PF1009 2024 Orbital Hybridisation and Bonding PDF

Pharmaceutical Chemistry Orbital Hybridisation and Bonding Dr. J.J. Keating 1  H2 – σ Bond Each H atom consists of a 1s electron in a spherical 1s orbital. H2 – its 1s electron pair occupies a single orbital that s...

Pharmaceutical Chemistry Orbital Hybridisation and Bonding Dr. J.J. Keating 1  H2 – σ Bond Each H atom consists of a 1s electron in a spherical 1s orbital. H2 – its 1s electron pair occupies a single orbital that spreads over both atoms (bonding orbital). Resulting sausage shape distribution of electrons – σ bond. Merging of two orbitals – orbital overlap. H2 hydrogen gas 2  Hybridisation – the C-atom C atom (ground state) – [He]2s22px12py12pz0. It appears that 4 bonds cannot be created to C as there are only two half-filled p orbitals. Promote a 2s electron into a higher energy 2p orbital. C atom (promoted state) – [He]2s12px12py12pz1. Promotion allowed if the energy investment is recovered by bond formation. C atom can now form 4 C–H bonds instead of 2. For C promotion, electron is transferred into an empty 2p orbital – requires little energy as the electron experiences little repulsion. At this stage, if 4 1s H orbitals are overlapped with the promoted C atom, two different σ bonds will be form of differing energies and bond lengths. Experimentally, the four C–H bonds of methane are equivalent. 3  Hybridisation – sp3 The 2s and three 2p orbitals of the promoted state of C hybridise to form 4 identical sp3 hybrid orbitals which point towards the vertices of a tetrahedron. C atom (promoted state) – [He]2s12px12py12pz1 → C atom (hybridised state) – [He](sp3)1(sp3)1(sp3)1(sp3)1 orbital diagram carbon 4  Hybridisation – sp3 : Carbon hybridise carbon 75% p-orbital, 25% s-orbital character [He]2s22px12py12pz0 [He](sp3)1(sp3)1(sp3)1(sp3)1 5  Hybridisation – sp3 : Carbon hybridise carbon 75% p-orbital, 25% s-orbital character 6  Hybridisation – sp3 : Carbon hybridise carbon 75% p-orbital, 25% s-orbital character CH4 Methane AX4 - tetrahedral 7  Hybridisation – sp3 : Nitrogen hybridise nitrogen [He]2s22px12py12pz1 [He](sp3)2(sp3)1(sp3)1(sp3)1 8  Hybridisation – sp3 : Nitrogen hybridise nitrogen E A B B B NH3 ammonia AB3E – trigonal pyramidal 9  Hybridisation – sp3 : Oxygen hybridise oxygen [He]2s22px22py12pz1 [He](sp3)2(sp3)2(sp3)1(sp3)1 10  Hybridisation – sp3 : Oxygen hybridise oxygen E A B E B H2O water AB2E2 – angular, bent 11  Hybridisation No. orbitals mixed Pattern Hybridisation Unhybridised type orbitals (C, N, O) 2 (1 x s, 1 x p) Linear sp 2xp 3 (1 x s, 2 x p) Trigonal planar sp2 1xp 4 (1 x s, 3 x p) Tetrahedral sp3 0 5 (1 x s, 3 x p, 1 x d) Trigonal sp3d - bipyrimadal 6 (1 x s, 3 x p, 2 x d) Octahedral sp3d2 - When > 4 electron pairs present in the valence shell of the central atom, d-orbitals required (PCl5). sp3 hybridised PCl5 phosphorous pentachloride AB5 – trigonal bipyramidal (5 x sp3d hybrid orbitals) B B B A B B 12  Hybridisation – sp2 Ethene (CH2=CH2) – each C atom exhibits trigonal planar geometry Molecule as a whole is planar C atom (hybridised state) – [He](sp2)1(sp2)1(sp2)12pz1 C2H4 ethene C atoms are sp2 hybridised, with one 2p unhybridised orbital (AB3)2 – trigonal planar π-bond σ-bond sp2 hybrid orbital – 67% p-orbital, 33% s-orbital character 13  Hybridisation – sp2 – Aspirin O OH carboxylic acid O acetyl O C9H8O4 aspirin acetylsalicylic acid OH O = sp 2 hybridised atoms 14  Hybridisation – sp Ethyne (CH≡CH) – each C atom exhibits linear geometry Molecule as a whole is linear C2H2 ethyne C atom (hybridised state) – [He](sp)1(sp)12py12pz1 (AB2)2 – linear C atoms are sp hybridised, with two 2p unhybridised orbitals per C atom π-bond σ-bond π-bond sp hybrid orbital – 50% p-orbital,50% s-orbital character 15  Hybridisation – sp3, sp2, sp – Terbinafine t-butyl methyl trans-alkene methylene N alkyne naphthalene tertiary amine N = sp hybridised = sp hybridised 2 = sp hybridised 3 16  Molecular Orbital Theory Exceptions Diborane – B2H6 12 valence electrons (2 x (3 x B), 6 x (1 x H)). Needs at least 7 bonds (14 electrons) to bind the 8 atoms. Too few electrons for it to be assigned a Lewis structure. O2 Paramagnetic. Property of unpaired electrons – contradicts Lewis structure which requires all electrons to be paired. Molecular orbital – orbital that spreads all over the atoms in a molecule. Bonding / anti-bonding orbitals 17  Bonding – Summary Bonds: Ionic Covalent Coordinate σ, π Single, double, triple Polar, nonpolar Orbitals: s-, p-, d-, f- orbitals sp, sp2, sp3, sp3d, sp3d2 hybridised orbitals Molecular orbitals, bonding and anti-bonding orbitals Electrons bonding, non-bonding, lone pair, valence, radical, σ, π Hybridisation Shape: Linear, angular, trigonal pyramidal, trigonal bipyramidal, trigonal planar, tetrahedral, octahedral, square planar VSEPR theory Lewis diagrams 18