types of reactions) i. ii. iii. iv. v.

Acid –base (neutralization reactions) Solvation or formation of addition compounds (adducts) Solvolysis or solvolytic reactions Metathetical or precipitation reactions Redox reaction

NON-AQUEOUS SOLVENTS

SOLVENTS  INTRODUCTION: In a very general sense, solvents are a class of chemical compounds that allow chemistry to occur. The concept of a solvent has significant ramifications because they serve as the matrix, medium, or carrier for solutes. They are necessary in a number of processes, reactions, and systems. It means that solvent molecules will surround the solute in such a way that a solution is formed; in other words, a homogeneous system is generated in which the solute is part of indistinguishable from the solvent network.

: Fig 1: Interaction of solute particles with solvent particles . Water molecules surround the solute particles in such a way that partial positive oxygen is attracted towards positive side and partial positive hydrogen is attracted towards negative end.

Most of the chemical reactions familiar to us take place in aqueous solution, since water, due to high value of its dielectric constant, has an increased ability to dissolve the ionic compounds and many other substances. However there are many chemical reactions which cannot be carried out in aqueous solution but can be studied in non-aqueous medium. The classification of solvents is as following:

 CLASSIFICATION OF SOLVENTS: Solvents have been classified in a. number of ways depending on their physical and chemical properties. INORGANIC CHEMISTRY

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NON-AQUEOUS SOLVENTS

1. FIRST CLASSIFICATION: This is a common classification which is based on protondonation and proton-acceptance property of the solvents. On the basis of this classification the solvents may be classified into three types: i. ii.

Protic or Protonic solvents Non-Protonic or Aprotic solvents

PROTIC SOLVENTS: Protic solvents are defined as following: “The solvents that either lose protons or gain them or can show both the tendencies are known as protic solvents” Protic solvent is a solvent that has a hydrogen atom bound to an oxygen (as in a hydroxyl group), a nitrogen (as in an amine group), or fluoride (as in hydrogen fluoride). In general terms, any solvent that contains a labile H+ is called a protic solvent. The molecules of such solvents readily donate protons (H+) to solutes, often via hydrogen bonding. Water is the most common protic solvent. Conversely, aprotic solvents cannot donate hydrogen. Major uses of polar solvents are in paints, paint removers, inks, and dry cleaning. Polar protic solvents are often used to dissolve salts. In general, these solvents have high dielectric constants and high polarity. CHARACTERISTICS: Common characteristics of protic solvents are as follows:  Solvents display hydrogen bonding.  Solvents have acidic hydrogen (although they may be very weak acids such as ethanol).  Solvents dissolve salts.  Cations by unshared free electron pairs.  Anions by hydrogen bonding.

INORGANIC CHEMISTRY

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NON-AQUEOUS SOLVENTS Examples include water, most alcohols, formic acid, hydrogen fluoride, and ammonia. Polar protic solvents are favorable for SN1 reactions, while polar aprotic solvents are favorable for SN2 reactions. Protic or Protonic solvents have hydrogen atom in their formula and are of three types. (a) Acidic (proto-genic) solvents. (b) Basic (proto-philic) solvents. (c) Amphi-protic solvents.  Acidic solvents have strong tendency to donate protons. Examples are H2S04, HF, CH3COOH, HCN etc.  Basic or protophilic solvents have strong tendency to accept protons. Examples are NH3, N2H4, NH2OH and amines.  Amphi-protic or amphoteric solvents. These have hydrogen in their formula and show dual character, can donate or accept protons depending on the nature of reacting species, e.g., H2O, alchohols, liq. NH3, CH3COOH etc. these can act both as acids and bases and consequently are amphoteric in nature. Amphoteric solvents undergo auto-ionization(self-ionization) in which a proton transfer between two similar neutral molecules takes place and a cation-anion pair of the solvent is obtained: Acid H3O

H2O + H2O NH3

+

+

NH4 +

NH3

CH3COOH +

Base +

+

B ase

A c id

CH 3COOH

HO

– –

NH 2 CH3COOH2

+

+

NON-PROTIC SOLVENTS: Aprotic or non-Protonic solvents are defined as following:

