Reactivity

• C-C and C-H s bonds are unreactive and most reactions occur at C=C multiple bonds or C-X bonds
• C-H next to a functional group may show reactivity
• Filled orbitals tend to be lower in energy than empty orbitals - orbital interactions can lead to a lower energy state
• Reactions usually proceed through the donation of a pair of electrons, excluding radical reactions
• HOMO - highest occupied molecular orbitals. Nucleophile donates electrons from its HOMO
• LUMO - lowest unoccupied molecular orbital. Electrophile accepts electrons into its LUMO
• Nucleophiles have a pair of electrons in a high energy orbital or a negative charge
• Electrophiles have an empty atomic orbital or a positive charge
• Curly arrows show the movement of the two electrons that comprise the bond
• For a C=C, the p bond is formed using pz on a C and another C - when this p bond breaks, the C takes both electrons into its px and uses them to bond to H+. The C is left with an empty pz and a positive charge, it rehybridises to sp3 to s bond to H
• An atom can use its lone pair (nucleophile) to bond to another with an empty orbital (electrophile)
• Reaction profile with I + CH3Br gives a transition state with a part broken C-Br and part formed I-C which is at the highest point on the energy pathway
• The reactive intermediate of a reaction exists briefly during a reaction and is rarely detected directly - C+ species is a carbenium ion intermediate
• For a two step reaction, there are two TS and two Eact. The first Eact is larger than the second as if the reaction have enough energy to get over the first, then is has more than enough energy to get over the second - step is the RLS
• Dienes contain two C=C
• If C=C in a diene are alternating, the diene is conjugated
• There is a small amount of overlap between the adjacent pz orbital on C2 and C3, so there is a partial double bond between C2 and C3 in a C4 diene
• The carbenium ion is stable:

  • three pz orbitals form an MO with 2 electrons that extend across 3 atoms
  • causes the positive charges to be delocalised over the two end C
  • single positive charge is delocalised and easier for the molecule to bear
• Delocalised structures are best explained by MO theory and are shown through resonance forms
• A carbenium ion next to a O or N is stabilised, so its energy is lowered by conjugation. If O is re-hybridised from sp3 to sp2, a p bond between O and C is possible - this spreads the positive charge over both atoms stabilising the ion