See fig.1.

The group which is removed with the bonding electron is called leaving group and is represented by `'L'`. The ease of removal of `L` (leaving tendency) is inversely related to the basic strength of `L`. Weaker bases are better leaving groups whereas stronger bases are poorer leaving groups. The basic strength of few leaving groups increases as below : See fig.2

(i) On the basis of basic strength we can say that halides are very good leaving groups and their leaving tendency is `I^(-) > Br^(-) > Cl^(-)` as their basic strength is `I^(-) < Br^(-) < Cl^(-) < F^(-)`. `F^(-)` is considered as bad leaving group among halides.

(ii) `RCOO^(-)` is weak base and hence a good leaving group. This group can be removed easily.

(iii) `HO^(-)` and `RO^(-)` are strong bases and hence bad leaving groups as such. They are not generally removed as such from neutral molecules but can be removed from anions. From neutral molecules, they can be removed very easily after their protonation in the form of `H_2O` and `ROH` which are very weak bases (almost neutral). See fig.3.

(iv) Removal of `text( )^(-)NH_2` is very difficult as it is a very strong base. It can not be removed as such from neutral molecules but can be removed (not so easily) from anions. It may also be removed (not so easily) from neutral molecules after its protonation in the form of ammonia which is a weak base. See fig.4.

(v) Removal of `H^(-), R^(-)` and `Ar^(-)` is very-very difficult as these are very-very strong bases and they can not be even protonated. Hence `H^(-), R^(-)` and `Ar^(-)` are not generally considered as leaving groups. Their removal is almost impossible (almost a last option) even from anion. Moreover, if at all they are removed from anions, their further reaction takes place in same step.

When two atoms or groups are removed from adjacent atoms, the bond cleavage of these two atoms or groups is generally heterolytic (rarely homolytic). The group which is removed with the bonding electron is called leaving group and is represented by `L`. See fig.1.

1,2-Eiimination mainly occurs via following mechanisms :

(a) `text(Unimolecular Elimination)` `(E_1)` : See fig.2.

In unimolecular elimination, leaving group is first expelled in slow (rate determining) step followed by abstraction of a proton. Some important characteristics of `E_1` mechanism are :

(i) Better the leaving group, faster will be rate of `E_1` reaction because leaving group is expelled in rate determining step.

`R- I > R- Br > R- Cl (E_1` `text(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 `E_1` reaction.

`3^o R-X > 2^o R-X > 1^o R-X`

`3^o R-OH > 2^o R-OH > 1^o 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 `E_1` reaction. Hence, `E_1` reaction is highly favoured in polar solvent. See fig.3.

(iv) As a carbocation (containing six valence electrons and a vacant orbital) is formed, rearrangements are possible before the loss of `H^(+)`.

(v) As proton has to be removed from adjacent atom of a carbocation (which is an easy process), a weak base like `H_2O, ROH` etc. can also act as base. A strong base is not required for `E_1` reaction. Moreover, increase in basic strength of the base or its concentration will not increase the rate of `E_1` reaction.

(vi) As carbocation is planar, removal of `H^(+)` from both the sides of plane is equally favoured. Therefore, `E_1` reaction can be `text(Anti E.limination)` (removal of two groups from opposite sides) or `text(Syn Elimination)` (removal of two groups from same sides).

(vii) If removal of a proton from two different adjacent atoms is possible, removal of proton occurs almost exclusively from more electronegative atom. If adj acent atoms are same, more stable (more substituted alkene) product is major product. See fig.4.

(viii) Unimolecular nucleophilic substitution `(S_N 1)` will be the competing reaction. See fig.5.

`text(Examples of)` `E_1` `text(are acid catalyzed dehydration of most alcohols except smaller primary alcohols.)`

(b) `text(Bimolecular Elimination)` `(E_2)` : See fig.1.

In bimolecular elimination, both atoms or groups from adjacent atoms are removed simultaneously in a single step. Some important characteristics of `E_2` mechanism are :

(i) Beller rhe leaving group, faster will be rare of `E_2` reaction because leaving group is expelled in rate determining step.

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

(ii) As the product of rate determining step in `E_2` reaction is alkene, more the stability of alkene, more will be rate of `E_2` reaction.

