Solvent effect in reaction mechanism
●A vast majority of organic reactions are carried out in solution.
• A quite small change in the solvent used can have profound effects on reaction rates and mechanism of a reaction.
• When there are polar intermediates forming in a reaction, the solvents play an important role.
• For example, carbocations and carbanions as ion pairs are greatelystabilized or not stabilized by the solvent employed.
• Polarity of solvent and its ion solvating capability does influence the stabilization of the intermediates. By contrast, the reactions involving radicals and the reactions having no intermediates are much less influenced by the nature of the solvent used.
• Changing of solvent may even result in change its mechanistic pathway.
• An SN1 reaction rate is increased in polar solvents because of their ion solvating ability.
• Solvolysis of tert butyl bromide- (CH3)3C-Br has 3x104times greater rate of reaction in aqueous ethanol compared to ethanol itself.
• In SN1 reactions, charge is developed and concentrated in the transition state compared to the starting material.
• R-Hal R-----Hal
• The energy required to effect such a process decreases as dielectric constant e of solvent increases. And is helped by increasing solvation and stabilization of ion-pair.
• SN1 reactions are not taking place in gas phase.
• They can be only carried out in solutions.
• While SN2 reactions are not much affected by change in solvent polarity.
• There is a slight decrease in rate in SN2 type reactions while going from non-polar to polar hydroxylated solvents.
• When a nucleophile is charged, it is stabilized more than the transition state.
• In other words, solvation of transition state is less effective than that of the initial nucleophile.
• This difference in effect of solvent on SN1 and SN2 reactions may be used to diagnose the type of reaction taking place.
• It is quite possible for same reaction to go by the SN1 mechanism in one solvent and SN2 in another solvent.
• Solvents may be classified as polar and non-polar solvents.
• Polar solvents are of two types:
• i) Polar hydroxylic solvents or polar protic solvents and
• ii) Polar non-hydroxylic solvents or aprotic solvents.
• Aprotic solvents are non-polar solvents also.
• Non-polar solvents are aprotic solvents.
• Carboxylic acids, water and alcohols are polar protic solvents.• Their ionizing rates are high. They are good for SN1 reactions.• CF3COOH is having one of the highest polarizing or ionizing power.• Methanol is having higher ionizing power and dielectric constant compare to ethanol.• Non-polar solvents are • Hexane• Benzene• Chloroform • Carbon tetrachloride• Other alkanes including cycloalkanes• Moderately polar solvents are:• Ether• Ethyl acetate• THF• Dioxane etc.• Polar solvents are two types as we have seen.• Hydroxylated or protic: alcohols, acids water.• Non-hydroxylated or aprotic:• DMSO, • DMF,• Acetonitrile, • pyridine etc.• In Sn2 reactions, change of solvent from polar protic to polar aprotic solvents has remarkable effects.• Protic solvent can form hydrogen bond with a nucleophile.• In aprotic solvents(such as DMF,DMSO) there is a better solvation but no hydrogen bonding. So rate of SN2 is increased in these solvents.N3 + CH3I --------> CH3N3 + I-• There is 4.5x104times increase when solvent is changed from
• Thus transfer of solvents from hydroxylated polar solvents to polar non-protic solvents can change the reaction mode from SN1 to SN2 by increase in the effectiveness of the nucleophile.• On the other hand, change in solvent from non-polar to polar hydroxylated solvents result in increase in their solvating property may change the reaction pathway from SN2 to SN1.
methanol to DMF.
●A vast majority of organic reactions are carried out in solution.
• A quite small change in the solvent used can have profound effects on
reaction rates and mechanism of a reaction.
• When there are polar intermediates forming in a reaction, the
solvents play an important role.
• For example, carbocations and carbanions as ion pairs are greately
stabilized or not stabilized by the solvent employed.
• Polarity of solvent and its ion solvating capability does influence the
stabilization of the intermediates. By contrast, the reactions involving radicals and the reactions having no intermediates are much less influenced by the nature of the solvent used.
• Changing of solvent may even result in change its mechanistic
pathway.
• An SN1 reaction rate is increased in polar solvents because of their
ion solvating ability.
• Solvolysis of tert butyl bromide- (CH3)3C-Br has 3x104
times greater rate of reaction in aqueous ethanol compared to ethanol itself.
• In SN1 reactions, charge is developed and concentrated in the transition
state compared to the starting material.
• R-Hal R-----Hal
• The energy required to effect such a process decreases as dielectric
constant e of solvent increases. And is helped by increasing solvation and
stabilization of ion-pair.
• SN1 reactions are not taking place in gas phase.
• They can be only carried out in solutions.
• While SN2 reactions are not much affected by change in solvent polarity.
• There is a slight decrease in rate in SN2 type reactions while going from
non-polar to polar hydroxylated solvents.
• When a nucleophile is charged, it is stabilized more than the
transition state.
• In other words, solvation of transition state is less effective than that
of the initial nucleophile.
• This difference in effect of solvent on SN1 and SN2 reactions may be
used to diagnose the type of reaction taking place.
• It is quite possible for same reaction to go by the SN1 mechanism in
one solvent and SN2 in another solvent.
• Solvents may be classified as polar and non-polar solvents.
• Polar solvents are of two types:
• i) Polar hydroxylic solvents or polar protic solvents and
• ii) Polar non-hydroxylic solvents or aprotic solvents.
• Aprotic solvents are non-polar solvents also.
• Non-polar solvents are aprotic solvents.
• Carboxylic acids, water and alcohols are polar protic solvents.
• Their ionizing rates are high. They are good for SN1 reactions.
• CF3COOH is having one of the highest polarizing or ionizing power.
• Methanol is having higher ionizing power and dielectric constant
compare to ethanol.
• Non-polar solvents are
• Hexane
• Benzene
• Chloroform
• Carbon tetrachloride
• Other alkanes including cycloalkanes
• Moderately polar solvents are:
• Ether
• Ethyl acetate
• THF
• Dioxane etc.
• Polar solvents are two types as we have seen.
• Hydroxylated or protic: alcohols, acids water.
• Non-hydroxylated or aprotic:
• DMSO,
• DMF,
• Acetonitrile,
• pyridine etc.
• In Sn2 reactions, change of solvent from polar protic to polar aprotic
solvents has remarkable effects.
• Protic solvent can form hydrogen bond with a nucleophile.
• In aprotic solvents(such as DMF,DMSO) there is a better solvation but
no hydrogen bonding. So rate of SN2 is increased in these solvents.
N3 + CH3
I --------> CH3N3 + I-
• There is 4.5x104
times increase when solvent is changed from
• Thus transfer of solvents from hydroxylated polar solvents to polar
non-protic solvents can change the reaction mode from SN1 to SN2
by increase in the effectiveness of the nucleophile.
• On the other hand, change in solvent from non-polar to polar
hydroxylated solvents result in increase in their solvating property
may change the reaction pathway from SN2 to SN1.
methanol to DMF.
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