Intermediate Types

We know that when a reaction is completed, all the reactants are completely converted into products. But the reactants do not directly convert into products. Rather, the reactants first convert to the transition state, and then from the transition state to the intermediate, then from the intermediate to the second transition state, and finally from the second transition state to the product. 

Hence "Intermediates are defined as a molecular entity which is formed inbetween reactants and products (It is neither be a reactant nor a product) or inbetween two transition states".

Fig:- Reaction Energy Diagram 

In a reaction mechanism (series of elementary reactions through which the overall reaction takes place), intermediate molecule is formed in one elementary step while consumed in another elementary step. Therefore, these intermediates are not present in the overall balanced equation. But still a part of every reaction.

Intermediates tend to be extremely reactive due to their short lifetime and represents lower concentration than the concentration of reactant and product in a chemical reaction. These intermediates may be stable or unstable in nature. 

For Example; 

A chemical reaction ,

              2NO(gas) + O₂ (gas) ➝ 2NO₂(gas) 

This is the overall balanced reaction for the formation of NO₂. It takes place in two elementary reaction steps, 

            Step1- 2NO(gas) ➝ N₂O₂ (gas) 

            Step2- N₂O₂(gas) + O₂(gas) ➝ 2NO₂(gas) 

                  Adding above two reactions;

                2NO + N₂O₂ + O₂ ➝ N₂O₂ + 2NO₂

After cancel the same term terms both the side then we have, 

            2NO(gas) + O₂ (gas) ➝ 2NO₂(gas) 

Here, 

      We saw that N₂O₂ Molecule is not present in the overall balanced equation as shown above. It was produced in the first step and then consumed in the second step. Hence, N₂O₂ acts as 'Intermediate'.

Types Of Intermediates

Generally, Intermediates are of four types and these are;

1) Free radicals 

2) Ions and polar substances 

3) Molecules 

4) Transition complexes

Let's understand these types of intermediates one by one; 

(1)Free Radicals

When a reaction is proceeding then between the reaction, intermediate molecules are formed. And when these molecules have an unpaired electron in their outer shell then these molecules are called as 'Free Radicals'. 

Free radicals are generally represented by a superscripts dot. Free radicals can also be atoms, molecules or ions that have an unpaired electron. Free radicals are formed when the bonds between reactant molecules split. 

For Example

       Cl₂ (in presence of UV rays) ➝ Cl• + Cl•

 Here, 

        " • "represents unpaired electron 

Generally, bonds don't split in a way that molecules contain an unpaired electron but in case of weak bonds when they are split, free radicals are formed. When the bonds of molecules split and free radicals are formed then the molecules loses their stability due to that unpaired electron and become unstable.

Since free radicals are highly unstable due to that unpaired electron therefore they react quickly with other compound or with itself and trying to capture the needed electron to gain stability. Most of the radicals are relatively stable at low concentration. Free radicals plays an important role in combustion, atmospheric chemistry, polymerisation, biochemistry etc.

For Example

Triphenyl methyl (this is the first organic free radical identified)

Other examples are - CH₃•, I•, CCl₃•, HO•, etc. 

(2)Ions and Polar substances

An electrically charge atoms  or a group of atoms, molecules or fragment of molecules are known as Ions. These may be positively or negatively charge ions based on the electron transfer.

For Example 

    N₃⁻, Na⁻, H₃O⁻, NH₄⁺, OH⁻, CH₃⁺,etc.

▶Molecule loses their paired electron are called Cations or positively charge ions. 

▶Molecules gain paired electron are called Anions or negatively charge ions. 

"The process in which an atom becomes an ion by losing and gaining of electrons is called ionisation".

While the substances which develop a dipole moment in their structure are called Polar substances and if this dipole moment is equal to zero then the substance is called Non-polar substances. 

Examples- H₂O, ethanol, NH₃, SO₂, CH₄, CCl₄, all noble gases, CO₂,etc.

(3)Molecules

Molecules are also acts as an  intermediates. They are formed directly by the reactant molecules.

          A(reactant)➝  R  ➝ S(product)

Here, 

      R represents intermediate which is not a free radicals or ion rather it acts as molecule. 

(4)Transition complexes

            A + B  ➝  AB* ➝  products

Here, 

       AB* is known as transition complex Which disintegrates easily into products due to their instability. 

These transition complexes is generally represented by "*". These transition complexes are formed by unstable association of molecules which decomposes easily into product.

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