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Physical Properties Affected by the Lone Pair - Polarity and Dipole Moment

Learning Objective: To study the impact of lone pair on a molecule's polarity and dipole moment. 

Prerequisites: 

 

Chapter: Fundamentals of Organic Reactions

Sub-topic: Lone Pair

 

Author's Note: The lone pair affects the physical properties of polarity and dipole moment. Lone pair affects differently when part of a hybrid or unhybridized orbital or when they are part of central or terminal atoms. Several examples, including the exceptions, explain this phenomenon. Here, I gather various observations under the roof of polarity, like – why CO2 is nonpolar, why the bond angle of PH3 is lesser than NH3 even though P is a congener of N, or why NF3 has a lesser dipole moment than NH3, and try to provide a satisfactory answer.

Physical Properties Affected by the Lone Pair - Polarity and Dipole Moment

Covalent bonds tie several atoms of varying electronegativity values to obtain a large variety of molecules. If one of the atoms holding the covalent bond is electronegative, it will pull the bond electrons towards itself, creating polar ends. Therefore, the bond polarity results from the difference in the electronegativity of atoms.

The asymmetric shape of molecules is an additional factor that determines their overall polarity. The dipole moment measures the molecule’s polarity and provides the values in Debye (D).  

 

Lone pair affects molecule’s shape contributing to Polarity

The lone pairs contribute to a molecule’s polarity if it is part of the central atom, and its presence affects the molecule’s shape. For example, Carbon dioxide (CO2) is a linear molecule, and the central carbon atom has no lone pairs. Still, another triatomic molecule, water (H2O), is bent due to the central Oxygen atoms’ two lone pairs. The lone pairs of Oxygen push the symmetry out of the molecule. Therefore, water is polar, and carbon dioxide isn’t. 

How lone pair affects the polarity and dipole moment in organic molecules

 

Orbital Occupancy of Lone Pair affects polarity

The lone pair can be part of hybrid orbitals (atomic orbitals involved in hybridization) or reside in pure s or p- atomic orbitals (atomic orbitals uninvolved in hybridization). 

For example, the.....

 

Directionality of Lone pair enhances or diminishes Polarity

The directionality of the lone pair in hybrid orbitals can enhance or reduce the contributions of bond dipoles. The bond dipole can add up or reduce to impact the total polarity of the molecule. 

For example, ....

Another example is the T‐shaped molecule series of ...

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Fundamentals of Organic Reactions
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