“Simple” model of covalent bonds

1. “Simple” model of covalent bonds Electrons in shells – 1st shell, 2nd shell, etc – ‘planetary model’ Dot-cross diagrams - Covalent bond is sharing of electron pairs Molecular shape predicted by VSEPR – still very useful Double/triple bond character not predicted – just 2 electron pairs

2. “More complex” model (Valence-bond model) Electrons in orbitals – 1s, 2s, 2p – different shapes – different energy levels Electrons have spin (represented by up/down arrows Covalent bond is overlap of orbitals Molecular shape is rationalised by hybridisation i.e. does not predict – explains what is observed Double/triple bond – actually s and p bonds – one is weaker than the other – spatial difference Try looking at http://www.chem.ox.ac.uk/vrchemistry/ (VSEPR) http://osxs.ch.liv.ac.uk/~ng/external/orbitalsethene.htm (hybridisation)

3. Short cut Sp3 hybridisation coordinated to 4 atoms 4 single bonds 109.5 Sp2 hybridisation coordinated to 3 atoms 1 sg / 1 double 120 Sp hybridation coordinated to 1 atom 1 sg / 1 triple 180

4. Formal charges Section 2.3 Resonance Section 2.4 Section 2.5-2.6 Conjugation Section 10.5 Section 14.1 (no need to do Molecular orbitals – top off p 485) Aromaticity Section 15.3, 15.5, 15.7

5. Resonance Can occur if lone pairs and double/triple bonds Resonance forms should conform to octet rule A molecule can have many resonance forms contributing to the true structure ‘Good’ resonance structures contribute a lot to the true structure ‘Bad’ resonance structures – those with creation of charges or with + on electronegative elements and vice versa – contribute relatively little to the structure Compounds with many ‘good’ resonance forms tend to be more stable relative to similar compounds with less ‘good’ resonance forms

6. Conjugation (chapter 14.1) Definition: stabilising efffect of alternating double bonds Implications: (1) electrons delocalised, (2) greater stability of conjugated alkenes Bond in between double bonds – also has double bond character Even though formally single bond

7. Hyperconjugation (Chapter 6.6 and 6.9) Definition: stabilising effect of C-H bond next to a double bond (sp2 carbon) Implications: (1) explains greater stability of substituted alkenes vs terminal alkenes (2) explains greater stability of tertiary>secondary>primary carbocations (3) explains electron-donating effect of methyl groups

8. Aromaticity (Chapter 15.3, 15.5, 15.7) Definition: stabilising efffect of molecules which obey Huckel’s rules Implications: greater stability of aromatic compounds Some hints to calculate the p electrons • If the atom has a double bond – it contributes one p electron • If the atom has only single bonds but one lone pair – it contributes two p electrons • If the atom has only single bonds but two lone pairs – it contributes two p electrons • The other lone pair is not involved in aromaticity

9. Ways of explaining stability of organic molecules Orbitals – s, p – different shapes Valence Bond model – hybridisation, s and p bonds Molecules with p bonds Molecules with sbonds Resonance (2.5,2.6) Stability if more forms Electronegativity (2.1) Inductive effects Conjugation (14.1) Alternate p bonds Aromaticity (15.3) Alternate p bonds and 4n+2 electrons Hyperconjugation (6.6) p bonds & C-H bonds Two examples of reactive intermediates alkyl carbocations aromatic/aryl carbocations