EFFECT OF CATALYST ON SUBSTITUTION REACTION

Catalytic effect on S_N¹ reaction---

The rate of S_N¹ reaction increases by the addition of Lewis acids such as AlX₃, Ag⁺ ion and the Bronsted acids. The more acidic the nucleophilic solvent, the faster is the rate of S_N¹ reaction. Ag⁺ or Al³⁺ has a stronger affinity for X^- than has a solvent molecule. The formation of AgX or AlX₄^- accelerates the dissociation of X^-, thus increases the S_N¹rate. This is an example of electrophilic catalysis. The effectiveness of the H bonding, a factor that accelerates the dissociation of X^-, increases with the acidity of the H of H―A, the Bronsted acid.

Catalytic effect on S_N² reaction---

Crown ethers and phase transfer catalyst accelerate the rate of S_N² reaction.

Crown Ethers

The crown ethers contains at least four oxygen (O) atoms, they are the heterocyclic poly ether. The naming of crown ethers is done by the following way x-crown-y, where ‘x’ represents the total number of atoms in the ring, and ‘y’ represents the number of oxygen atoms (O). The structure of crown ethers are:

Crown Ethers

Crown ethers strongly complex with metallic cation in the cavity of the ring by forming ion-dipole bonds. 18-crown-6 strongly complexes and traps K⁺ as shown in the figure.

18-crown-6 traps potassium cation

There are a host-guest relationship between the crown ether and the ion that is transports. There host is crown ether, and guest is the coordinated cation. Similarly, 12-crown-4 forms complex with Li⁺ and 15-crown-5 forms complex with Na⁺ ion.

Ion pairing diminishes the reactivity of the anion which is intended to act as a nucleophile in substitution reaction. By complexing the cation, the crown ethers leave a ‘bare’ anion with a greatly enhanced reactivity. Since the nucleophile comes after the rate determining step in an S_N¹ reaction, enhancement of nucleophilic character of an anion does not influence the rate of an S_N¹ reaction, but it increases the rate of S_N² reaction. For example, the rate of the reaction---

CH₃―CH₂―Br + KF -------> CH₃―CH₂―F

Increases several times by the addition of 18-crown-6.

Phase Transfer Catalyst

A difficulty that occasionally arises when carrying out nucleophilic substitution reaction is that the reactants do not mix. For a reaction to take place the reacting molecule is usually insoluble in water and other polar solvents, while the nucleophile is often an anion which is soluble in water, but the nucleophile is insoluble in the substrate or other organic solvents. To overcome this problem is to use a solvent that will dissolve both species. A dipolar aprotic solvent may serve this purpose. Another way, which is used may often is phase transfer catalysts.

An example of phase transfer catalyst (Q⁺X^-) is usually a quaternary ammonium halide (R₄N⁺X^-), such as tetra butyl ammonium chloride (Bu₄N⁺Cl^-), benzyl triethyl ammonium chloride [Ph-CH₂-N⁺ (CH₂CH₃)₃Cl^-]. These are soluble in organic phase because of the four hydrocarbons substituents on nitrogen as well as these are soluble in aqueous phase due to the ionic in nature. The transfer of the nucleophile (for example CN^-) as an ion pair (Q⁺CN^-) into the organic phase, is causes by the phase transfer catalyst. The reaction between the nucleophile of the ion pair (CN^-) and the organic substrate RX, occur in the organic phase. The cation (Q⁺) then migrates back into the aqueous phase to complete the cycle. Until all of the nucleophile or the organic substrate has reacted, the process goes continues.

Phase transfer catalyst

An example of a nucleophilic substitution reaction carried out with phase transfer catalysis is the reaction of 1-chlorooctane (in decane) and sodium cyanide (in water). The reaction (at 105⁰C) is complete in less than 2 hours and gives a 95% yield of the substitution product.

Another example of phase transfer catalyst is crown ethers. They render many salt soluble in non-polar solvents. Salts such as KF, KCN and CH₃COOK for example can be transferred into aprotic solvents by using 18-crown-6. A nucleophilic substitution reaction carry out by the relatively unsolvated anions of these salts on an organic substrate in the organic phase.