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“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.

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“Simple” model of covalent bonds

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Presentation Transcript

“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


“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 (VSEPR) (hybridisation)


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


Formal charges

Section 2.3


Section 2.4

Section 2.5-2.6


Section 10.5

Section 14.1 (no need to do Molecular orbitals – top off p 485)


Section 15.3, 15.5, 15.7



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


(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 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

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



Stability if more forms



Inductive effects



Alternate p bonds



Alternate p bonds and

4n+2 electrons



p bonds & C-H bonds

Two examples of reactive intermediates