Problem 14 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is the electron deficient molecule? (a) \(\mathrm{C}_{2} \mathrm{H}_{6}\) (b) \(\mathrm{B}_{2} \mathrm{H}_{6}\) (c) \(\mathrm{SiH}_{4}\) (d) \(\mathrm{PH}_{3}\)

Step-by-Step Solution

Answer

(b) \(B_{2}H_{6}\) is the electron deficient molecule.

Step 1: Understanding Electron Deficiency

An electron deficient molecule has fewer electrons available for bonding than would be expected from the typical valence electron model. This often leads to unusual bonding such as multi-center bonds (where more than two atoms share a pair of electrons). Look for boron, which typically forms electron-deficient compounds because it has three valence electrons, fewer than a typical octet.

Step 2: Analyze Each Option

- Option (a) \(C_{2}H_{6}\): Ethane, a saturated hydrocarbon, has typical covalent bonds without electron deficiency. - Option (b) \(B_{2}H_{6}\): Borane, known for its electron-deficient structure involving three-center two-electron bonds. - Option (c) \(SiH_{4}\): Silane, similar in structure to methane, with typical covalent bonds and no electron deficiency. - Option (d) \(PH_{3}\): Phosphine, a pnictogen hydride with normal covalent bonds and no electron deficiency.

Step 3: Identify Electron Deficient Molecule

From our analysis in Step 2, \(B_{2}H_{6}\) has an electron-deficient structure due to the presence of three-center two-electron bonds, which is a characteristic of electron-deficient compounds. Borane is the well-known example of an electron-deficient molecule.

Key Concepts

Boron Chemistry

Boron chemistry is a fascinating area of study due to its unique bonding characteristics and tendency to form electron-deficient compounds. Boron, unlike carbon and many other elements in the periodic table, has only three valence electrons. This means it doesn't have enough electrons to form typical four-electron covalent bonds with octet completion. Instead, boron often forms compounds with multi-center bonds, allowing it to form stable complexes even when it appears to lack sufficient electrons.

Because of these distinctive properties, boron compounds are often used in industrial applications, as catalysts, and even in developing new materials. Its complexity in bonding also makes it an interesting subject of study in advanced inorganic and organic chemistry courses. Exploring boron's ability to bond in unusual ways helps deepen our understanding of chemical bonding and molecular structure.

Three-Center Two-Electron Bonds

In the world of chemistry, bonds are a critical concept to understand. **Three-Center Two-Electron (3c-2e) bonds** are one of those fascinating bonding configurations seen in electron-deficient molecules, particularly with boron. Unlike typical two-center two-electron bonds, where a pair of electrons is shared between two atoms, a 3c-2e bond involves three atoms sharing just two electrons.

This formation is most notable in compounds like diborane (\( B_2H_6 \)). In these compounds, the three-center bond helps to stabilize the molecule despite the electron deficiency. These bonds are crucial for the stability and existence of boranes and many other complex structures. Scientists and students often find this concept intriguing because it challenges the classical understanding of molecular bonds.

Understanding 3c-2e bonds is key for anyone studying advanced chemistry, as it opens doors to complex molecular interactions and designs.

Boron Hydrides

Boron hydrides, or boranes, are a group of compounds consisting of boron and hydrogen atoms. The most well-known of these is diborane (\( B_2H_6 \)), which serves as a prime example of an electron-deficient compound.

These hydrides are characterized by their unique bonding structures—often incorporating three-center two-electron bonds. This type of bonding helps compensate for the electron deficiency of boron, allowing it to bond with hydrogen and stabilize the overall structure. Boron hydrides are not just structurally interesting but also exhibit diverse chemical reactivity.

Thanks to these properties, boranes play a crucial role in chemical synthesis and reactions where unique catalytic activities are required. Being able to understand boron hydrides means being able to anticipate their behavior in various chemical environments, making it an essential concept in advanced chemical studies.

Chemical Bonding

Chemical bonding refers to the force that holds atoms together in a molecule. Typically, chemical bonds are formed from the sharing or transferring of electrons. For most elements, the goal is to achieve an electron configuration similar to the nearest noble gas, often by fulfilling the octet rule.

However, as seen in electron-deficient molecules like boron compounds, this rule can be bent. Instead of failing to form stable structures, elements like boron form multi-center bonds. These bonds involve a different distribution of electrons than traditional bonds, allowing for stability in less conventional ways.

Understanding the nuances of chemical bonding can provide a deeper insight into molecular geometry, reactivity, and properties. These insights are fundamental for predicting the behavior of complex molecules in chemistry.

Valence Electrons

Valence electrons are the outermost electrons of an atom. They determine an element's chemical properties and its ability to bond with other elements. For most elements, having a complete valence shell results in stability, often conforming to the octet rule for the second and third rows of the periodic table.

Boron, however, challenges this typical understanding. With only three valence electrons, it often partakes in unconventional bonding to satisfy its structural needs. Its valence electrons can form unique bond types such as multi-center bonds, leading to stable albeit electron-deficient compounds.

The concept of valence electrons is fundamental to the study of chemistry. It plays a key role in understanding how and why atoms interact to form molecules. Knowing the behavior of valence electrons helps predict the kinds of bonds an atom can form and, by extension, the type of compounds that result from those bondings.