Structure and bonding

• P-orbitals have a positive half and a negative half, so there is a phase change. Each sub-orbital can hold up to 2e-.

  P-orbitals must be of the same phase to form a bond
• A covalent bond is formed by the overlap of 1s orbitals on different H and sharing of electrons - overlap of the AOs give a MO with 2e- filling this MO
• The anti-bonding MO is formed at higher energy and is usually empty but can exist and be populated with an electron
• Quantum mechanics calculates the bonding MO energy to be 436kJmol-1 lower than the free H 1s orbital energy
• Overlap head-to-head gives a sigma MO
• P-orbitals do not overlap head-to-head and instead prefer a sideways pi orbital
• Each MO is a single orbital
• Lower energy means more stable
• Experimental geometries of C compounds are found to be tetrahedral(Td), trigonal planar (Tg) or linear (Lr)
• Correct C geometry can be obtained from AOs by hybridisation, giving 4 new equivalent hybrid orbitals each with one electron
• Overlap of a C sp3 hybrid with H 1s gives C-H s bond
• sp2 hybridisation is obtained by part hybridisation, the mixing of 2s, 2px, 2py and not 2pz giving 3 new equivalent hybrid orbitals in a planar geometry with one electron in each orbital
• sp2 hybrid has trigonal planar geometry
• For Csp2-Csp2 pi bonds, two pz orbitals are left over so there is a sideways overlap of these unhybridised pz AOs giving C-CpMO/ pi bond containing the last 2e-. This MO is both above and below the plane
• For a C=C, the MOs are formed from sp2 orbitals - the energy levels are overall closer for pz compared to sp2 and are generally higher
• sp hybridisation is obtained by mixing 2s and 2px, not 2py or 2pz, giving two new hybrid orbitals with one electron each
• Sideways overlap of unhybridised pz AOs gives C-C p MO above and below the plane
• Sideways overlap of unhybridised py AO gives C-C p MO in front and behind the plane
• sp3 hybridisation at N shapes shows a tetrahedron
• Hybridisations are populated singly before in pairs - remaining single occupied to hybrid bond to H 1s or C
• Bonding sp3 O to sp3 C gives methanol, methyl amine and fluoromethane
• Bonding sp3 O to sp2 C gives vinyl alcohol, amino ethene and fluoroethene
• sp2 hybridisation of O gives 3 new equivalent hybrid orbitals with 2pz unhybridised. First 4e-s fill each orbital single to being, 5th e- pairs up in an sp2, 6th e- pairs up in another sp2
• Osp2-Csp2 sigma bond is formed from sp2 with single electrons overlapping with Csp2. Unhybridised pz orbitals overlap sideways to O-C p bonding MO
• An electronegative atom will pull a shared electron towards itself so the electron spends more time on the side of the most electronegative atom. The result is a slight-charge on F and a slight positive charge on H - this is shown qualitatively by distorting the orbital
• Hybridisation can be applied to atoms with fewer electrons than C
• sp2 hybridisation is favoured as it keeps the bonding pairs furthest apart, but can re-hybridised to sp3 is necessary
• The atom providing the filled orbital gains a +1 charge, atoms providing the empty orbital gains -1 charge
• Filled species is the donor/nucleophile/lewis base
• Empty species is the acceptor/electrophile/lewis acid