Problem 10 - A Complete Resource Book in Chemistry for JEE Main

Question

Reactivity of borazole is greater than that of benzene because (a) it has localized electron in it (b) borazole is electron deficient compound (c) borazole is polar compound (d) borazole is non-polar compound

Step-by-Step Solution

Answer

Borazole is more reactive than benzene because it is an electron deficient and polar compound.

Step 1: Understand Borazole Structure

Borazole, also known as borazine, has the chemical formula \(B_3N_3H_6\). It is a six-membered ring consisting of alternating boron (B) and nitrogen (N) atoms, similar to the structure of benzene.

Step 2: Examine Benzene Structure

Benzene is a six-membered carbon ring with the chemical formula \(C_6H_6\). It contains alternating single and double carbon-carbon bonds, resulting in a stable, aromatic structure with delocalized electrons.

Step 3: Compare Electron Delocalization

In benzene, electrons are delocalized over the entire carbon ring, providing exceptional stability. In borazole, electron delocalization is limited due to the presence of electronegative nitrogen atoms withdrawing electron density from the boron atoms.

Step 4: Analyze Electronegativity Differences

Nitrogen in borazole is more electronegative than boron, causing electron density to be pulled towards nitrogen. This makes the B-N bond polar, leading to an overall polar compound.

Step 5: Evaluate Reactivity

Borazole's greater reactivity compared to benzene is due to its electron deficiency and polarization. The boron centers, being electron deficient, are more prone to reactions compared to benzene's stable, fully delocalized system.

Key Concepts

Borazole Structure

Borazole, also known as borazine, is an interesting compound with a structure resembling the famous benzene ring. Its formula is \(B_3N_3H_6\), indicating a ring composed of alternating boron (B) and nitrogen (N) atoms.
Unlike benzene, which is made entirely of carbon atoms, borazole's alternating pattern results in differing polarities across the compound.

• The boron atoms have an electron deficiency.
• Nitrogen atoms are more electronegative.

This difference is fundamental in understanding borazole’s unique properties and reactivity compared to benzene.

Electron Delocalization

Electron delocalization plays a crucial role in the stability of chemical structures. In benzene, electrons are shared equally across all atoms, forming a stable system.
In contrast, delocalization in borazole is less pronounced due to the alternating nature of boron and nitrogen within its ring.

• Benzene electrons are spread uniformly, providing immense stability.
• Borazole experiences partial delocalization, as nitrogen pulls electrons away due to its higher electronegativity.

This limited delocalization impacts the reactivity of borazole.

Electronegativity Differences

Electronegativity refers to an atom's ability to attract and hold onto electrons.
In borazole, nitrogen atoms are significantly more electronegative than boron atoms.

• This creates a scenario where electrons are unequally shared, leading to polar B-N bonds.
• The higher electronegativity of nitrogen results in a shift of electron density toward itself.

This creates areas of partial negative charge on nitrogen and partial positive on boron, adding to the compound's overall polarity.

Polarization

Polarization in a molecule arises from uneven electron distribution. In borazole, this is primarily due to the alternating boron and nitrogen atoms. This structure causes the B-N bonds to be polar.
This polarity contributes to borazole's increased reactivity, as molecules tend to react to minimize potential energy differences.

• Boron ends up with a slight positive charge.
• Nitrogen has a partial negative charge.

These differences make borazole more reactive as it seeks to stabilize these charges.

Benzene Structure

Benzene, a classic example of aromaticity, is a six-membered ring of carbon atoms with a chemical formula \(C_6H_6\).
Its structure includes alternating single and double bonds; however, these bonds are not fixed but delocalized over the whole ring, creating a resonance hybrid.

• This delocalization makes benzene exceptionally stable.
• The electron cloud above and below the plane of the atoms contributes to this stability.

Benzene's widespread electron delocalization and symmetry provide it with a low reactivity compared to borazole.