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Nucleophilic Substitution - SN1

When tert-Butyl iodide was treated with sodium acetate a substitution reaction occured in which the acetate ion (\(CH3CO_2^-\)) substituted the iodide ion (\(I^−\)). Kinetic study of this reaction showed that the rate of this reaction depends only on the concentration of alkyl halide.

\[Rate \ \alpha \ [(CH_3)_3CI]\]

As the kinetic study suggests, the concentration of the nucleophile is not affecting the rate of the reaction. This means that there could be more than one step in the reaction mechanism and the step that involves the nucleophile does not affect the rate of the reaction. The other step involving the alkyl iodide could be the slower rate determining one.

So, a straight forward two step mechanism could be thought of, first step of which involves a slow ionization of the alkyl iodide into a carbocation and iodide ion followed by the fast attack on the cation by the nucleophile. As the first step involves only bond cleavage, it must be slower than the second step which involves only bond formation. This is called SN1 (Unimolecular Nucleophilc Substitution).

The reaction proceeds through the formation of a carbocation. The carbon atom carrying the positive charge is \(sp^2\) hybridized. Its geometry is trigonal planar in which the empty p-orbital is perpendicular to the plane of the molecular ion.

The nucleophile that attcks this can do so from either side leading to the formation of two products with different stereochemistry. For example, if the alkyl halide with which the reaction is started with is optically active, then the product of its SN1 substitution will be racemized i.e. will have retention as well as inversion about the stereochemical centre. If the incoming nucleophile occupies the same position as the leaving group the configuration will be retained and if it occupies a position opposite to that of the leaving group the configuration will be inverted.

It has been found that inversion is slightly more than retention of configuration in reactions proceeding through SN1 mechanism. Further investigation of the reaction mechanism reveals that before the formation of the free carbocation, there are two short lived stages which mainly undergoes inversion.

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Nucleophilic Substitution - The Mechanisms

The functional group in Alkyl Halides is the polar C - X bond. The electronegativity difference between the halogen and the carbon atom is the key factor that determines the chemical properties of alkyl halides.

Alkyl halides are generally found to undergo elimination or substitution reactions. When they undergo elimination reaction they form an alkene and when they undergo substitution they form a variety of compounds based on the nucleophile used.

A negatively charged species has two roles to play; 1. the role of a base and 2. the role of a nucleophile. So, when an alkyl halide is treated with a negatively charged species either elimination or substitution will be the major outcome. If the negatively charged species acts as a base, the major outcome will be an elimination reaction and if the negatively charged species acts as a nucleophile the major outcome will be a substitution reaction.

During the course of our discussion we will talk about the differences between as base and a nucleophile. For now, we will talk about the substitution reactions. In a substitution reaction, our main focus is on the nucleophile and the leaving group.

In a typical substitution reaction of an alkyl halide the nucleophile forms a bond with the electrophilic carbon and the leaving group, the halide ion leaves the molecule.

When methyl iodide was treated with sodium acetate a substitution reaction occurred in which the acetate ion (\(CH_3CO_2^-\)) substituted the iodide ion (\(I^-\)). Kinetic study of this reaction showed that the rate of this reaction depends both on the concentration of the alkyl halide and the nucleophile.

\[Rate \propto[CH_3I][CH_3CO_2^-]\]

When tert-Butyl iodide was treated with sodium acetate as substitution reaction occured in which the acetate ion (\(CH_3CO_2^-\)) substituted the iodide ion (\(I^-\)). Kinetic study of this reaction showed that the rate of this reaction depends only on the concentration of alkyl halide.

\[Rate \propto [(CH_3)_3CI]\]

Two types of kinetic observation gives us the idea that there should at least be two reaction mechanisms for the nucleophilic substitution reaction.

What are they?

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Nucleophilic Substitution - Introduction

Nucleophile can be broadly described as a species which is nucleus loving and is in search of electron deficient sites to bond with. There are many organic molecules that have an electron deficient site, usually a carbon atom. Following are some examples of organic compounds having an electrophilic carbon atom.

Let us focus on Alkyl Halides and have a closer look at it.

There is an electronegativity difference between the halogen atom and the carbon atom. Here chlorine is more electronegative than carbon and hence it shows the negative inductive effect. It pulls the electron density in the bond, it forms with carbon, towards it. This results in a small positive charge on the carbon atom and a small negative charge on the chlorine atom.

The carbon atom directly connected to the chlorine atom thus becomes electron deficient and call be called as an electrophilic centre. Similarly, if we look at the other examples that we have taken above, the carbon atom connected to the electronegative atom in them (Oxygen and Nitrogen) is electron deficient and can be called as electrophilic centre. This electrophilic centre can attract a suitable nucleophile and if the conditions permit may substitute the halogen atom.

One suitable condition for such nucleophilic substitution is that the group that is being substituted, referred to as “leaving group“ be ready to leave when the nucleophile approaches. Halogen atoms are good leaving groups and hence alkyl halides readily undergo nucleophilic substitution reactions.