`3^o R-X > 2^o R- X > 1^o R-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 fig.2 in energy profile of rate determining step). Therefore, increase in solvent polarity increases the activation energy and decreases the rate of `E_2` reaction. Hence `E_2` reaction is less favoured in polar solvent.

(iv) No rearrangements are observed during `E_2` elimination.

(v) As proton has to be removed from a carbon of neutral species (a difficult job), a strong base is required for `E_2` reaction. Moreover, increase in the basic strength or its concentration will increase rate of `E_2` elimination as base is involved in r.d.s.

(vi) `E_2` elimination is preferably 'anti' although 'syn' elimination may also take place if anti configuration is not possible. See fig.3.

This is because when the base approaches the proton to be abstracted, it repels the bonding electrons away from leaving group in syn configuration and towards the leaving group in anti conformation.

(vii) If removal of proton from two adjacent atoms is possible, major product depends on the stability of product (alkene) as well as steric hindrance.

If active base is small `(1^o R-O^(-))`, steric hindrance can be neglected and the effect of stability decide the major product (Saytzeff rule). The elimination is called Saytzeff elimination and the product is called Saytzeff product.

If active base is bulky `(3^o R-O^(-))`, steric hindrance will be large and dominating and will decide the major product (Hoffmann's Rule). The elimination is called Hoffmann's elimination and the product is called Hoffmann's product. See fig.4.

(viii) Nucleophilic substitution and `E_1` are the competing reactions because strong base is generally a good nucleophile also.

`text(Examples of)` `E_2 :`

(i) Dehydrohalogenation of alkyl halides in basic medium : See fig.5.

(ii) Dehalogenation of vicinal dihalide using magnesium or zinc : See fig.6.

(iii) Acid catalyzed dehydration of smaller primary alcohols : See fig.7.

Normal primary alkyl halides undergo only `E_2` elimination reactions. They cannot undergo `E_1` reactions because of the difficulty encountered in forming primary carbocations. Secondary and tertiary alkyl halides can undergo either `E_1` or `E_2` reactions.

For those alkyl halides that can undergo both `E_1` and `E_2` reactions, the`E_2` reaction is favoured by a high concentration of a strong base and polar aprotic solvent while `E_1` reaction is favoured by a weak base and a polar protic solvent.

See fig.1.

In `E_(1CB)` mechanism, a proton is initially abstracted to generate conjugate base which then loses `L^(-)` giving the elimination product in rate determining step.

(i) As the proton has to be removed from a carbon of neutral species (a difficult job), a strong base is needed for `E_(1CB)` reaction as in `E_2` reaction. Moreover, increase in basic strength and the concentration of base increases the rate of `E_(1CB)` reaction.

Rate = `(k_1 xx k_2)/k_(-1)` [`text(Substrate)`] [`B^(-)`]

(ii) If removal of `H^(+)` from two adjacent atoms is possible, more acidic hydrogen is mainly removed.

(iii) Better the leaving group, faster will be `E_(1CB)` reaction (if `E_(1CB)` is operating).

`E_(1CB)` mechanism operates when either the abstraction of proton is very easy (removal of `H^+` from highly electronegative atom or formation of resonance stabilized conjugate base) or the expulsion of leaving group is difficult (leaving group is bad like `F^(-), OH^(-), RO^(-)` etc. or if leaving group has partial double bond due to resonance).

`text(Examples of)` `E_(1CB) :`

(i) Dehydrofluorination : Fluoride is the poorest leaving group among all halides (strongest base). See fig.2.

(ii) Dehydrohalogenation when resultant carbanion is resonance stabilized. See fig.3.

(iii) Dehydrohalogenation when halogen has partial double bond due to resonance as in vinyl halide & aryl halides. See fig.4.

(iv) Dehydration and deamination in basic medium. See fig.5.

When two atoms or groups are removed from non adjacent atoms, the elimination is called 1, n-elimination (also called cyclization). Such eliminations result in the formation of cyclic compounds. These can result into formation of rings which can be `3` member or larger. Before discussing 1, n-Eiimination we need to understand thermodynamic and kinetic aspect of ring formation.

Thermodynamic ease of ring formation : We know that the `6` member ring is strain free and most stable. Smaller rings have some strain in the ring which is very large in `3` member ring, large in `4` member ring and negligible in `5` member ring. Therefore, thermodynamic ease of ring formation is in the following order : See fig.1.

