**Isomerism in Di-substituted Borazine (B3N3H4X2)**

*Introduction:*
Di-substituted borazine (B3N3H4X2) is a compound that contains three boron (B) atoms, three nitrogen (N) atoms, four hydrogen (H) atoms, and two substituent groups represented by X. Isomerism refers to the phenomenon where compounds have the same molecular formula but differ in their connectivity or spatial arrangement of atoms. In the case of di-substituted borazine, isomerism arises due to the different possible arrangements of the two substituent groups.

**Explanation:**
To determine the number of isomeric structures of di-substituted borazine, we need to consider the different possibilities for the arrangement of the substituent groups (X). Let's analyze the different cases:

1. **Case 1: Identical substituent groups (X1 = X2)**
In this case, the two substituent groups are the same, which means the di-substituted borazine will have a symmetrical structure. Therefore, there is only one isomeric structure possible.

2. **Case 2: Different substituent groups (X1 ≠ X2)**
In this case, the two substituent groups are different, which allows for more structural possibilities. We need to consider the different arrangements of the two substituent groups around the borazine ring.

a. **Case 2a: Adjacent substitution**
If the two substituent groups are adjacent to each other on the borazine ring, there are two possible arrangements. These arrangements can be obtained by placing X1 and X2 on either side of the ring.

b. **Case 2b: Non-adjacent substitution**
If the two substituent groups are non-adjacent to each other on the borazine ring, there are two possible arrangements. These arrangements can be obtained by placing X1 and X2 on opposite sides of the ring.

Therefore, in case 2, there are a total of four isomeric structures possible.

**Conclusion:**
In summary, the number of isomeric structures of di-substituted borazine (B3N3H4X2) is four. This is because when the two substituent groups are identical, there is one isomeric structure, and when the substituent groups are different, there are three additional isomeric structures.