Cyanogen bromide (BrCN) is an inorganic compound that is widely needed in organic reactions. BrCN is a specific reagent that digests peptides by cutting at the C-terminus of methionine.
It is an important reagent in a reaction involving modification of biopolymers, fragmentation of proteins, and peptides, and synthesis of other organic compounds.
In this blog, we will study the lewis structure of BrCN and its steps to draw it. Apart from this, we will explore about the reason of its geometrical shape, hybridization, and polarity.
Lewis Structure of BrCN
Lewis structures or Lewis dot diagrams of compounds is a simple yet effective method of indicating the chemical bonds, especially the covalent bonds between the atoms of a compound.
It uses the symbol of an element (for example, C for carbon) to depict an atom of that element, dots to depict electrons in the valence shell, and dashes to depict covalent bonds.
Chemical bonding and the formation of compounds take place out of the necessity for atoms to have an octet in their valence shell.
This means the outermost shell in an atom (except, hydrogen and helium) must have eight electrons to be stable.
To achieve an octet in their valence shells, atoms either donate or receive electrons from other atoms. Electrons can also be shared between two atoms. Based on this, chemical bonding is categorized into two types ionic and covalent.
In ionic bonding, electrons from one atom are completely donated to another atom. The electron donor and the electron acceptor species (or atom) depends on the number of electrons present in the valence shell of the atoms of an element.
If the number of electrons in the valence shell is less than four in an element, the atoms of this element are likely to form electron donors, while the atoms of elements with more than four electrons in their valence shell form electron acceptors.
For example, sodium has 11 electrons. Bohr’s theory contains 3 electron shells with 2 electrons in the first shell, 8 in the second shell, and the remaining one electron in the outermost shell.
If sodium donates this electron, the atom would have 2 electron shells with eight electrons on the now outermost shell.
Similarly, chlorine has 7 electrons in the valence shells. Chlorine atoms can therefore easily attain an octet in their outermost shell by accepting one electron from another atom.
Atoms of elements like carbon that have four electrons in their valence shell tend to form covalent bonds.
In covalent bonds, two atoms come close to each other such that they can share electrons.
The shared electrons count as one electron for both the atoms participating in a covalent bond.
Carbon forms extensive covalent bonds with hydrogen, nitrogen, oxygen, and halogens (fluorine, chlorine, bromine, and iodines).
In the case more than one electron is shared between two atoms, the atoms are said to have multiple covalent bonds.
For example, oxygen has 6 electrons in its valence shell and therefore may form two covalent bonds with a carbon atom to complete its octet.
This is referred to as a double bond. In double bonds, the first covalent bond is called a sigma bond while the second covalent bond is called the pi bond.
Nitrogen often forms triple bonds with carbon by sharing three electrons as it contains 5 electrons in its outer shell.
The first bond is called a sigma bond in triple bonds, while the other two are called pi bonds.
Steps to draw the lewis dot structure of BrCN
In cyanogen bromide, a carbon atom forms a triple bond with a nitrogen atom (cyanogen group) and a single covalent bond with bromine, a halogen.
Step 1: First, the skeleton structure of the compound must be drawn, using the symbol of the elements and joining them by single bonds only.
For BrCN, the skeleton structure would be Br-C-N.
Step 2: Next, all the valence electrons, that is, the electrons present in the valence shells of all the atoms in the compound must be added to receive a sum of all the valence electrons present on the compound.
Positive or negative charges present on any of the atoms must also be denoted at this point.
The total number of valence electrons in BrCN is 16 ie;7 from Br, 4 from C, and 5 from N.
Step 3: Then, the electrons must be added to all the non-cental atoms (the central atom is the atom to which all the other atoms are bonded).
This must be done remembering that all atoms, except hydrogen and helium, want a total of 8 electrons, that is the octet. Hydrogen requires only two electrons.
Every single bond counts as two shared electrons, therefore, hydrogen does not need any more electrons once it is sharing a single covalent bond with another atom.
Upon adding electrons to all the atoms, if any atom does not have eight electrons after all the valence electrons have been accounted for, these atoms must be given double or triple covalent bonds, as required.
