E1 vs E2 Elimination reactions, Elimination vs Substitution:
Competition between E2 and E1 reactions:
In an E1 mechanism reaction, the rate determining step is unimolecular and does not involve the base. The leaving group depart in the first step, while the proton is removed in the second step. In contrast, an E2 reaction has a bimolecular rate determining step requiring the base. Loss of the leaving group is simultaneous with the removal of a proton by the base.
Primary alkyl halide undergoes only E2 elimination reaction. They cannot undergo E1 reactions because of the difficulty encountered in forming primary carbocations. Secondary and tertiary alkyl halide undergo both E2 and E1 mechanism reaction. For those halides that can undergo both E2 and E1 reaction, an E2 reaction is favoured by the same factor that favour an SN2 reaction, and an E1 reaction is favoured by the same factors that favour an SN1 reaction. Thus an E2 reaction is favoured by the high concentration of a strong base and an aprotic polar solvent (example DMSO or DMF), whereas an E1 reaction is favoured by a weak base and protic polar solvent (example H2O or ROH).
If the attacking reagent is a good base, it abstracts the β-proton and causes elimination whereas a good nucleophile attacks the α-carbon causing substitution.
Elimination vs Substitution:
The following factors influence the extent of elimination and substitution: (i) structure of the reactant (ii) nature of the base (iii) nature of the solvent (iv) effect of temperature.
Structure of the reactant: Generally where elimination and substitution are competing reactions, the proportion of elimination increases with increase chain branching of the reactant. One reason for this preference may be ascribed to greater stability of highly substituted alkene formed from substrates such as alkyl halides. The other reason is purely steric and is applicable only to a Unimolecular reaction where Carbocations are the common intermediates.
Carbocations are planar in structures with a bond angle of 1200, and with the increase in branching, they will show more resistance to the decrease in bond angle (1200 to 109.28o) brought about by the substitution. On the other hand, elimination will be preferred as it does not involve any change in bond angles.
Nature of the base: A good nucleophile may not necessarily be a good base or vice versa. The basicity of some reagents are found to be in the order:
H2O < I– < Cl– < CH3OO– < SH– < OH–
Whereas, the nucleophilicity is of the same reagents follow different order:
H2O < CH3OO– < Cl– < OH– < I– < SH–
To some extent, the elimination/substitution ratio depends upon the nature of the base involved. In bimolecular reaction (E2 and SN2), for example, if we have a weak base but strong nucleophile like iodine, there will be more substitution than elimination. On increasing the concentration and strength of the base, the unimolecular reaction tends to bimolecularility. This change in molecularity is more pronounced in E1 than in SN1 reaction, leading to a higher proportion of the elimination product. This is precisely the reason for employing strong bases at high concentration in the preparative method or alkenes.
Nature of the solvent: A change in the polarity of the solvent also affects the course of a reaction. In bimolecular reaction, for example, E2/SN2 ratio depends on how well the solvent is able to solvate the two transition states of the same reactant. Usually, a bimolecular reaction is favoured by a decrease in the polarity of solvents, but this effect is more pronounced in E2 reaction. Thus a less polar solvent not only favours bimolecular reaction but also E2 reaction over SN2 reactions.
Effect of temperature: Elimination reactions usually have higher activation energy than the accompanying substitution reactions. Therefore increase in the temperature of the reaction increases the extent of elimination.