INORGANIC CHEMISTRY

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-

CH3COO

NON-AQUEOUS SOLVENTS “These may or may not have hydrogen in their formula and neither donates nor accepts protons” Examples are C 6H6, CHCI3, S02, CCI4, and BrF3 etc. Aprotic solvents are solvents that lack acidic hydrogen. Consequently, they are not Hydrogen donors. These solvents generally have intermediate dielectric constants and polarity. Although discouraging use of the term "polar aprotic", IUPAC describes such solvents as having both high dielectric constants and high dipole moments, an example being acetonitrile. Other solvents meeting IUPAC's criteria include pyridine, ethyl acetate, DMF, HMPA, and DMSO. CHARACTERISTICS: Common characteristics of aprotic solvents:  solvents that can accept hydrogen bonds  solvents that do not have acidic hydrogen  solvents that can dissolve salts The criteria are relative and very qualitative. A range of acidities are recognized for aprotic solvents. Their ability to dissolve salts depends strongly on the nature of the salt. Polar aprotic solvents are generally incompatible with strong bases, such as Grignard reagents or t-butyllithium. These reagents require ethers, not nitriles, amides, sulfoxides, etc. The strong base may even deprotonate them (such as methyl anion as a base having a pKaH of 50, and sulfoxides have pKa of approximately 35.

2. SECOND CLASSIFICATION: This classification is based on polar and non-polar nature of the solvents. This classification is based on the fact that "like dissolves like”. Thus this classification gives the following types of solvents: i. Ionising or Polar solvents ii. Non-ionising or non-polar solvents

INORGANIC CHEMISTRY

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NON-AQUEOUS SOLVENTS

IONISING SOLVENTS: Ionising solvents are polar or ionic in nature and hence dissolve ionic compounds and initiate ionic reactions. These exist as ions in their pure state and thus are weak conductors of electricity. These have high values of dielectric constants. Because of their polar nature they have strong tendency to form associated structures. Examples: H20, NH3, HF, S02 etc. These solvents undergo self-ionisation as shown below:

B ase

A c id HF

+

HF

SO2

+

SO2

A c id B a s e – + H2F + F SO

+2

+

SO3

-2

NON- IONIZING SOLVENTS: These are non-polar or non-ionic in nature and hence dissolve only non-polar or neutral compounds and do not initiate ionic reactions. These have low dielectric constants. They have little associating and solvating tendency between the solute and solvents. Examples: C6H6, CCI4 etc. These solvents do not undergo self-ionisation.

3.THIRD CLASSIFICATION: Third classification includes the following: i. Aqueous solvent ii. Non- aqueous solvent The solvents other than water are called non-aqueous solvents. The groups obtained as a result of self-ionisation of H20 and non-aqueous solvents are analogous to each other, e.g., H3O+and OH- groups obtained from the selfionisation of H20 are analogous to NH4+ and NH2 - groups respectively resulted from the self-ionisation of liq. NH3. This is called parent solvent concept and a large number of chemical reactions have been correlated in terms of this concept.

INORGANIC CHEMISTRY

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NON-AQUEOUS SOLVENTS

NON-AQUEOUS SOLVENTS  INTRODUCTION: Most of the chemical reactions familiar to us take place in aqueous solution, since water, due to high value of its dielectric constant, has an increased ability to dissolve the ionic compounds and many other substances. However there are many chemical reactions which cannot be carried out in aqueous solution but can be studied in non-aqueous medium. A large no of non-aqueous solvents have been discovered in the last few years. These solvents have sizeable values of their dielectric constant and hence dissolve many substances. Examples of such solvents are anhydrous liq. NH3, liq. SO2, HF and N2O4.

 IMPORTANCE: The importance of solvents is described as following: Most of the chemical reactions take place in aqueous solutions. The reacting substances should be in a dispersed condition i.e., as ions, atoms or molecules) in a suitable medium (SOLVENT') before appreciable interaction can take place. It is rather unfortunate that we usually lay stress on reacting substances only arid ignore the importance of the solvent on the course of a chemical reaction. By changing the solvent, the reactants may give altogether different products or even the course of a chemical reaction may be reversed. e.g. NH 4CI and LiN03 do not react in aqueous solution but give a precipitate of LiCl in liq. NH 3.So solvent is of great importance by every respect. Similarly AgNO3 and BaCl2 react in aqueous solution to give ppt. of AgCl, but in water the reverse reaction occurs. Following is given the examples of such reactions:

