See fig.1.

In unimolecular Nucleophilic substitution reaction, leaving group (a nucleophile) is first expelled in slow (rate determining) step followed by attack of another nucleophile. Some important characteristics of `SN^1` mechanism are :

(i) Better the leaving group, faster will be rare of `SN^1` reaction because leaving group is expelled in rate determining step.

`R-I > R-Br > R-Cl` (`SN^1` reactivity)

(ii) As the product of rate determining step is carbocation, we can interpret that more the stability of carbocation, more will be rate of `SN^1` reaction.

`3° R-X > 2° R-X > 1° R-X`

`3° R-OH > 2° R-OH > 1° R-OH`

(iii) When solvent polarity is increased, increase in solvation of neutral reactant is almost unaffected, solvation of partially charged transition state increases slightly but the solvation of charged intermediate increases considerably (as shown in energy profile of rate determining step). Therefore, increase in solvent polarity decreases the activation energy and increases the rate of `SN^1` reaction. Hence, `SN^1` reaction is highly favoured in polar solvent. See fig.2.

(iv) As a carbocation (containing six valence electrons and a vacant orbital) is formed, rearrangements are possible before the attack of nucleophile.

(v) `Nu^(-)` can be a weaker nucleophile than `L^(-)`. A strong `Nu^-` is not required for `SN^(1)` reaction. Moreover, increase in nucleophilic strength of `Nu^(-)` or its concentration will not increase the rate of `SN^(1)` reaction.

(vi) As carbocation is planar, attack of `Nu^-` from both the sides of plane is equally favoured. Therefore, `SN^1` reaction proceeds with 50% inversion of configuration (attack of nucleophile from opposite side of leaving group) and 50% retention of configuration (attack of nucleophile from the side of leaving group) i.e. `SN^1` reaction proceeds with 100% racemization (racemization is the process of formation of racemic mixture which is the equilibrium mixture of two mirror images of each other). See fig.3.

However, it must be remembered that even if solvent is highly polar (which is generally used for doing `SN^1` reactions), the attractive force between the carbocation and `L^-` cannot be completely destroyed. They will have some attractive force and will have some extent of ion pairing (although very small). Therefore, attack of nucleophile from opposite side of leaving group will be more favoured (though very slightly). In other words, inversion of configuration will be slightly more than retention of configuration i.e. `S_N 1` reaction leads to almost 100% racemization with little inversion.

(vii) Unimolecular elimination `(E_1)` will be the competing reaction.

`text(Examples of)` `SN^1 :`

`(i)` `text(Acid catalyzed nucleophilic substitution on most alcohols except smaller primary alcohols :)` See fig.4.

`(ii)` `text(Solvolysis)` `text((hydrolysis & alcoholysis))` `text(of most Alkyl halides except smaller)` `1^o` `text(Alkyl halides.)` See fig.5.

See fig.1.

In bimolecular nucleophilic substitution, the attack of nucleophile as well as removal of leaving group occurs simultaneously in a single step. Some important characteristics of `SN^2` mechanism are :

(i) Better the leaving group, faster will be race of `SN^2` reaction because leaving group is expelled in rate determining step.

`R- I > R- Br > R- Cl`

(ii) As the product of `SN^2` reaction is saturated, stability of the product (highly stable) is of little importance. However, the transition state is highly crowded (five groups attached to carbon), steric hindrance is of large importance. More the steric hindrance, lesser will be the rate of `S_N2` reaction.

`3^o R- X < 2^o R- X < 1^o R- X < CH_3- X`

(iii) When solvent polarity is increased, increase in solvation of negatively charged reactant and negatively charged products is considerable, while solvation of partially charged transition state is slight (as shown in energy profile of rate determining step). Therefore, increase in solvent polarity increases the activation energy and decreases the rate of `SN^2` reaction. Hence, `SN^2` reaction is less favoured in polar solvent. See fig.2.

(iv) No rearrangements are observed during `S_N2` reaction.

(v) As `Nu^(-)` has to remove `L^(-), Nu^(-)` has to be a stronger nucleophile than `L^(-)`. Moreover, increase in the nucleophilic strength or its concentration will increase rate of `S_N2` reaction.

(vi) Nucleophile attacks from the opposite side of leaving group due to which `S_N2` reaction proceeds with `100%` inversion of configuration (it is called Walden inversion). See fig.3.

(vii) `S_N2` and elimination reactions are the competing reactions because strong nucleophiles are generally strong bases also.

`text(Examples of)` `S_N2` :

(i) Nucleophilic substitution in lower `1°` alcohol (`CH_3OH, EtOH, C_3H_7OH` etc) in acidic medium. See fig.4.

(ii) Solvolysis (hydrolysis & alcoholysis) of smaller `1°` Alkyl halides like `CH_3X, C_2H_5X, C_3H_7X` etc. See fig.5.

(iii) Most nucleophilic substitution reactions of alkyl halides except `3°` alkyl halides. See fig.6.

This type of mechanism is observed when leaving group is attached to an unsaturated carbon having small amount of positive charge (to facilitate the attack of nucleophile on `pi` electron rich system). See fig.1.

Some important aspects of AN+ DN reaction are :

(i) `Nu^-` can be a weaker nucleophile than `L^-` because Nu has not expelled `L^-`. However, `Nu^-` must be a strong base (bad leaving group) than `L^-` so that removal of `L^-` can occur in preference to `Nu^-`. See fig.2.

Moreover, as nucleophile is involved in r.d.s., higher the nucleophilic strength or concentration of `Nu^-` higher will be rate of AN+DN reaction.

(ii) Higher the amount of positive charge on the carbon faster will be attack of nucleophile i.e. r.d.s and faster will be AN+DN reaction. See fig.3.

(iii) Nucleophilic addition will be the competing reaction. Here it can be generalized that if `L` is even a bad leaving group like `NH_2`, it can be removed from anions. However, removal of `H^-, R^-` and `Ar^-` is almost impossible (almost a last option) even from anions. Therefore, AN+DN reaction will mainly occur if `L` is `X, OCOR, OH, OR` or `NH_2` but Nucleophilic addition will dominate if `L` is `H`, `R`, `Ar` (i.e. in aldehydes and ketones). See fig.4.

`text(Examples of AN+DN Reactions :)`

(i) `text(Nucleophilic substitutions in Acid halides, acid anhydrides, esters and amides etc)`. See fig.5.

(ii) `text(AN+DN mechanism is also observed when leaving group is attached to an atom having vacant orbital)`

.However, in such examples attack of nucleophile is not rate determining step because it does not involve any bond cleavage and only bond formation is never a slow i.e. rate determining step. See fig.6.

See Table