Polar Aprotic - Protic table

Protic  Table

Polar Protic solvents are those solvents that are capable of hydrogen-bonding interactions. Most of these solvents have OH groups,

Examples: Water, Formic acid, Methanol, and Acetic acid.

Polar aprotic solvents are solvents do not have an acidic hydrogen. Consequently, they are not capable of hydrogen-bonding interactions. Most of these solvents do not have OH groups. These solvents generally have intermediate polarity and dielectric constant.

Examples: dimethyl sulfoxide,   N, N -dimethylformamide, and acetonitrile 

Polar Protic solvents	Polar Aprotic solvents
HCOOH CH3COOH CH3OH H2O	(CH3)2S=O CH3CN CH3NCOH

The polarity of a solvent is related to its dielectric constant (ε), which is a measure of the ability of a material to moderate the force of attraction between oppositely charged particles. The standard dielectric is a vacuum, assigned a value ε of exactly 1, to which the polarities of other materials are then compared. The higher the dielectric constant ε, the better the medium is able to support separated positively and negatively charged species. Solvents with high dielectric constants are classified as polar solvents; those with low dielectric constants are nonpolar.

Solvent plays an important role in determining the acidity and basicity of a substance. 
A change from a protic to an aprotic solvent can also affect the acidity or basicity, since there is a difference in solvation of anions by a protic solvent and an aprotic one. Example, in DMF, picric acid is stronger than HBr, although in water HBr is far stronger.

Protic  Table

Polar Protic solvents	Dielectric constant
H2O	80
HCOOH	58
CH3OH	33
C2H5OH	24.55
CH3COOH	6.2

  
EFFECT OF SOLVENT ON NUCLEOPHILIC SUBSTITUTION REACTION MECHANISM
Changing the solvent in which a reaction is carried out often exerts a profound effect on its rate and may, indeed, even result in a change in its mechanistic pathway.Thus for a halide that undergoes hydrolysis by the SN1 mode, increase  in the polarity of the solvent and its ion-solvating ability is found to result in a very marked increase in reaction rate.

From the table given below, the rate of solvolysis of tert -butyl chloride (which is equal to its rate of ionization) increases dramatically as the solvent becomes more polar.

This occurs because , in SN1 mode , charge is developed and concentrated in the T.S. compared with the starting material:

The energy required to effect such a process decreases as dielectric constant rises; The process is also facilitated by increasing solvation and consequent stabilization of the developing ion pair compared with the starting material.That such effects, particularly solvation are of prime importance is borne out by the fact that SN1 type of reactions are extremely uncommon in the gas phase.

A solvent with a low dielectric constant has little effect on the energy of the transition state, whereas one with a high dielectric constant stabilizes the charge-separated transition state, lowers the activation energy, and increases the rate of the reaction.

If the polar solvent is a protic one, stabilization of the transition state is even more pronounced because of the hydrogen bonding that develops as the leaving group becomes negatively charged.

EFFECT OF SOLVENT ON SN2 MECHANISM

Polar solvents are required in typical bimolecular substitutions because ionic substances or polar molecules are not sufficiently soluble in non-polar solvents to give a high enough concentration of the nucleophile to allow the reaction to occur at a rapid rate. However for SN2 mode, increasing solvation polarity is found to have a much less marked effect, resulting in a slight decrease in reaction rate. This is because in this particular example new charge is not developed and existing charge is dispersed, in the T.S. compared with the starting material:

Thus solvation of the T.S. is likely to be somewhat less effective than that of the initial nucleophile- hence the slight decrease. This differing behaviour of SN1 and SN2 modes to changes of solvent can be used to some extent diagnostically. What is more important is whether the polar solvent is protic or Aprotic. Protic solvents such as water, methanol all have -OH groups that allow them to form hydrogen bonds to ionic nucleophiles.

This clustering of protic solvent molecules (solvation) around it suppresses the nucleophilicity of the anion and retards the rate of bimolecular substitution.

Aprotic solvents, on the other hand, lack -OH groups and do not solvate anions very strongly, leaving the anions much more able to express their nucleophilic character. 
So far as actual changes of mechanistic pathway with change of solvent are concerned , increase in solvent polarity and ion-solvating ability may (not always) change the reaction mode from SN1 to SN2. Transfer from hydroxylic to polar , non-protic solvents can (often do) , change  the reaction mode from SN1 to SN2 by enormously increasing the effectiveness of the nucleophile in the system.