INORGANIC CHEMISTRY

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NON-AQUEOUS SOLVENTS

NH 4Cl

+

LiNO 3

a q .s o l

N o r e a c tio n

B u t in liq . a m m o n ia t h e y r e a c t t o f o r m L iC l a n d N H 4N O 3: NH 4Cl

+

LiNO 3

NH

3

LiCl

+

NH 4NO 3

A g N O 3 a n d B a C l2 in a q . s o l r e a c t t o f o r m p p t o f A g C l:

+

2AgNO 3

BaCl 2

H 2O

2AgCl

+

Ba(NO 3)2

B u t in liq . a m m o n ia r e v e r s e r e a c t io n t a k e s p la c e : 2AgCl

+

Ba(NO 3) 2

NH

3

BaCl 2

+

2AgNO 3

 PHYSICAL PROPERTIES: Some of the physical properties of some ionising solvents are given in Table 1.1

Table 1.2 INORGANIC CHEMISTRY

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NON-AQUEOUS SOLVENTS

1.Dipole moment: Dipole moment is defined as following: “The product of charge and distance between the two charges is called dipole moment.” Greater is the polarity of the bond, greater will be the charge separation and higher will be the dipole moment. Solvents having higher values of dipole moment dissolve the polar substances (i.e. solutes) readily. This is because of the fact that greater the polarity of a solvent molecule, greater is the solvation energy released on dissolution of a solute. Dipole moment value of solvent also gives an idea about the extent of association in the liquid state and hence its liquid temperature range.

2.Dielectric constant Columbic force of attraction, F between a cation and anion in an ionic crystal is given by the expression: q−¿ D r2 ¿ q F=¿

+¿

Where q + and q- = charges on the cation and anion respectively, r= distance between the cation and anion and D=a constant called dielectric constant INORGANIC CHEMISTRY

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NON-AQUEOUS SOLVENTS which depends on the nature of the solvent in which the ionic crystal is kept. The above expression shows that for the same values of q+, q- and r, if F is small, D will be large, i e., a solvent having a high value of D will reduce F. In other words a solvent with a high value of D will weaken the force holding the ions of ionic crystal together and ultimately will dissolve the ionic crystal in it, e.g. water which has greater value of dielectric constant (=78.5) than liq. NH3( = 22.0) is a better solvent for dissolving ionic compounds like NaCl. When NaCI is dissolved in water; the positive ions Na + ions are attracted by the negative dipolar end of H20 and the negative ions CI - ions are attracted by the positive end of H20 molecules and thus NaCI passes into H20 and Na+ and ClIons removed from the ionic crystal. It is imperative that in order to have a solute dissolve in an ionic solvent the solute itself must be ionic. This explains why a non-polar substance does not dissolve in ionic solvents like water. The dissolution of an ionic compound in a polar solvent occurs only when the energy of solvent ion or the ions exceeds the lattice energy of the crystal. It should be noted that solvent-solute interaction leading to solubility is more involved than would appear from a series of dielectric constants alone." Other factors that might influence, in specific cases, are ion-dipole, dipole-dipole, and hydrogen bonding and even pi complexes interactions. RELATION BETWEEN DIELECTRIC CONSTANT AND SOLUBILITY: As columbic force is given by following expression: −¿ r2+¿ +r ¿ K¿ a−¿ +¿× ¿ F= ¿ c a−¿ =¿ +¿ × K r2 ¿ c ¿ ¿

Here K is di- electric constant. The above equation shows that F and K are inversely proportional to each other. This means that if value of K for solvent is INORGANIC CHEMISTRY

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NON-AQUEOUS SOLVENTS high, the force between ions of solute is very low and hence solvent will dissolve the solute quickly and hence solvent of high dielectric constant has increased ability to dissolve ionic or polar compound.

3.Electrical conductance: These solvents are self-ionising; hence electrical conductance gives an idea about the extent of such ionisation. Greater is the ionisation, easier it is for acidbase reactions to occur in the solvent.

4.Viscosity: Viscosity is an important property of a liquid solvent. Some solvents are highly fluid, e.g. water, low molecular weight alcohols, liq. NH3, while some are viscous e.g. anhydrous HF, SO2 etc.

