Chapter 7, Problem 7.143QP

Carbon monoxide (CO) is a poisonous compound due to its ability to bind strongly to Fe²⁺ in the hemoglobin molecule. The molecular orbitals of CO have the same energy order as those of the N₂ molecule, (a) Draw a Lewis structure of CO and assign formal charges. Explain why CO has a rather small dipole moment of 0.12 D. (b) Compare the bond order of CO with that from molecular orbital theory, (c) Which of the atoms (C or O) is more likely to form bonds with the Fe²⁺ ion in hemoglobin?

Interpretation Introduction

Interpretation:

The Lewis structure of the carbon monoxide and its corresponding formal charges should be drawn.  The bond order of carbon monoxide with that of molecular orbital theory should be compared.  Also to found out which atom of carbon monoxide forms bond with ${\text{Fe}}^{\text{2+}}$ of hemoglobin.

Concept Introduction:

• Lewis structures which also known as Lewis dot structures represents the bonding between atoms of a molecule and the lone pairs of electrons that may exist in the molecule.
• A formal charge (FC) is the charge assigned to an atom in a molecule, irrespective of relative electronegativity by thinking that electrons in all chemical bonds are shared equally among atoms.
• Formal charge of an atom can be determined by the given formula.

$\text{Formal}\text{charge}\left(\text{F}\text{C}\right)\text{=}\left(\begin{array}{l}\text{no}\text{.of}\text{valence}\\ \text{electron}\text{in}\text{atom}\end{array}\right)-\frac{\text{1}}{\text{2}}\left(\begin{array}{l}\text{no}\text{.of}\text{bonding}\\ \text{electrons}\end{array}\right)-\left(\begin{array}{l}\text{no}\text{.of}\text{non-bonding}\\ \text{electrons}\end{array}\right)$

• Dipole moment occurs when there is a difference in the electronegativity of atoms in a bond which results in charge separation.

Answer to Problem 7.143QP

1. (a) Lewis structure of the carbon monoxide and its corresponding formal charges is
2. (c) It was found out that carbon atom of carbon monoxide forms bond with ${\text{Fe}}^{\text{2+}}$ of hemoglobin.

Explanation of Solution

Given molecule is $\text{CO}$.

Lewis structure of carbon monoxide is drawn below.

• The total number of valence electrons is found to be 10, where carbon and oxygen atom contribute 4 and 6 electrons respectively.

The 8 electrons getting after reducing two electrons for each bond from the total valence electron are distributed on atom to complete the octet. After the distribution of electrons, atoms does not obey octet rule so triple bond is formed carbon and oxygen atoms.

The formal charge carbon monoxide

The formal charge carbon monoxide is calculated,

• Carbon atom

$\begin{array}{l}\text{Number of }\text{valence}\text{electron}\text{=}\text{4}\\ \text{Number of }\text{bonding}\text{electron}\text{=}\text{6}\\ \text{Number of }\text{non-bonding}\text{electron}\text{=}\text{2}\end{array}$

Substituting these values to the equation,

$\begin{array}{l}\text{FC=}\text{4-}\left(\frac{\text{1}}{\text{2}}\text{×6}\right)\text{-2}\\ \text{=-1}\end{array}$

• Oxygen atom (a)

  $\begin{array}{l}\text{Number of }\text{valence}\text{electron}\text{=}6\\ \text{Number of }\text{bonding}\text{electron}\text{=}\text{6}\\ \text{Number of }\text{non-bonding}\text{electron}\text{=}\text{2}\end{array}$

Substituting these values to the equation,

$\begin{array}{l}\text{FC=}6\text{-}\left(\frac{\text{1}}{\text{2}}\text{×6}\right)\text{-2}\\ \text{=+1}\end{array}$

Due to the difference in the electronegativity of carbon and oxygen atoms, the $\text{C-O}$ bond is polar. So carbon atom gets a partial positive charge whereas oxygen atoms gets partial negative charge which means concentration of electron density is expected to have on oxygen atom. After calculating the formal charges, the negative charge is assigned on the carbon atom. Thus carbon monoxide is expected to have low dipole moment.

Interpretation Introduction

Interpretation:

The Lewis structure of the carbon monoxide and its corresponding formal charges should be drawn.  The bond order of carbon monoxide with that of molecular orbital theory should be compared.  Also to found out which atom of carbon monoxide forms bond with ${\text{Fe}}^{\text{2+}}$ of hemoglobin.

Concept Introduction:

• Lewis structures which also known as Lewis dot structures represents the bonding between atoms of a molecule and the lone pairs of electrons that may exist in the molecule.
• A formal charge (FC) is the charge assigned to an atom in a molecule, irrespective of relative electronegativity by thinking that electrons in all chemical bonds are shared equally among atoms.
• Formal charge of an atom can be determined by the given formula.

$\text{Formal}\text{charge}\left(\text{F}\text{C}\right)\text{=}\left(\begin{array}{l}\text{no}\text{.of}\text{valence}\\ \text{electron}\text{in}\text{atom}\end{array}\right)-\frac{\text{1}}{\text{2}}\left(\begin{array}{l}\text{no}\text{.of}\text{bonding}\\ \text{electrons}\end{array}\right)-\left(\begin{array}{l}\text{no}\text{.of}\text{non-bonding}\\ \text{electrons}\end{array}\right)$

• Dipole moment occurs when there is a difference in the electronegativity of atoms in a bond which results in charge separation.

