Solvent plays a crucial role in the reaction. So we should smartly work with solvent. This presentation tries to summarizes the role as well as the effects of the solvent on the different reactions.

1. 11
Hardik MistryHardik Mistry
L M college of pharmacyL M college of pharmacy
Pharmaceutical chemistry dept.Pharmaceutical chemistry dept.

2. 22
SOLVENTSOLVENT
It is the component of the reaction system that
seldom appear in our equation , but that is
nearly always present.

3. 33
Importance of the solventImportance of the solvent
The presence of the solvent can speed up or slow
down the reaction by a factor 1020
Change from the one solvent to another can bring
about millionfold change in reaction rate.
Solvent effect is more powerful than any other factors
like steric factor,polarity factor or symphoric factor.

4. 44
Solute molecules and ions do not exist in the solution
as nacked particles but always solvated and each
dissolved particles is a cluster of solvent molecules.

5. 55
Any solvent cluster help to make up the structure and
determine their stabilities which help us to predict the
chemical behavior of the structure.
Through the secondary bonding ,solvent help us to
determining the shape of the of large molecules like
protein and nucleic acids and inturn their biological
activities.

6. 66
Secondary bondingSecondary bonding
A) Ion-Ion bonds
B) Dipole-Dipole bonds
+ - + - + -+ - + - + -
- +- +
+ _

7. 77
C) Van der Waals forces
D) Ion-Dipole bonds
+ - + - + - + - + - +
- + - + - + - + - + -
+
+
+
+
+
+
+
_
_
_
_
_
_
_
+ _
_
_
_
+
_
+
+
+
+
_

8. 88

9. 99
Role of solvent inRole of solvent in
SOLUBILITYSOLUBILITY
1) Non ionic solute:
Solubility of it depends upon their polarity and in
particular their ability to form hydrogen bond.

10. 1010
“Like dissolves like “ apply here
Example : hydrocarbons dissolve in benzene while alkyl
halide dissolve in solvent like chloroform or carbon
tetrachloride.

11. 1111
In case of alcohol , it composite alkane-like alkyl group
and water like hyroxyl group.
Hydroxyl group is polar with high electronegativity of
oxygen element.
So alcohols are soluble in other hydroxyl compounds like
water.

12. 1212
2) Ionic solutes.
Here the forces holding together an ionic lattice are
powerful, and to break them ,energy is supplied by the
formation of the ion-dipole bonds and the solvent.

13. 1313
To dissolve ionic compounds
- solvent must be highly polar
- it must have high dielectric constant
Means it must be good insulator to lower the
attraction between oppositely charged Ions once they
are solvated.

14. 1414
Water is the universal and most superior solvent due to
its polarity and high Dielectric constant.
Solvating power(the ability to form strong bonds to
dissolved ions) is very important and so there are other
liquids having large dipole moments and high
Dielectric constant are still poor solvent.

15. 1515
Cations are attracted to theCations are attracted to the
negative pole of the solventnegative pole of the solvent
Look in water here
O
H
H
+δ
+δ
-δ
+

16. 1616
Anions are attracted to theAnions are attracted to the
positive pole of the solventpositive pole of the solvent
H O
H
_
+δ
-δ
+δ

17. 1717

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Types of the solventTypes of the solvent
1) Protic solvent
solvents containing hydrogen that is attached to the
oxygen and nitrogen and hence acidic enough to form
hydrogen bond.
Example: water and methanol

20. 2020
Other protic solvents solvate ions in the same manner as
that water does:
Cations: through unshared pairs
Anions: through hydrogen bonding

21. 2121
2) Aprotic solvents
solvents with moderately high dielectric constant
and do not contain acidic hydrogen.
Example: DMSO,DMF,HMPT

22. 2222
The aprotic solvents dissolve ionic compounds, but
they cannot form hydrogen bond to anions as in
protic solvents.
Aprotic solvents are highly polar and thus they
dissolve ionic compounds through their solvation of
the cations.

23. 2323
Selection of the solventSelection of the solvent
Example: water and methanol
Water dissolves ionic compounds very well but poor
for organic one
while non polar solvents dissolves organic compounds
but poor for inorganic salts.

24. 2424
Alcohol ,particularly smaller ones like methanol and
ethanol due to their alkyl group dissolve non polar
organic compounds and due to their hydroxyl group
dissolve polar compounds.

25. 2525
Problem with alcoholProblem with alcohol
Water and alcohol are protic solvents and through the
hydrogen bonding they solvates the anions strongly
so they stabilizes the anions and lower their
reactivity and so weakens its basicity and nucleophilic
power.

26. 2626
So we require aprotic solvent
and reaction that ,in protic solvent proceed slowly
at high temperature to give low yields may be found
,In aprotic solvent, to proceed rapidly often at room
temperature-to give high yields.