5.Density: Density is also very important physical property. Density is defined as below: SOLVENT

DENSITY(g/mol )

Water

1.0 at (4◦C)

Ammonia

0.68 at b.p

Sulphur dioxide

17.4 at (6◦C)

 CHEMICAL PROPERTIES: Chemical properties include chemical reactions that occur in solvents. Following are chemical reactions that occur non-aqueous solvents:

1. NEUTRALIZATION REACTION: INORGANIC CHEMISTRY

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NON-AQUEOUS SOLVENTS The reaction in which acid combines with base and form salt and water is known as neutralization reaction.

Neutralization occurs in non- aqueous solvent in such a way: “That compound containing or making available acidic specie combines with compound containing or making available basic specie to form salt and solvent” Neutralization reaction can also be defined as the combination of solvent cation and solvent anion to form the un-ionized solvent. Following are the examples of neutralization reaction in liq.NH3 and liq. SO2: EXAMPLES:

INORGANIC CHEMISTRY

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NON-AQUEOUS SOLVENTS

N e u t r a liz a t io n r e a c t io n in liq . N H 3:

B ase

A c id

+

NH 4Cl

KNH 2

S o lv e n t

S a lt

liq .N H

3

+

KCl

2NH 3

A c tu a lly th e a b o v e r e a c tio n ta k e s p la c e th r o u g h f o llo w in g s te p s : NH 4Cl KNH 2 NH4Cl

+

liq .N H

3

liq .N H

3

KNH2

+

NH4 + +

+

K

liq .N H

3

K

+

+

Cl

Cl

– –

NH2 –

+

O th e r E x a m p le s : NH 4X

+

2NH 4X

NaNH 2

+

3NH 4 X

INORGANIC CHEMISTRY

+

PbNH BiN

liq .N H

3

liq .N H

liq .N H

3

3

Page 12

–

NH4 + NH2

+

NaX

+

PbX 2

+

3NH 3

BiX 3

+

4NH 3

2NH 3

NON-AQUEOUS SOLVENTS

N e u t r a l i z a t i o n r e a c t i o n in l iq . S O 2:

B ase

A c id SOCl 2

+

liq . S 0 2

Cs2 SO3

S o lv e n t

S a lt 2CsCl +

2SO 2

A c t u a lly t h e a b o v e r e a c t io n ta k e p la c e in t h e f o llo w in g s t e p s : liq . S 0 2

SOCl 2

liq . S 0

2

+2

-

-

+2

+ 2Cl +2 + 2Cs + SO2

liq . S 0 2

Cs 2SO 3 SOCl 2+ Cs2SO 3

SO

+

2Cs Cl + SO + SO 3

-2

0 t h e r E x a m p le s : SO(SCN) 2

+

K 2SO3

liq . S 0

SOBr 2 + [N(CH 3) 4]2SO 3 l i q . S 0 2

2

2K(SCN)

+

2SO 2

2[N(CH 3)4 ] 2Br

+

2SO 2

2. FORMATION OF ADDITION COMPOUNDS/SOLVATION: Solvation reaction is defined as following: “ The reaction in which a solute(cation, an anion or a neutral molecule) react with one or more molecules of solvent to form product in which the solute and solvent species are attached to each other by a H-bond or coordinate bond, the reaction is known as solvation reaction.”

The products obtained as a result of solvation reactions are called solvates. The solvation reaction taking place in water, liq. NH3, alcohol, hydrazine and ether are known as hydration, ammoniation, alchoholation, hydrazination and etheration respectively and the solvates obtained in these reactions are called hydrates, ammoniates, alcoholates, hydrozinates and etherates respectively. INORGANIC CHEMISTRY

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NON-AQUEOUS SOLVENTS The attachment of the solvent molecule to the solute species may be through ion-dipole, hydrogen bonding or coordinate bonding. In the last type of bonding the solvent may act as a Lewis base or as a Lewis acid depending on the acceptor or donor property of the solute respectively, e.g.,

S o lu te ( L e w is a c id )

S o lv e n t

S o lv a te s

( L e w is b a s e )

(a d d u c ts )

SO 3

+

2NH 3

SO3.2NH 3 ( 1 : 2 a d d u c t )

SiF 4

+

2NH 3

SiF4 .2NH3 ( 1 : 2 a d d u c t )

BF 3 KI

+ +

NH 3

BF3 .NH3 ( 1 : 1 a d d u c t )

4SO 2

K [(SO 2) 4I] ( 1 : 1 a d d u c t )