Nucleophilic Substitution Reactions
Nucleophilic substitution reaction is a type of organic reaction in which nucleophile (an electron pair donor) reacts with an electrophile (an electron pair acceptor). The word substitution means to replace or to substitute; in this type of reaction one group or atom is replaced by other group or atom. There are two types of nucleophilic substitution reactions that are SN1 and SN2 reactions.
A nucleophilic substitution reaction that follows first order kinetics is called SN1 reaction. It is also known as unimolecular substitution reaction. For example: Hydrolysis of tert-butyl bromide to tert-butyl alcohol by NaOH.
The rate of this reaction only depends on the concentration of (CH3)3-Br, i.e.
Rate = K1 [(CH3)3-Br]
This is the first order reaction because it only depends on the concentration of one component.
Mechanism of SN1 reaction
SN1 reaction proceeds by a two step mechanism. It involves:
Step-1: Ionization of tert-butyl bromide to tert-butyl carbocation due to dipole interaction with polar solvent. This is a slow step.
Step-2: Fast reaction of carbocation with HO–.
Energy Profile for SN1 reaction
The energy profile shows two transition states. In the first transition state, C-Br dissociates and it results in the formation of carbocation during a slow step. In the second transition state, there is a formation of new bond C-OH in a rapid step.
There is also a formation of intermediate (carbocation) between two transition state and this intermediate is more stable than both the transition states.
Stereochemistry of SN1 reactions
The SN1 reaction involves the formation of carbocation intermediate. It is a trigonal planar and the C+ carbon is sp2 hybridized. It has an empty p-orbital which is perpendicular to the plane of three sp2 hybridized orbitals. Either lobe of the p-orbital can interact with the nucleophile, so the attack of either lobe is equal. Thus, a carbocation formed from asymmetric carbon atom would give equal proportions of products with retention and inversion of configuration, i.e. the reaction will results in the racemization. It is practically true for very stable carbocation, i.e. tertiary carbocation.
But with secondary carbocation, some inversion also takes place and the product is not pure racemic mixture. It is due to the presence of ion pair like species (A) during ionization that makes easy, backside attack giving product with some inversion of configuration, e.g.
A nucleophilic substitution reaction that follows second order kinetics is called SN2 reaction. It is also known as bimolecular nucleophilic substitution reaction. For example:
The rate of reaction is dependent on the concentration of both CH3-Br and HO–.
Rate = K2 [CH3-Br] [HO–]
This is a second order reaction because reaction depends on the concentration of two components.
Mechanism of SN2 reaction
The SN2 reaction take place in one step process, in which there is no any formation of intermediate occurs during the reaction. The process of bond making and bond breaking takes place simultaneously. The nucleophile attacks the substrate from the back of the leaving of group. The reaction takes place by following mechanism.
Energy Profile of SN2 reaction
In SN2 reaction, there is only one transition state and there is no any intermediate formed during the reaction.
Stereochemistry of SN2 reaction
The backside attack is favored in SN2 reaction mechanism. Oxygen and bromine atoms are well separated in the transition state, i.e. the charge (δ–) is dispersed. The front side attack would not disperse the negative charge to the same extent and hence, would be less favorable.
An SN2 reaction between cis-1-chloro-3-methylcyclopentane and HO– would give cis-alcohol by front side attack but the trans-alcohol by backside attack. The actual product is trans-alcohol. Thus, it proves the backside attack.
The backside attack predict the inversion of configuration when substitution occurs at a chiral carbon. It is found in many cases.
For example: the hydrolysis of 2-bromobutane produces alcohol having the configuration opposite to the halide.
The SN2 reaction proceeds with the stereochemical inversion. This inversion of configuration is called Walden inversion.