27. 2727
Role of solvent in SRole of solvent in SNN1 reaction1 reaction
Here one substrate reacts faster than another chiefly
because of a lower Eact .
In SN1, for example the difference in rate between
tertiary and secondary substrate corrosponds to a
difference in Eact of about 15 kcal.
C CH3
CH3
H3C
Br
+ OH-
C CH3
CH3
H3C
OH
+ Br-
Rate= k[RBr]

28. 2828
In the SN1 reaction in transition state there is a
heterolysis means bond breaking takes place without
any bond forming to balance it to provide energy.
And in the gas phase bond dissociation energy shows
that heterolysis of an alkyl halide requires great deal
of energy of about 149 Kcal/mole for tert-butyl
bromide.

29. 2929
Where does this great amount of energy comes from?Where does this great amount of energy comes from?
This great amount of energy comes from formation of
the many bonds: bonds between the ions produced
and solvent.
The ions are not generated as the naked particles but
they are generated as the solvated ions.

30. 3030
Ions are products of heterolysis.
Individually, each of these ion-dipole bonds is
relatively weak, but altogether they provide great
deal of energy.
+
+
+
+
+
+
+
_
_
_
_
_
_
_
+ _
_
_
_
+
_
+
+
+
+
_

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The reactant has dipole moment, and forms dipole-
dipole bonds to solvent molecules.
R R XX R+
+ X-+δ −δ
Reactant transition state
product

32. 3232
Transition state has great dipole moment than reactant
and can form strong dipole-dipole bonds to the
solvent.
The solvent thus stabilizes the transition state more
than it does the reactant, lowers the Eact and speeds up
the reaction.

33. 3333
Clearly the effect of the solvent :it lowers the Eact by
130 Kcal or more and thus allow the reaction to takes
place.

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Kinds of the solvent that is best for SKinds of the solvent that is best for SNN1 reaction1 reaction
More polar solvent, the stronger the ion-dipole
moment.
So the SN1 reaction of neutral substrates in water
than ethanol. they go faster in 20% ethanol than
80% ethanol.

35. 3535
Role of solvent in SRole of solvent in SNN2 reaction2 reaction
SN2 reaction: reaction between alkyl halide and hydroxyl ion
Most of the energy needed to break the bond to the leaving
group is supplied by the making of the bond to the
nucleophile. in attach by OH-
, C-X bond is being broken
and simultaneously C-O bond is being formed.
CH3Br + OH- CH3OH + Br-
Rate = k [CH3OH] [OH-
]

36. 3636
Among the protic solvents, ionizing power is highest
for those solvent which can form most stronger
hydrogen bond among all of them.
example:- 2,2,2 tri fluoroethanol
- formic acid
- trifluoroacetic acid

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Solvents which speeds up the SN1 reaction enormously
slows down the SN2 reaction by the factor 1020
!
The alkyl halide has dipole moment and forms weak
dipole-dipole bonds with the solvent. hydroxyl ion
has fully negative charge and forms very powerful
ion-dipole bond to the solvent.

38. 3838
The transition state carries a full negative charge too
and charge is divided between alkylhalide and
hydroxyl ion and so the solvent stabilizes the reactants
more than that in the transition state and raises the
Eact and slows down the reaction.
R R XX OH + X-
−δ
Reactant transition state
product
OH-
+ HO
+δ
R

39. 3939
Solvation of the anionic nucleophile is thus overriding
factor here. By stabilizing it-relative to the transition
state-the solvent deactivates the nucleophile.

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Kinds of the solvent that is best for SKinds of the solvent that is best for SNN22
reactionreaction
The greater the polarity slower the SN2 reaction.
SN2 reaction in the solvents like dimethyl sulphoxide
(DMSO), dimethylformamide (DMF) or
hexamethylphosphorotriamide (HMPT) go millions
times faster than in alcohol or alcohol-water mixture.

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The SThe SNN2 reaction:2 reaction:
Phase transfer catalysisPhase transfer catalysis
Example:
The reaction of an alkyl halide with sodium cynide to
yield alkyl cynide (nitrile).
Traditional way is to use solvent which dissolves both
reagents.

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Consider solution of alkyl halide in nonpolar organic
solvent and a solution of sodiumcynide in water.
Then
The solvents are immisible, two separate layers and
on heating also no reaction takes place.

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On the addition of small amount of quaternary
ammonium salt to the reaction mixture, alkyl halide
and cynide still seperated in its own phase-react
rapidly and give high yield of the nitrile.
This type of the reaction is called as the phase
transfer catalysis.

44. 4444
Mechanism
Q+
Z-
+ R-X R-Z + Q+
X-
Substrate product
organic phase
Q+
Z-
+ Na+
X- Na+
Z-
+ Q+
X- Aqueous phase

45. 4545
Ions of the alkyl halides are in the organic phase while
cynide ions are in the aqueous phase.
Q+
ion carry cynide ion from aqueous to organic phase
and reaction of alkyl halide and cynide ion takes
place and form alkyl cynide.
And Q+
carry halide displaced from alkylhalide back to
the aqueous phase.