Kinetic ease of ring formation : Based on collision theory, we know that the reaction occurs only if the reacting molecules are in proper orientation. First and third atoms in every molecule (no `sp` hybrid atom) of a sample are close to each other (in proper orientation) and can form a bond with each other most easily. Therefore, `3` member ring formation is kinetically highly favoured.

First and fourth atoms in every molecule (due to rotation of bond between `2nd` and `3rd` atom) of a sample are not close to each other (not in proper orientation) and can not form a bond with each other easily. Therefore, `4` member ring formation is not kinetically favoured.

Similarly, larger the size of the ring, fewer will be the molecules in proper orientation (due to rotation of several bonds) and lesser it will be kinetically favoured. See fig.2.

`text(Based on above discussion we may conclude :)`

(i) 3 member ring formation takes place readily as its formation is kinetically favoured.

(ii) 4 member ring formation takes place rarely as its formation is favoured neither kinetically nor thermodynamically.

(iii) 5, 6, 7,.... member rings are formed easily as their formation is favoured thermodynamically.

(iv) When 4 member ring formation competes with other ring formation, formation of other rings dominate.

(v) When 5 member ring formation competes with larger than 5 member ring formation, formation of 5 member ring generally dominates as it is kinetically favoured and the stability of 5 member ring is comparable to larger rings.

(vi) When 6 member ring formation competes with other larger ring formation, formation of 6 member ring generally dominates because it is favoured kinetically as well as thermodynamically.

(vii) When 5 member ring formation competes with 6 member ring formation, it is very difficult to get a general answer because 5 member ring is slightly more favoured kinetically and 5 member ring formation is slightly more favoured thermodynamically. However, thermodynamically favoured product generally dominates at high temperature.


`text(Examples of)` `1, n-` `text(Eiimination :)`


(i) 1, n-Dehydrohalogenation of alkyl halide in basic medium when removal of `H^+` from nth atom is specially favoured (when nth atom is highly electronegative like `O`, `S`, `N` etc. or when anion on nth atom is resonance stabilized). See fig.1.

(ii) 1, n-Dehalogenation of 1, n-Dihaloalkane in presence of Magnesium or Zinc : See fig.2.

When two atoms or groups are removed from same atom. Such eliminations result in the formation of carbene or nitrene as we have already discussed during formation of intermediate. Such elimination occurs only when 1, n-Eiimination reactions are not specially favoured and 1, 2-elimination reactions are not possible. The carbene or nitrene intermediates formed in this way prefer to undergo rearrangements as discussed earlier.

`text(Dehydrohalogenation of Alkyl halide using a base :)` See fig.1.

`text(Dehalogenation of geminal dihalides using Mg or Zn :)` See fig.2.

If 1, 2-Dehalogenation is also possible. 1, 2-Dehalogenation will dominate. See fig.3.

It is observed that if two leaving groups are attached to one carbon, they are unstable provided at least one of the leaving groups has acidic hydrogen attached to directly linked atom. Such compounds are unstable even at room temperature and cannot be stored in pure form at room temperature because they undergo spontaneous elimination reactions even at room temperature. Although the reactions are endothermic but the increase in entropy favours the elimination reaction even at room temperature.

The following compounds are stable at room temperature inspite of more than one leaving group on same carbon. This is because the directly linked atom is not attached to acidic hydrogen. See fig.1.

The following compounds are unstable at room temperature because of more than one leaving group on same carbon as well as acidic hydrogen attached to the directly linked atom. See fig.2.

These reactions are catalyzed by acids as well as bases. The mechanism of the reaction is `E_1` in acidic medium and `E_(1CB)` in basic medium. The mechanisms are shown here to have a clear view about the reaction. In `E_1` mechanism, protonation is followed by removal of the leaving group which is then followed by loss of proton. In `E_(1CB)` mechanism, base first removes the proton and this step is followed by removal of leaving group from the conjugate base. Even strong bases like `HO^(-), RO^(-)` and `NH_2^(-)` etc. can be removed from anions. See fig.3.

However, in special cases, such two leaving groups may be stable at room temperature and require heating to undergo elimination reactions. The stabilizing factors may be intramolecular hydrogen bonding, lesser strain etc. that can increase their stability. This makes dissociation (elimination) more endothermic thus requiring higher temperature to make reaction spontaneous in forward direction. See fig.4.