Problem 79 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following are formed when \(\mathrm{BCl}_{3}\) is treated with \(\mathrm{NH}_{3}\). (a) \(\mathrm{B}\left(\mathrm{NH}_{2}\right)_{3}\) (b) \(\mathrm{B}_{2}\left(\mathrm{NH}_{3}\right)_{2}\) (c) \(\mathrm{BN}\) (d) All of these

Step-by-Step Solution

Answer

(a) \(\mathrm{B\left(NH_2\right)_3}\) and (c) \(\mathrm{BN}\) are formed.

Step 1: Understanding the Reaction

When \(\mathrm{BCl}_3\) is treated with \(\mathrm{NH}_3\), \(\mathrm{BCl}_3\) acts as a Lewis acid (electron pair acceptor) and \(\mathrm{NH}_3\) acts as a Lewis base (electron pair donor). The reaction involves \(\mathrm{NH}_3\) donating a lone pair to \(\mathrm{BCl}_3\) to form an adduct.

Step 2: Formation of Adduct

Initially, \(\mathrm{BCl}_3\) and \(\mathrm{NH}_3\) combine to form the adduct \(\mathrm{Cl}_3\mathrm{B}-\mathrm{NH}_3\). This complex can lead to further reactions, eventually leading to polymeric nitrogen-boron compounds.

Step 3: Decomposition to Other Products

The interaction can lead to the loss of hydrogen chloride (\(\mathrm{HCl}\)), forming \(\mathrm{B\left(NH_2\right)_3}\) after elimination of \(\mathrm{HCl}\) molecules. Thermal decomposition of these adducts can sometimes yield \(\mathrm{BN}\) in a polymeric form.

Step 4: Conclusion on Possible Products

Both \(\mathrm{B\left(NH_2\right)_3}\) and \(\mathrm{BN}\) are typically formed during the reaction process. There is no mention of \(\mathrm{B_2\left(NH_3\right)_2}\) as a common product without detailed conditions; thus it is not included in typical reaction predictions.

Key Concepts

Lewis Acids and Bases

In the realm of inorganic chemistry, the terms "Lewis acids" and "Lewis bases" are crucial to understanding molecular interactions. A Lewis acid is defined as a substance that can accept a pair of electrons. On the other hand, a Lewis base is an entity that donates electron pairs.

• Take \( \mathrm{BCl}_3 \) for example. It is a renowned Lewis acid because of its electron-deficient boron center.
• Ammonia, \( \mathrm{NH}_3 \), serves as a Lewis base since it has a lone pair of electrons available for donation.

This interaction is not merely theoretical. It's exemplified when \( \mathrm{BCl}_3 \) comes into contact with \( \mathrm{NH}_3 \). The reaction is initiated as \( \mathrm{NH}_3 \) donates its lone pair to the vacant orbital of \( \mathrm{BCl}_3 \). This results in the formation of a new bond between boron and nitrogen. In practical chemistry, identifying which compounds serve as Lewis acids and bases helps in predicting reactions and understanding the nature of product formation.

Adduct Formation

Adduct formation is a fundamental concept in chemistry, particularly in reactions involving Lewis acids and bases. When talking about adduct formation, it refers to the creation of a complex where two or more distinct molecules come together.

• Initially, when \( \mathrm{BCl}_3 \) and \( \mathrm{NH}_3 \) react, they form an adduct by combining into \( \mathrm{Cl}_3\mathrm{B}-\mathrm{NH}_3 \).
• This complex manifests itself through the new covalent bond between boron and nitrogen, highlighting the transfer of an electron pair from nitrogen to boron.

Adduct formation is not just a straightforward endpoint but often a stepping stone to other chemical transformations. These new complexes can engage in further reactions or breakdown under suitable conditions to form various products, as seen when they undergo changes leading to polymeric compounds. This step is crucial because it showcases the dynamic nature of chemical reactions, where initial adducts can evolve into more complex structures.

Polymeric Compounds

Polymeric compounds, often resulting from chemical reactions involving simple molecules, have a wide array of applications. An interesting example is when the adduct \( \mathrm{Cl}_3\mathrm{B}-\mathrm{NH}_3 \) undergoes further reaction steps.

• Through the release of hydrochloric acid (\( \mathrm{HCl} \)), one possible product is \( \mathrm{B\left(NH_2\right)_3} \). This can be considered a step towards polymerization.
• At higher temperatures, these intermediates can decompose further into polymeric boron nitride (\( \mathrm{BN} \)).

Boron nitride bears significant industrial importance due to its unique properties similar to carbon-based polymers like graphite and diamond. It's notable for its electrical insulation and thermal conductivity, making it suitable for specialized material applications. Understanding the formation of polymeric compounds in such reactions opens the door to material science innovations, demonstrating how simple molecule interactions can lead to complex and useful materials.