Nucleophilic Substitution

NUCLEOPHILIC SUBSTITUTION

What is nucleophilic substitution?

Nucleophilic substitution is a class of reactions in which an electron nucleophile selectively bonds with or attacks a positive or partially positively charged atom (an electrophile).

The most general form for the reaction can be given as

Nuc: + R-LG → R-Nuc +LG:

The electron pair (:) from the nucleophile (Nuc) attacks the substrate (R-LG) forming a new bond. While this happens, the leaving group (LG) departs with an electron pair. The main product in this case is (R-Nuc). The nucleophile may be electrically neutral or negatively charged whereas the substrate is typically neutral or positively charged.

Here is a typical hydrolysis of an alkyl bromide, it can be used as an example for nucelophilic substitution.

R-Br + OH⁻ → R-OH +Br⁻

In this example, the attacking nucleophile is the OH⁻ ion and the leaving group is Br⁻.

Nucleophilic substitution reactions are very common in organic chemistry and they are broadly categorised as taking place as a saturated aliphatic carbon or at (less often) a saturated aromatic or other unsaturated carbon centre.

SN1 and SN2 reactions

There are two main mechanisms at work in a nucleophilic substitution, both competing with each other. The mechanisms are known as SN1 and SN2. S stands for chemical substitution, N stands for nucleophilic and the number represents the kinetic order of the reaction.

In the SN2 reaction, the addition of the nucleophile and the elimination of the leaving group take place at the same time. SN2 reactions occur where the central carbon atom is easily accessible to the nucleophile.

The SN1 reaction involves two steps. this reaction tends to be important when the central carbon atom…