1.7: Electronegativity and bond - PowerPoint Presentation

1. 1.7: Electronegativity and bond polarityCHE2060 Topic 1: Atoms, orbitals & bonding

2. Bond polarity & electronegativityCovalent bonds can be either polar or nonpolar depending on how equally(or unequally) their e- pair bonds are shared between the 2 bonded atoms.Electronegativity is used to determine polarity of a bond between two atoms.“Electronegativity is the ability of an atom in a molecule to attract the molecule’s electrons toward itself.” Higher EN = stronger attraction. More protons create more pull on e-. More shells move ve- further from nucleus.Bruice (2007)

3. Calculating bond polarityNon-polar bonds share electrons equally (or evenly) between the 2 atoms.Polar bonds share electrons unequally.Degree of polarity is calculated: polarity (ΔEN) = | ENA – ENB|Type of bondΔENIonic> 2.0Polar covalent0.5 – 2.0Nonpolar covalent< 0.5Crossed arrows are used to show the direction in which electrons are pulled.E- move toward the arrow head, leaving behind a positive charge (the cross). ...C – F: H |H - C - O - H | HH \ C = O /H H |H - C - H | H........Use EN values to determine the polarity of each bond.....A dipolar charge is a partial positive ornegative charge on atoms joined by a polar covalent bond. Small delta (δ) indicates dipolar charge.Here δ are shown only for polar bonds.+-+-+-

4. Exercise: non-polar, polar or ionic?For each pair of atoms:a) Calculate bond polaritiesb) Identify each bond as ionic, polar covalent or non-polar covalentHBrH2OLiIBrClNH3KF|2.1 – 2.8| = 0.7 polar covalent|2.1 – 3.5| = 1.4 polar covalent|1.0 – 2.5| = 1.5 polar covalent|2.8 – 3.0| = 0.2 non-polar covalent|3.0 – 2.1| = 0.9 polar covalent|0.8 – 4.0| = 3.2 ionic

5. Visualizing polarityThe polarity of bonds (or within molecules) can be visualized aselectron density in electrostatic potential maps. “Hot” colors indicate higher electron density.EPMs also showrelative atomic size.H - I2.1 – 2.5H - Br2.1 – 2.8H - Cl2.1 – 3.0H - F2.1 – 4.0δ+δ-Bruice (2007)

6. While the yellow atoms are farapart if the peel is stretched out,they are close when its 3Dstructure is restored. So theyaffect one another’s polarityNeighbors can affect your polarity!Inductive effects occur when the polarity of bond (A) is increased if its neighboring bond (B) has a much higher polarity.Ethane’s C-Cbond is non-polar. H H | |H - C – C – H | | H H H H | |H - C – C – Cl: | | H H....vsBut 1-chloroethane has anatom (Cl) with a higher EN.The polar C-Cl bondpolarizes the neighboringC-C bond a bit.This new polarity is induced. Field effect polarization can also be caused by a molecule’s environment. Image placing a large magnet near one end of a long non-polar molecule.Field effects are similar, but occur when influencing neighbors are far from one another in terms of bonds, but are close in 3D structure.D&D p.56

7. Formal charge shows extreme polaritiesIf a molecule carries a charge, we calculate formal charge to show where thatcharge is located; which atom carries the charge. Start with a Lewis structure.Formal charge = (# ve-) – (dots + sticks) Calculated for each atom.H = 1 – (0+1) = 0C = 4 – (0 + 4) = 0Makes sense since there is no charge.Draw Lewis structures & calculate formal charges for these molecules:CH4H3O+1CH3O-1H = 1 – (0+1) = 0O = 6 – (2 + 3) = +1So O carries the charge. H | H - C – H | H H - O – H | H .. H | H - C – O: | H ....H = 1 – (0+1) = 0C = 4 – (0 + 4) = 0 O = 6 – (6 + 1) = -1So O carries the charge.Generally, the mostEN atom carriesthe charge.D&D p.56-9

8. Exercise: calculating formal chargeDraw a Lewis structure & calculate formal charges for each atom.CH3FCH3OH2+1CH3+1NH3BF3-1 :CH3:CH2 HNO2Start by countingnumber of ve- H | H - C – F: | H H = 1 – (0+1) = 0C = 4 – (0 + 4) = 0F = 7 – (6 + 1) = 0.... H | H - C – O – H | | H H H = 1 – (0+1) = 0C = 4 – (0 + 4) = 0O = 6 – (2 + 3) = +1.. H | H – C: | H H = 1 – (0+1) = 0C = 4 – (2 + 3) = -1 H :F: | | H – N – B – F: | | H :F: H = 1 – (0+1) = 0N = 5 – (0 + 4) = +1B = 3 – (0 + 4) = -1F = 7 – (6 + 1) = 0Net = zero........ H | H – C | H H = 1 – (0+1) = 0C = 4 – (0 + 3) = +1H = 1 – (0+1) = 0C = 4 – (2 + 2) = 0 H | H – C: H – N = O | :O:......H = 1 – (0+1) = 0N = 5 – (0 + 4) = +1Single O = 6 – (6 + 1) = -1Double O = 6 – (4 + 2) = oNet = zeroD&D p.59

9. Dipole moment: polarity & geometryDipole moments are permanent partial charges created byunsymmetrical separation of charges.Which of these molecules has a permanent dipole moment?Here the dipoles cancel eachother producing no overallmolecular polarity, ordipole moment.H&P p.7C CHHClClC CHClHClδ-δ-δ+δ+δ-δ-δ+δ+

10. Polarity varies with atomic contextThe electronegativity values of H & O are constant, and always differ by 1.4.However, the degree of molecular polarity also depends on overallmolecular charge, and to some extent, geometry & shape.Which of these molecules is the most reactive? The least reactive?Bruice (2007)These molecules all havedipole moments.most reactiveleast reactive

11. Polarities of typical organic bondsNote that bond dipole takes molecular geometry into account along with differences in electronegativity values.Bond Bond dipole (D)C - F1.53C - Cl1.59C - Br1.48C - I1.29C - N0.22C - O0.85C - H0.35Bond Bond dipole (D)H - F1.82H - Cl1.08H- Br0.82H - I0.44H - N1.32H - O1.54What trends do you see here?Critical point to remember:Chemical reactivity of a bond increases with its increasing polarity!D&D p.54-5

12. Can you?Explain what electronegativity measures? And why electronegativity increases up and across the periodic table?Calculate electronegativity differences and use them to determine bond polarity and to classify bonds as nonpolar covalent, polar covalent or ionic?Define the terms ‘inductive effect’ and ‘field effect’ and describe the difference between the two?Define the term dipole moment?Explain how bond polarity relates to the reactivity of bonds?Aristotle1700’s gas pump experiment