In molecular orbital theory, when the bonding takes place the atomic orbitals that take part combine to get a new orbital that has the properties of the whole molecule. The newly formed orbitals are known as molecular orbitals and only contain a maximum of two electrons. The number of newly formed molecular orbital is equal to the number of atomic orbitals involved in the bonding.

There are two types of molecular orbitals,

1. a) Bonding molecular orbitals: sharing of electron density is between the nuclei and has comparatively lower energy and fills first.
2. b) Antibonding molecular orbitals: Two nuclei are pulled by the electrons density in opposite direction and have higher energy comparing to bonding molecular orbital.

Explanation of Solution

Total number of electrons in carbon monoxide is 14 where carbon and oxygen atom have 6 and 8 electrons respectively.

Electronic configuration of $\text{CO}$ is ${\text{(σ1s)}}^{\text{2}}{\text{(σ1s*)}}^{\text{2}}{\text{(σ2s)}}^{\text{2}}{\text{(σ2s*)}}^{\text{2}}{{\text{(π2p}}_{\text{x}}\text{)}}^{\text{2}}{{\text{(π2p}}_{\text{y}}\text{)}}^{\text{2}}{{\text{(σp}}_{\text{z}}\text{)}}^{\text{2}}$.

The bond order of $\text{CO}$ is determined.

The bond order gives an idea about the stability of a molecule. It can be calculated using the molecular orbital theory. The stability of a molecule increase as the bond order increases.

$\text{Bond order =}\frac{\text{(No}\text{.of}{\text{e}}^{\text{-}}\text{in}\text{bonding}\text{orbitals)-(No}\text{.of}{\text{e}}^{\text{-}}\text{in}\text{anti}\text{bonding}\text{orbitals)}}{\text{2}}$

$\begin{array}{l}\text{No}\text{.of}{\text{e}}^{\text{-}}\text{in}\text{bonding}\text{orbitals}\text{=8}\\ \text{No}\text{.of}{\text{e}}^{\text{-}}\text{in}\text{anti}\text{bonding}\text{orbitals}=2\end{array}$

$\begin{array}{l}\text{Bond}\text{order}\text{=}\frac{\text{1}}{\text{2}}\text{(8-2)}\\ \text{=}\text{3}\end{array}$

The bond order of $\text{CO}$ is 3 which is similar to the triple bond that drawn in Lewis structure. $\text{CO}$ and ${\text{N}}_{\text{2}}$ is isoelectronic which means they  have same number of valence electrons and atoms.

Conclusion

The bond order of carbon monoxide with that of molecular orbital theory was compared.

Interpretation Introduction

Interpretation:

The Lewis structure of the carbon monoxide and its corresponding formal charges should be drawn.  The bond order of carbon monoxide with that of molecular orbital theory should be compared.  Also to found out which atom of carbon monoxide forms bond with ${\text{Fe}}^{\text{2+}}$ of hemoglobin.

Concept Introduction:

• Lewis structures which also known as Lewis dot structures represents the bonding between atoms of a molecule and the lone pairs of electrons that may exist in the molecule.
• A formal charge (FC) is the charge assigned to an atom in a molecule, irrespective of relative electronegativity by thinking that electrons in all chemical bonds are shared equally among atoms.
• Formal charge of an atom can be determined by the given formula.

$\text{Formal}\text{charge}\left(\text{F}\text{C}\right)\text{=}\left(\begin{array}{l}\text{no}\text{.of}\text{valence}\\ \text{electron}\text{in}\text{atom}\end{array}\right)-\frac{\text{1}}{\text{2}}\left(\begin{array}{l}\text{no}\text{.of}\text{bonding}\\ \text{electrons}\end{array}\right)-\left(\begin{array}{l}\text{no}\text{.of}\text{non-bonding}\\ \text{electrons}\end{array}\right)$

• Dipole moment occurs when there is a difference in the electronegativity of atoms in a bond which results in charge separation.

Answer to Problem 7.143QP

1. (d) It was found out that carbon atom of carbon monoxide forms bond with ${\text{Fe}}^{\text{2+}}$ of hemoglobin.

Explanation of Solution

${\text{Fe}}^{\text{2+}}$ is the metal presence in hemoglobin. When carbon monoxide enters the blood stream ${\text{Fe}}^{\text{2+}}$ cation forms bond with a negatively charged atom. Carbon monoxide is a neutral compound. While calculating the formal charge of $\text{CO}$, it is found that carbon has negative charge. So ${\text{Fe}}^{\text{2+}}$ tend to make bond with carbon atom ${\text{(OC-Fe}}^{\text{2+}})$ instead of oxygen atom ${\text{(CO-Fe}}^{\text{2+}})$.

Conclusion

The Lewis structure of the carbon monoxide and its corresponding formal charges was drawn. Also it was found out that carbon atom of carbon monoxide forms bond with ${\text{Fe}}^{\text{2+}}$ of hemoglobin.