Polarity Of Bonds And Molecules
Polarity in organic chemistry refers to a separation of charge and can describe a bond or an entire molecule. Experimentally, bond polarity is measured by its dipole moment. Bonds connecting atoms of different electronegativity are polar with a higher density of bonding electrons around the more electronegative atom giving it a partial negative charge (designated as d-). The less electronegative atom has some of its electron density taken away giving it a partial positive charge (d+). This polarization of charge in the H-Cl bond is due to different electronegativities of chlorine and hydrogen. The polarity in the bond can also be represented by a arrow indicating a dipole (two charges separated by a distance). The tip of the arrow points toward the more electronegative atom. The polarity of the molecule is the sum of all of the bond polarities in the molecule. Since the dipole moment (m, measured in Debyes (D)) is a vector (a quantitiy with both magnitude and direction), the molecular dipole moment is the vector sum of the individual dipole moments. If we compare the molecular dipole moments of formaldehyde and carbon dioxide, both containing a polar carbonyl (C=O) group, we find that formaldehyde is highly polar while carbon dioxide is nonpolar . Since CO2 is a linear molecule, the dipoles cancel each other. Water is a bent molecule with polar O-H bonds. The bond dipole moments add to give a resultant dipole (m = 1.85 D) directed toward the more electronegative oxygen. The polarity of chloromethanes reveals the importance of symmetry. All of these compounds contain polar C-Cl bonds but the tetrahedral symmetry of CCl4 causes the bond dipoles to cancel giving a nonpolar molecule.
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