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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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slide1

“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

slide2

“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)

slide3

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

slide4

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

slide5

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

slide6

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

slide7

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

slide8

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
slide9

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