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 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.