Question

Which of the following does not exist in free state? (a) \(\mathrm{BCl}_{3}\) (b) \(\mathrm{BH}_{3}\) (c) \(\mathrm{BF}_{3}\) (d) \(\mathrm{BBr}_{3}\)

Short answer

\(\mathrm{BH}_{3}\) does not exist in free state because it dimerizes to form \(\mathrm{B}_{2}\mathrm{H}_{6}\).

Step-by-step solution

Understanding the Concept of Free State

The term 'free state' refers to molecules that can exist independently as distinct entities without forming complexes or polymers with themselves or other molecules. Molecules with a tendency to dimerize or polymerize usually do not exist in a free state.

Analyzing Boron Compounds

Among the given options, we have four boron-containing compounds: - \(\mathrm{BCl}_{3}\)- \(\mathrm{BH}_{3}\)- \(\mathrm{BF}_{3}\)- \(\mathrm{BBr}_{3}\)Boron tends to form three bonds and sometimes can have a tendency to dimerize, specifically when hydrogen is involved.

Evaluating Existence of Free State

Boron trihalides like \(\mathrm{BCl}_{3}\), \(\mathrm{BF}_{3}\), and \(\mathrm{BBr}_{3}\) commonly exist as monomers in the free state. However, \(\mathrm{BH}_{3}\) is unique due to its electron deficiency, which often leads to dimerization to form diborane, \(\mathrm{B}_{2}\mathrm{H}_{6}\), rather than existing as a monomer.

Determining the Compound Not Existing in Free State

Given the electron-deficient nature of \(\mathrm{BH}_{3}\), it forms \(\mathrm{B}_{2}\mathrm{H}_{6}\) by dimerizing and does not exist freely as \(\mathrm{BH}_{3}\). In contrast, the other boron halides do not dimerize under normal conditions and exist in the free state.

Key concepts

Dimerization

When we talk about dimerization in chemistry, we're referring to the process where two identical molecules combine to form a dimer. This happens often with certain compounds that find it energetically favorable to pair up rather than existing alone. For example, a compound like \( \mathrm{BH}_3 \) (borane) doesn't exist as a single molecule in nature.

Instead, it tends to form a dimer, \( \mathrm{B_2H_6} \) (diborane), to fulfill its need for electrons, thereby becoming more stable.

This dimer formation is particularly notable in compounds with electron-deficient centers, which leads us to our next concept: electron deficiency.

Electron Deficiency

Electron deficiency refers to a molecule having fewer electrons than are needed for the formation of simple covalent bonds. Boron compounds often exhibit thisphenomenon because boron has only three electrons available for bonding, whereas four are typically needed for a full octet.

This shortage pushes a compound like \( \mathrm{BH}_3 \) to seek an electron-rich partner, leading to dimerization.

With dimerization, \( \mathrm{B_2H_6} \) compensates for the electron deficiency by sharing electrons between the two boron atoms, forming bridging hydrogen bonds.

• This results in a stable dimer where the electron deficiency is effectively alleviated.
• On the other hand, boron trihalides such as \( \mathrm{BCl_3} \), \( \mathrm{BF_3} \), and \( \mathrm{BBr_3} \) manage their electron requirements through different strategies.

Boron Trihalides

Boron trihalides, including \( \mathrm{BCl_3} \), \( \mathrm{BF_3} \), and \( \mathrm{BBr_3} \), are an interesting set of compounds where boron is bonded with three halogen atoms. Unlike \( \mathrm{BH_3} \), these molecules do not dimerize under normal conditions. This difference is primarily due to two factors:

• Total availability of electrons from attached halogen atoms which can engage in back bonding.
• The size and electronegativity of the halogen can stabilize the boron center.

This results in stable monomeric forms for boron trihalides, allowing them to exist freely as individual molecules in nature.

Molecular Structure

The molecular structure of boron compounds greatly influences their chemical behavior and stability. For example, \( \mathrm{BH_3} \) has a trigonal planar geometry which makes it electron-deficient and highly reactive.

To stabilize, it rearranges to form diborane with a unique structure featuring bridging hydrogen atoms.

• This provides additional pairing to facilitate electron sharing.

Conversely, the \( \mathrm{BX_3} \) compounds, like \( \mathrm{BCl_3} \), have similarly trigonal planar structures but are stabilized through electron-sharing via halogens.

The interactions between these atoms, tailored by bonding pairs and back bonding, minimize any electron deficiency, giving these structures a more stable existence singly.

Free State

The concept of a 'free state' in chemistry refers to a molecule's ability to exist independently without needing to bond with other molecules. Some compounds are simply too eager to react with others and don't stay single for long.

In our context, \( \mathrm{BH_3} \) doesn't hang around by itself due to its electron deficiency, quickly forming \( \mathrm{B_2H_6} \).

On the flip side, compounds like \( \mathrm{BF_3} \), \( \mathrm{BCl_3} \), and \( \mathrm{BBr_3} \) are totally comfortable chilling as singles in the free state. This is due to the stability offered by their full octet achieved through bonding with halogens. Hence, understanding whether a molecule can exist in its free state tells us a lot about its bonding needs and stability.