The geometry around boron in the product 'B' formed from the following reaction is $ BF _3+ NaH \xrightarrow{450 K } A + NaF$ $ A + NMe _3 \rightarrow B$
- trigonal planar
- tetrahedral
- pyramidal
- square planar
The Correct Option is B
Solution and Explanation
Let's analyze the given reaction to determine the geometry around the boron atom in the product 'B'.
- The initial reaction provided is: \(BF_3 + NaH \xrightarrow{450 K} A + NaF\).
- Boron trifluoride (\(BF_3\)) is initially a trigonal planar molecule with boron making three bonds with fluorine.
- In the presence of sodium hydride (\(NaH\)), a hydride ion reacts with \(BF_3\), resulting in the formation of a compound 'A' (likely \(BH_3\)) and sodium fluoride (\(NaF\)).
- The compound 'A' then reacts with \(NMe_3\): \(A + NMe_3 \rightarrow B\).
- \(NMe_3\) is trimethylamine, a Lewis base, which donates an electron pair to form a dative bond with 'A', which is \(BH_3\).
- The reaction essentially forms \(BH_3 \cdot NMe_3\).
- The complex \(BH_3 \cdot NMe_3\) has a tetrahedral geometry because:
- The \(BH_3\) group allows the formation of three sigma bonds with the fluorine atoms. Additionally, it forms a coordinate covalent bond with the lone pair of trimethylamine.
- The dative bond from \(NMe_3\) to \(BH_3\) results in a total of four bonded pairs around the boron atom.
- Owing to the VSEPR (Valence Shell Electron Pair Repulsion) theory, the four pairs of electrons (three from \(H\) atoms and one from the dative bond) arrange themselves around the central boron to minimize repulsions, resulting in a tetrahedral geometry.
- Based on the above reasoning, the geometry around boron in the product 'B' (\(BH_3 \cdot NMe_3\)) is tetrahedral.
Thus, the correct answer is tetrahedral.
Let's rule out the other options:
- Trigonal planar: Applies to \(BF_3\) not after forming a bond with \(NMe_3\).
- Pyramidal: Typically exists in \(sp^3\) hybridization with a lone pair, which isn't the configuration here.
- Square planar: Typically associated with coordination complexes of transition metals, not applicable in this scenario.
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Concepts Used:
Chemical Bonding and Molecular Structure
Such a group of atoms is called a molecule. Obviously, there must be some force that holds these constituent atoms together in the molecules. The attractive force which holds various constituents (atoms, ions, etc.) together in different chemical species is called a chemical bond.
Types of Chemical Bonds:
There are 4 types of chemical bonds which are formed by atoms or molecules to yield compounds.
- Ionic Bonds - Ionic bonding is a type of chemical bonding which involves a transfer of electrons from one atom or molecule to another.
- Covalent Bonds - Compounds that contain carbon commonly exhibit this type of chemical bonding.
- Hydrogen Bonds - It is a type of polar covalent bonding between oxygen and hydrogen wherein the hydrogen develops a partial positive charge
- Polar Bonds - In Polar Covalent chemical bonding, electrons are shared unequally since the more electronegative atom pulls the electron pair closer to itself and away from the less electronegative atom.
Factors Affecting Bond Enthalpy in Chemical Bonding:
- Size of the Atom
- Multiplicity of Bonds
- Number of Lone Pair of Electrons Present
- Bond Angle