Polarization Student Notes PDF

Polarization Dr Lynda Storey Level 5, Room 548 X2528 [email protected]  Learning Objectives Understand the property, polarization, and how it affects molecules. Be able to identify dipole moments. Distinguish between polar and non-polar molecules. Have an appreciatio...

Polarization Dr Lynda Storey Level 5, Room 548 X2528 [email protected]  Learning Objectives Understand the property, polarization, and how it affects molecules. Be able to identify dipole moments. Distinguish between polar and non-polar molecules. Have an appreciation of Dielectric Constants.  Dielectric Constant (D) or Relative Permitivity (e) A measure of the ability of a molecule to resist charge separation. A measure of polarity for organic solvents. Is dimensionless  Background Before we can gain an understanding of dielectric constants, we must have an appreciation of how electrons are distributed in molecules. If we consider a covalent bond as being made up of a pair of electrons residing between two atoms then we can draw structures in the following way: H H H C C H H 3C CH3 H H abreviated stick structure fully drawn out structure  Electronegativity The ability of an atom in a molecule to attract electrons to itself. H 2.1 Li C N O F 1.0 2.5 3.0 3.5 4.0 Increasing Na Mg Cl 0.9 1.2 3.0 electronegativity Increasing electronegativity The higher the number, the better the atom is at attracting electron density towards itself. Remember: Carbon is the least electronegative element except for Hydrogen & the metals.  Electron Distribution In the case of ethane, we would expect the electron density making up the C-C bond to be exactly midway between the carbon atoms, since each C has an equal pull on the electrons. H H H C C H This is indeed the case H H  Electron Distribution In the case of the C-H bonds we would expect the electron density to be slightly pulled towards the carbon. H H H C C H H H Because of the similarity in electronegativity the effect is very small. This effect is known as polarisation.  Bond Polarisation Bond Dipole Electron Electron Rich Poor C c = 2.5 c = 2.1 A polar covalent bond. The Greek letter d stands for partial charge.  Bond Polarities Two atoms in a covalent bond may have different electronegativities () (the ability of an atom to pull electrons towards itself). This results in the bond being polarised with electrons concentrated at one end. No polarisation Slight H H H d+ H =2.1 polarisation =2.5 e - H C C H H C O d- d+ =3.5 H H H e- =2.5 =2.1 C-O and O-H bonds are polarised  Examples of Bond Polarities H 3C Cl H 3C C Br H 3C I H2 chloromethane bromoethane iodomethane H H 3C H 3C O O O H 3C C N H H H 3C formaldehyde ethanal propanone acetonitrile (methanaldehyde) (acetone) (methyl cyanide) CH3 H 3C MgBr H 3C Si CH3 H 3C Li CH3 methyl magnesium tetramethyl methyl bromide silane lithium  More Examples Draw, name and show bond polarisation for the following: O H H N Ethanol or ethyl alcohol H Propyl amine NH2 Phenyl amine Or aniline O N Pethidine O  Molecular Dipole Moment (m) The molecular dipole moment (m) is the vector sum of its individual bond dipoles. F Cl B C Cl O C O F F Cl Cl Linear (180) Trigonal Planar (120) Tetrahedral (109.5) CO2, BF3 & CCl4 therefore each have 0 dipole moment. They are therefore non- polar.  Molecular Dipole Moment (m) Other molecules, such as water and ammonia, have polar bonds and dipole moments greater than 0. N O H H H H H They are therefore polar molecules.  Inductive Effect This is the influence on electron distribution in sigma bonds by polarising species. The effect can be seen over more than one bond. E.g. Cl Cl C C C CH3 H2 H2 Cl degree of +ve character decreases  Permanent Dipoles & Dielectric Constants If a molecule has a permanent dipole then its dielectric constant will be fairly high. It will have minimal resistance to charge separation. A solvent is considered polar if its dielectric constant is greater than 15. Eg. H2O, formic acid, methanol, ethanol, acetic acid. These are all protic solvents, i.e. they have a relatively acidic proton.  Dielectric Constants If the dielectric constant is less than 5, it is considered non-polar. A non-polar molecule can resist charge separation. If the dielectric constant falls between 5 & 15 it is considered semi polar.  Dielectric Constants Dielectric Solvent Structure Constant Polar (>15) water H2O 79 formic acid HCOOH 59 glycerol HO - CH2 - CH(OH) - CH2 - OH 43 acetonitrile CH3 - CN 38 methanol CH3 - OH 33 acetone CH3(CO)CH3 21 Semi-Polar (5 – 15) dichloromethane CH2Cl2 9 acetic acid CH3 – COOH 6 Non-Polar (