PF1009 2024 7 Lewis Diagrams and Resonance Forms PDF
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Uploaded by FragrantSpessartine
University College Cork
Dr. J.J. Keating
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This document covers Lewis diagrams and resonance in Pharmaceutical Chemistry. It includes examples and explanations of various molecules like water, ammonia, and methane.
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Pharmaceutical Chemistry Lewis Diagrams and Resonance Forms Dr. J.J. Keating 1 Lewis Diagrams – Chemical Bonds Way of keeping track of the valence electrons of an atom....
Pharmaceutical Chemistry Lewis Diagrams and Resonance Forms Dr. J.J. Keating 1 Lewis Diagrams – Chemical Bonds Way of keeping track of the valence electrons of an atom. H2 (hydrogen gas) Cl2 (chlorine gas) HCl (hydrogen chloride) 2 Lewis Diagrams – Covalent Compounds H2O (water) NH3 (ammonia) CH4 (methane) 3 Lewis Diagrams – Ionic Compounds NaCl (sodium chloride) MgO (magnesium oxide) CaCl2 (calcium chloride) 4 Lewis Diagrams – Chemical Bonds CO2 (carbon dioxide) N2 (nitrogen gas) C2H3Br (bromoethene) C2HI (iodoethyne) HCN (hydrogen cyanide) 5 Covalent Bonds – The Octet Rule Atoms proceed as far as possible towards completing their octets by sharing electron pairs. N has 5 valence electrons and needs 3 more to complete its octet. Period 2 – no d-orbitals available for occupation – C, N, O, F Period 3 – empty d-orbitals available – > 8 electrons can be accommodated around each atom – expanded octet. 3d orbitals are only slightly higher in energy than 3s and 3p orbitals. Later elements can also form expanded octets. Size of the central atom – P atom big enough for ≥ 5 Cl atoms to fit around the nucleus. N is too small. PCl5 (phosphorous pentachloride), PCl3, PCl4+, PCl6– (phosphorous hexachloride). B – forms compounds with only 6 valence electrons – BF3 – incomplete octet. Odd electron molecules. NO – 11 valence electrons – radical. 6 Resonance – Nitrate Multiple bonds may be written in several equivalent locations. Resonance hybrid – structure resulting from resonance. Resonance is not a flickering of a molecule between Lewis structures, but a blending. Resonance form – A possible Lewis diagram form of a molecule, substructure or ion. Resonance hybrid – a structure resulting from resonance Nitrate – NO3– [ ] resonance forms resonance hybrid 7 Resonance – Sulfur Dioxide Sulfur dioxide – SO2 [ ] resonance forms 8 Resonance – Benzene resonance forms 1.4 Å resonance hybrid 1 Å = 10–10 m curly arrow A curly arrow indicates the movement of a pair of electrons electron pair 9 Bond Strength Measured by bond enthalpy – ΔHB H2 → 2H (dissociation) ΔH = + 436 kJ/mol (endothermic). ΔHB (H–H) = + 436 kJ/mol. Bond enthalpy of any particular bond depends on adjacent atoms: H–O–H = + 494 kJ/mol Bond ΔHB Bond ΔHB N≡N 944 H–F 565 CH3–O–H = + 435 kJ/mol O=O 496 H–Cl 431 C–O 1074 H–Br 366 F–F 158 H–I 299 Cl–Cl 242 C–C 348 Br–Br 193 C=C 612 I–I 151 C≡C 837 10 Bond Length Heavy atoms have more electrons and > atomic radii. Multiple bonds are shorter than single bonds between the same pair of elements as additional bonding electrons bind the atoms together more closely. Covalent radii increase down a group. 1 pm = 10–12m. Bond Length (pm) Bond Average Length (pm) H2 74 C–C 154 N2 110 C=C 134 O2 121 C-/=C 139 (benzene) F2 142 C≡C 120 Cl2 199 C–O 143 Br2 228 C=O 112 I2 268 11
