Question

The number of isomers possible for disubstituted borazine, \(\mathrm{B}_{3} \mathrm{~N}_{3} \mathrm{HX}_{2}\) is (a) 2 (b) 3 (c) 4 (d) 5

Short answer

The number of disubstituted borazine isomers is 3.

Step-by-step solution

Understanding the Molecule

Firstly, let's understand that borazine, also known as 'inorganic benzene,' has a similar structure to benzene, with alternating boron and nitrogen atoms. In the case of disubstituted borazine, the substitutions happen on the hydrogen atoms that are originally bonded to the boron or nitrogen atoms.

Identify Potential Substitution Positions

Borazine has a hexagonal ring composed of alternating Boron (B) and Nitrogen (N) atoms. Therefore, there are six positions around the ring: three positions on the boron atoms and three on the nitrogen atoms where substitutions can occur.

Evaluate Symmetry for Isomer Counting

Next, consider the ring symmetry of borazine which affects how we count distinct isomers. Like benzene, borazine is a symmetrical molecule. When different groups replace two hydrogen atoms (X in this case), the positions of substitution (ortho, meta, or para) matter, but not all sequences will produce unique isomers.

Count the Possible Disubstituted Isomers

For disubstituted borazine, there are three fundamental orientations for substitutions relative to each other: ortho (adjacent), meta (one position apart on the same ring), and para (opposite sides of the ring). Due to symmetry, these orientations will produce different isomers. Upon evaluation, these three give distinct isomers.

Verify All Unique Isomers

Re-examine to ensure no duplication among these possibilities due to the ring's symmetrical nature. Since borazine’s hexagonal shape and alternating bond arrangement don't introduce new distinct separations beyond those predicted, no additional isomers exist beyond those already counted.

Key concepts

Borazine Structure

Borazine, often called "inorganic benzene", is an interesting compound that plays a significant role in chemistry. It shares a hexagonal structure similar to benzene but consists of alternating boron (B) and nitrogen (N) atoms. This arrangement gives borazine both aromatic properties and unique characteristics distinct from organic benzene. In the borazine ring, each boron atom is bonded to a nitrogen atom and a hydrogen atom, while each nitrogen atom is bonded to two boron atoms. Understanding this structure is crucial, as it sets the foundation for comprehending more complex topics like isomerism and substitution.

Disubstituted Compounds

When talking about disubstituted borazine, we refer to a borazine molecule where two hydrogen atoms are replaced by other groups, referred to typically as 'X'. Disubstitution can occur on different positions around the ring. The two hydrogen atoms originally bonded to either boron or nitrogen atoms are the sites of substitution. This leads to the formation of different isomers, each with its own unique spatial arrangement. Recognizing these positions is essential for understanding how these substitutions affect the compound and contribute to the overall number of possible isomers.

Molecular Symmetry

Molecular symmetry plays a vital role in determining how many distinct isomers can be formed during substitutions. In chemistry, the concept of symmetry refers to how identical a molecule looks from different perspectives. Borazine, like benzene, is highly symmetrical. Because of this symmetry, certain substitutions might appear different but result in identical structures, ultimately reducing the number of possible isomers. For instance, swapping two groups adjacent to each other in the borazine ring might seem unique but could be indistinct due to rotational and reflectional symmetrical properties of the ring.

Isomer Counting

Counting distinct isomers requires an understanding of both the substitution positions and molecular symmetry. In the case of disubstituted borazine, the possible orientations are often described using terms borrowed from benzene chemistry: ortho, meta, and para. Ortho refers to adjacent positions, meta refers to positions one step apart around the ring, and para being opposite each other across the ring. These three orientations typically yield distinct isomers. However, due to borazine’s symmetric nature, some combinations may overlap and appear identical after considering all possible rotations and reflections. Thus, careful examination is needed to count only unique isomers.

Boron and Nitrogen Substitution

In creating isomers through substitution, it's crucial to recognize how boron and nitrogen act differently during the process. Each atom type presents different affinities and preferences for substitution. Within borazine's alternating pattern, understanding how replacements on boron atoms compare to nitrogen is essential. Though they are part of the same ring, chemical and electronic properties may lead to variations in how substitutions are stabilized. For students, it’s important to probe these nuances for a fuller grasp of isomer formation and to predict the properties of the resultant compounds. This deeper dive reinforces the significance of boron and nitrogen's roles when substituted, beyond just the counting of isomers.