MOLECULAR POLARITY
Covalent Compounds  Elmhurst College
Electrostatic Potential Organic Functional Groups Functional Gps vs. Boiling Pt.  Chemistry Department
Simple Inorganics Organic Chain Length  Virtual ChemBook

 Polarity vs. Chain Length in Organic Compounds

Principles of Polarity:

The greater the electronegativity difference between atoms in a bond, the more polar the bond. Partial negative charges are found on the most electronegative atoms, the others are partially positive.

The combination of carbons and hydrogens as in hydrocarbons or in the hydrocarbon portion of a molecule with a functional group is always NON-POLAR.

The molecular electrostatic potential is the potential energy of a proton at a particular location near a molecule.
Negative electrostatic potential corresponds to:
partial negative charges (colored in shades of red).
Positive electrostatic potential corresponds to:
partial positive charges (colored in shades of blue).

What happens to the polarity of a molecule with a constant functional group such as an alcohol, but the number of carbons and hydrogens increases i.e. chain length increases?

Let us look at this effect through the use of the molecular electrostatic potential.

Methanol is a polar molecule:

See graphic on the left. The alcohol (-OH) group dominates the molecule making it definitely polar. The electrostatic potential clearly shows that the oxygen is partially negative whereas the carbon and hydrogens are partially positive.

Methanol - Chime in new window

Propanol is a about equal polar and non-polar molecule:

See graphic on the left. The alcohol (-OH) group still gives the polar effect. The electrostatic potential clearly shows that the oxygen is partially negative. When compared to the methanol, the total of three carbons and and multiple hydrogens it begins to show quite a bit more non-polar properties with larger areas of gray.

Propanol - Chime in new window

Hexanol is mostly non-polar with a some polar properties:

See graphic on the left. The alcohol (-OH) group still gives a small polar effect. The electrostatic potential clearly shows that the oxygen is partially negative. When compared to the methanol and propanol, the total of six carbons and and multiple hydrogens shows the dominance of the non-polar properties by the large amount of gray areas.

Note: The band of blue and red on end of the molecule should be all gray to reflect the non-polar property.

Hexanol - Chime in new window