Bonding 1

Polarization

• Polarization is a property that affects molecules.
• Students need to understand the property and how it affects molecules.
• Dipole moments need to be identified.
• Students need to distinguish between polar and non-polar molecules.
• Students need to understand the concept of dielectric constants.

Dielectric Constant (or Relative Permittivity)

• A measurement of a molecule's ability to resist charge separation.
• A measurement of polarity for organic solvents.
• Dimensionless.

Background

• To understand dielectric constants, knowledge of electron distribution in molecules is necessary.
• Covalent bonds are composed of electron pairs residing between atoms.
• Molecular structures can be represented in various ways: fully drawn out, abbreviated, or stick structures. Example: H₃C-CH₃

Electronegativity

• The ability of an atom to attract electrons to itself.
• Electronegativity values increase across a row and up a column in the periodic table.
• Carbon is the least electronegative, except for hydrogen and metals.

Electron Distribution

• In ethane, electron density in the C-C bond is exactly midway between the carbon atoms.
• Each carbon has an equal pull on the electrons.

Electron Distribution (C-H bonds)

• Electron density is slightly pulled towards the carbon atom in the C-H bond.
• This is due to the slight difference in electronegativity between carbon and hydrogen.
• The effect is small and it is called polarization.

Bond Polarization

• A polar covalent bond has a partial positive and partial negative charge.
• The Greek letter δ represents the partial charge.
• One atom is electron rich, and the other is electron poor. Example: C-H bond, C is δ- and H is δ+.

Bond Polarities

• Atoms in a covalent bond can have different electronegativities.
• This leads to polarization, concentrating electrons at one end. Example: C-C, C-H, C-O and O-H bonds.

Examples of Bond Polarities

• Many examples of molecules with different bonds like chloromethane, bromoethane, iodomethane, formaldehyde, ethanol, propanone, acetonitrile, methyl magnesium bromide, tetramethyl silane, and methyl lithium.

More Examples

• Structures and bond polarizations need to be drawn for specific molecules like ethanol, propyl amine, phenyl amine (or aniline), and pethidine.

Molecular Dipole Moment (μ)

• The vector sum of individual bond dipoles.
• CO₂, BF₃, and CCl₄ have zero dipole moment, making them non-polar.
• Other molecules like water and ammonia have polar bonds and dipole moments greater than 0, which makes them polar.

Inductive Effect

• The influence of polarising species on electron distribution in sigma bonds.
• The effect can span more than one bond. Example: presence of chlorine atoms affecting electron distribution in the carbon chain.

Permanent Dipoles & Dielectric Constants

• Molecules with permanent dipoles often have high dielectric constants.
• These have low resistance to charge separation.
• Polar solvents (e.g., water, formic acid, methanol, ethanol, acetic acid) have dielectric constants greater than 15.
• Protic solvents have a relatively acidic proton.

Dielectric Constants

• Dielectric constant < 5 = Non-Polar
• 5 < dielectric constant < 15 = Semi-polar

Dielectric Constants (Table)

• A table listing solvents, their structures, and their dielectric constants. (Polar >15, Semi-polar 5-15, Non-polar <5)

Solubility of Organic Compounds

• C-C and C-H bonds are low in polarity.
• Polarity of organic molecules is often low.
• Organic molecules are soluble in non-polar solvents (not water).
• Molecules with polar functional groups (e.g., -CH₂OH, -CH₂NH₂) can be soluble in water.

In Summary

• Polar covalent bonds are formed from unequal electron sharing determined by electronegativity.
• Polarity is indicated by δ+ (partial positive) and δ- (partial negative) charges.
• Higher dielectric constant indicates greater polarity.
• Polar compounds dissolve in polar solvents; non-polar in non-polar solvents.