Substitution Reaction

Substitution Reaction Definition and Meaning

Have you ever observed how the application of anesthetic agents into your body reduces the pain? Did you know that these agents are produced by the process of substitution reaction?Substitution Reaction

The substitution reaction is defined as “a reaction in which the functional group of one chemical compound is substituted by another group or it is a reaction which involves the replacement of one atom or a molecule of a compound with another atom or molecule.

These type of reactions are said to possess primary importance in the field of organic chemistry. For example, when CH3Cl is reacted with the hydroxyl ion (OH-), it will lead to the formation of the original molecule called as methanol with that hydroxyl ion. The following reaction is as shown below-

CH3Cl + (−OH) ————-CH3OH( methanol) + Cl-

One more example would be the reaction of Ethanol with the hydrogen iodide which forms iodoethane along with water. The reaction is as shown-

CH3CH2OH + HI———— CH3CH2OI + H2O

Substitution Reaction Conditions

In order to substitution reaction to occur there are certain conditions that have to be used. They are-

  • Maintaining low temperatures such as room temperature
  • The strong base such as NaOH has to be in dilute form. Suppose if the base is of higher concentration, there are chances of dehydrohalogenation taking place
  • The solution needs to be in an aqueous state such as water

Substitution Reactions- Types

Substitution Reactions are of two types naming nucleophilic reaction and electrophilic reactions. These two types of reactions mainly differ in the kind of atom which is attached to its original molecule. In the nucleophilic reactions the atom is said to be electron-rich species, whereas, in the electrophilic reaction, the atom is an electron-deficient species. A brief explanation of the two types of reactions are as given below-

  1. Nucleophilic substitution reaction-

Firstly what are nucleophiles? Nucleophiles are those species in the form of ion or a molecule which are strongly attached to the region of a positive charge. These are said to be fully charged or have negative ions present on a molecule. The common examples of nucleophiles are cyanide ions, water, hydroxide ions, and ammonia.

A Nucleophilic substitution reaction in organic chemistry is a type of reaction where a nucleophile gets attached to the positive charged atoms or molecules of the other substance. A good example of a nucleophilic substitution reaction is the hydrolysis of alkyl bromide (R-Br), under the basic conditions, wherein the nucleophile is nothing but the base OH, whereas the leaving group is the Br. The reaction for the following is as given below R-Br + OH ————– R-OH + Br

Nucleophilic reactions are as important in the field of organic chemistry, and these reactions are broadly classified as to occur at the place of a carbon atom of a saturated aliphatic carbon compound.

             2. Electrophilic substitution reactions-

The electrophilic substitution reaction involves the electrophiles. Electrophiles are those which donate a pair of electrons in the formation of a covalent bond. The Electrophilic reactions occur mostly with the aromatic compounds. These compounds have about an excess of electrons that can be shared throughout the system of reaction.

The Electrophilic substitution reactions are basically defined as those chemical reactions where the electrophile replaces the functional group in a compound but not the hydrogen atom. Some of the examples of species of electrophiles include hydronium ion (H3O+), halides of hydrogen such as HCl, HBr, HI, sulfur trioxide (SO3), the nitronium ion (NO2+), etc.

The other types of substitution reactions include radical reactions and organometallic substitution reaction.

Practise This Question

The decreasing order of solubility of methanal (A), propanaldehyde (B), benzaldehyde (C) and acetophenone (D):