Molecular polarity

Examples of Molecular polarity in the following topics:

• Bond Polarity

  • Molecular polarity is dependent on the presence of polar covalent bonds and the molecule's three-dimensional structure.
  • Molecular polarity: when an entire molecule, which can be made out of several covalent bonds, has a net polarity, with one end having a higher concentration of negative charge and another end having a surplus of positive charge.
  • In molecules containing more than one polar bond, the molecular dipole moment is just the vector addition of the individual bond dipole moments.
  • H2O, by contrast, has a very large molecular dipole moment which results from the two polar H–O bonds forming an angle of 104.5° between them.
  • Apply knowledge of bond polarity and molecular geometry to identify the dipole moment of molecules

• Boiling & Melting Points

  • First there is molecular size.
  • Molecular shape is also important, as the second group of compounds illustrate.
  • Finally, permanent molecular dipoles generated by polar covalent bonds result in even greater attractive forces between molecules, provided they have the mobility to line up in appropriate orientations.
  • The last entries in the table compare non-polar hydrocarbons with equal-sized compounds having polar bonds to oxygen and nitrogen.
  • Halogens also form polar bonds to carbon, but they also increase the molecular mass, making it difficult to distinguish among these factors.

• Properties of Aldehydes and Ketones

  • Because of the greater electronegativity of oxygen, the carbonyl group is polar, and aldehydes and ketones have larger molecular dipole moments (D) than do alkenes.
  • The resonance structures in the first diagram below illustrate this polarity, and the relative dipole moments of formaldehyde, other aldehydes and ketones confirm the stabilizing influence that alkyl substituents have on carbocations (the larger the dipole moment the greater the polar character of the carbonyl group).
  • The polarity of the carbonyl group also has a profound effect on its chemical reactivity, compared with the non-polar double bonds of alkenes.
  • The inherent polarity of the carbonyl group, together with its increased basicity (compared with alkenes), lowers the transition state energy for both reactions, with a resulting increase in rate.

• The Shape of Molecules

  • The best way to study the three-dimensional shapes of molecules is by using molecular models.
  • This powerful visualization tool allows the user to move a molecular stucture in any way desired.
  • A molecule which has one or more polar covalent bonds may have a dipole moment as a result of the accumulated bond dipoles.
  • In the case of water, we know that the O-H covalent bond is polar, due to the different electronegativities of hydrogen and oxygen.
  • The bond dipoles are colored magenta and the resulting molecular dipole is colored blue.

• Dipole Moment

  • The bond dipole moment uses the idea of the electric dipole moment to measure a chemical bond's polarity within a molecule.
  • This is the case with polar compounds like hydrogen fluoride (HF), where the atoms unequally share electron density.
  • Debye was the first to extensively study molecular dipoles.
  • Molecules with only two atoms contain only one (single or multiple) bond, so the bond dipole moment is the molecular dipole moment.
  • The two carbon to oxygen bonds are polar, but they are 180° apart from each other and will cancel.

• Charge Distribution in Molecules

  • Although this is true for diatomic elements such as H2, N2 and O2, most covalent compounds show some degree of local charge separation, resulting in bond and / or molecular dipoles.
  • Similarly, nitromethane has a positive-charged nitrogen and a negative-charged oxygen, the total molecular charge again being zero.
  • Such a covalent bond is polar, and will have a dipole (one end is positive and the other end negative).
  • Although there is a small electronegativity difference between carbon and hydrogen, the C–H bond is regarded as weakly polar at best, and hydrocarbons in general are considered to be non-polar compounds.
  • Methane is essentially non-acidic, since the C–H bond is nearly non-polar.

• Hydrogen Bonding

  • If we compare the boiling points of methane (CH4) -161ºC, ammonia (NH3) -33ºC, water (H2O) 100ºC and hydrogen fluoride (HF) 19ºC, we see a greater variation for these similar sized molecules than expected from the data presented above for polar compounds.
  • Most of the simple hydrides of group IV, V, VI & VII elements display the expected rise in boiling point with molecular mass, but the hydrides of the most electronegative elements (nitrogen, oxygen and fluorine) have abnormally high boiling points for their mass.
  • Hydrogen forms polar covalent bonds to more electronegative atoms such as oxygen, and because a hydrogen atom is quite small, the positive end of the bond dipole (the hydrogen) can approach neighboring nucleophilic or basic sites more closely than can other polar bonds.
  • The molecule providing a polar hydrogen for a hydrogen bond is called a donor.

• Dipole-Dipole Force

  • Molecules that contain dipoles are called polar molecules and are very abundant in nature.
  • Molecules often contain polar bonds because of electronegativity differences but have no overall dipole moment if they are symmetrical.
  • However, these carbon-chlorine dipoles cancel each other out because the molecular is symmetrical, and CCl4 has no overall dipole movement.
  • Attractions between polar molecules vary.
  • Why does polarity have an effect on the strength of attraction between molecules?

• Covalent Bonds

  • For atoms with equal electronegativity, the bond between them will be a non-polar covalent interaction.
  • In non-polar covalent bonds, the electrons are equally shared between the two atoms.
  • For atoms with differing electronegativity, the bond will be a polar covalent interaction, where the electrons will not be shared equally.

• Background and Properties

  • An aldehyde and ketone of equivalent molecular weight are also listed for comparison.
  • The relatively high boiling points of equivalent 3º-amides and nitriles are probably due to the high polarity of these functions.