Question

Which of the following compounds has low dipole moment?
1. Propane
2. Ethyl fluoride
3. dimethyl ether
4. Ethanol

Answer 1

Correct Answer - Option 1 : Propane

Concept:

Bond Polarity –

• In a covalent chemical bond, the more electronegative the element more is its tendency to attract the electron cloud towards itself.
• Thus one of the atoms (more electronegative one) attains a partial negative charge, and the other (less electronegative) attains a partial positive charge creating a dipole.
• The stronger the separation of charges, the stronger is the polarity of the bond.
• When the electronegativity difference is zero, the bond is nonpolar covalent.

Dipole moment: 

• Dipole moment arises in a polar bond be it ionic or covalent.
• The greater the separation of charges between the atoms in bonds, the greater is the value of the dipole moment.
• The dipole moment is a vector quantity that means it has a direction as well as magnitude.
• The direction of the dipole moment in a bond is towards the more electronegative atom.
• The direction of the dipole in a molecule is the resultant of all the bond dipole moments.

Explanation: 

• Propane is an alkene and the difference in electronegativity between carbon and Hydrogen is very low which is only .35.
• The bond between carbon and fluorine in ethyl fluoride is very polar with an electronegativity difference of 1.4.
• The total bond moment acts upwards towards the Fluorine atom and does not cancel out with the rest of the molecule.
• In dimethyl ether, the difference in electronegativity between carbon and oxygen is .9.
• The bond between carbon and hydrogen is also polar in ethanol. 
• The highest polarity and the molecules are seen in ethyl fluoride and the lowest is in propane.

Hence, propane has the least dipole moment.

• It has to be noted that the bonds in a molecule can be polar but the molecule can have the net dipole moment as zero. The difference between them is listed below.

Bond polarity	Molecular polarity
Bond polarity is a concept that produces polarity in covalent bonds.	Molecular polarity is a phenomenon that produces polarity in covalent compounds.
Depends on the difference in E.N values of the bonded atoms.	Depends on the geometry of the molecules and types of atoms present.