If there are excess electrons left after the octet of all the atoms are fulfilled, the extra electrons must be added to the central atom, even if it means that the number of electrons exceeds 8 in this atom.
The completed lewis dot structure of BrCN would therefore be as below.
BrCN Molecular Geometry
The VSEPR theory is employed to predict the geometry of the molecules of a compound based on the number of electron pairs present around the central atoms of the molecule, as well as the covalent bonds formed among these atoms.
This theory depends on the assumption that the geometry of a molecule is motivated solely to attain the minimum amount of electronic repulsion in the valence shell.
Electronic repulsion is generated by negative-negative interaction between lone pairs (unbonded electrons) as well as between electrons in covalent bonds (bond pairs) and lone pairs.
The Valence Shell Electron Pair Repulsion Theory or VSEPR theory suggests that the atoms in a compound would always tend to arrange themselves such that the electron pair repulsion is minimalized.
One such similar example is CO2, you can take a look at the article written on lewis structure and geometry of CO2.
This, in turn, determines the geometry of the resultant molecule.
The postulates of the VSEPR theory:
1. In molecules that are made up of three or more atoms, one of the atoms is considered the central atom.
All the other atoms in the molecule are linked to the central atom.
2. The shape of the molecule is determined by the total number of electron lone pairs in all the atoms of the molecule and the electronic repulsion between them.
Lone pairs always tend to orient themselves such that there is maximal space between them to minimize the electron repulsion.
3. The electron repulsion between two lone pairs is stronger than the electron repulsion between a lone pair and a bond pair.
4. Lone pairs of the central atom lead to the distortion of the bond angles between the central atom and the other atoms.
5. The magnitude of the electron repulsion between two bond pairs depends on the difference in electronegativity between the central atom and the other atoms.
6. Triple bonds cause the highest amount of electron repulsion due to greater electron density while single bonds produce the least amount of electron repulsion due to low electron density.
7. A greater distance between electron pairs results in lowered electron repulsions between them which lower the energy of the molecule.
Based on these postulates, the geometry of molecules is determined as follows:
In BrCN, two bondings, a single covalent bond between carbon and bromine, and a triple covalent bond between carbon and nitrogen are present.
These account for two-electron domains. Carbon forms the central atom. It contains no lone pairs that might lead to any distortions in the bond angles.
Therefore, BrCN has a perfect linear geometry with carbon in the center, and bromine and nitrogen on the two sides at 180°.
Hybridization refers to the interaction of two atomic orbitals in the same energy level to produce degenerated new types of orbitals based on quantum mechanics.
Hybridizing orbitals must always be at an identical energy level.
During hybridization, atomic orbitals like the s, p, or d orbitals, that has identical energies, come together to form hybridized orbitals called sp3, sp2, sp, sp3d, sp3d2, and sp3d3.
BrCN exhibits sp hybridization. sp hybridization takes place when 1 s-orbital and 1 p-orbital of an atom, that is in the same energy level, interact to form 2 new equivalent orbitals.
The new orbitals thus formed are referred to as sp hybridized orbitals.
Each sp hybridized orbital exhibits 50% s and 50% p character. sp hybridized orbitals form linear molecules with a bond angle of 180°.
Polarity refers to the presence of two poles, positive and negative, in a molecule.
Polarity in molecules is developed when one region of a molecule exhibits a positive charge while another region exhibits a negative charge.
This takes place due to the presence of uneven charge distribution within a molecule.
The polarity of a covalent bond can be determined by calculating the difference between the electronegativity of the two atoms involved in the covalent bond.
A result between 0.4 – 1.7 indicates that the covalent bond is polar.
BrCN is a polar molecule due to the uneven distribution of charge.
Qualitatively, this takes place as the nitrogen end of the molecules is less electronegative than the bromine end.
This is because nitrogen tends to push away electrons, while bromine attracts them. The bromine atom in BrCN also consists of three lone pairs which increase the electronegativity in this end.
Cyanogen bromide (CNBr) is a polar compound but is soluble in both polar and nonpolar solvents. In polar solvents, it does not ionize rapidly. It is called a pseudohalogen as it exhibits many chemical properties of a halogen while being a polyatomic compound.
It has a linear molecular geometry with sp hybridization.