“Simple” model of covalent bonds Electrons in shells – 1 st shell, 2 nd 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

“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  and  bonds – one is weaker than the other – spatial difference Try looking at (VSEPR) (hybridisation)

Short cut Sp3 hybridisationcoordinated to 4 atoms 4 single bonds109.5 Sp2 hybridisationcoordinated to 3 atoms1 sg / 1 double120 Sp hybridationcoordinated to 1 atom1 sg / 1 triple180

Formal charges Section 2.3 Resonance Section 2.4 Section 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

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

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

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

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  electrons -If the atom has a double bond – it contributes one  electron -If the atom has only single bonds but one lone pair – it contributes two  electrons -If the atom has only single bonds but two lone pairs – it contributes two  electrons The other lone pair is not involved in aromaticity

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