RI 2022 Chemical Bonding II v2.0

Raffles Institution Year 5 H2 Chemistry 2022 Lecture Notes 8 - Chemical Bonding II (Hybridisation and R,:sonance) Learning outcomes Students should be able to: (a) describe sp3 hybridisation, as in ethane molecule, sp2 hybridisation, as in ethene and benzene molecules, and sp hybridisation, as in ethyne molecule (b) explain the shapes of, and bond angles in, the ethane, ethene, benzene, and ethyne molecules in relation to cr and 1t carbon-carbon bonds (c) predict the shapes of, and bond angles in, molecules analogous to those specified in (b) (d) interpret, and use the term "delocalisation" associated with organic reactivities. Contents 1. Hybridisation ........................................................................................................................ 1 1.1 sp3 hybridisation of carbon in methane, CH4 ............................................................................................... 2 1.2 sp2 hybridisation of carbon in ethene, C2H4 .................................................................. 3 1.3 sp hybridisation of carbon in ethyne, C2H2 .................................................................... 4 1.4 Effect of Hybridisation on Bond Length and Bond Strength ......................................... 7 2. Resonance .......................................................................................................................... 8 3. Structures of some giant molecular structures and their properties ................................. 12 1 Hybridisation • The theory of covalent bond formation via overlapping of atomic orbitals does not always give good agreement with observations . • Hybridisation is a concept used to explain the observed shapes of molecules ( determined by experimental studies) . • On hybridisation , a set of atomic orbitals are mixed to generate a set of equivalent hybrid orbitals with the same shape and energy. (Note: number of hybrid orbitals formed always equals number of atomic orbitals used for mixing.) • Consider the electron ic configuration of carbon at ground state (1 s2 2s2 2p2). (i) (ii) (iii) To form a normal covalent bond, an orbital containing one electron is required . These orbitals overlap to form a covalent bond. Carbon in its ground state contains only two singly occupied orbitals, thus it should form 2 covalent bonds. However, carbon tends to form 4 covalent bonds . To have four singly occupied orbitals for forming 4 covalent bonds , an electron must first be promoted from the 2s orbital to the empty 2p orbital. 1s 2s 2p C in ground state [!] I 1 11 I C in excited state 1s [I] 2s [I] 2p 11 11 11 I Carbon now has 4 unpaired electrons and can form 4 covalent bonds, but the atomic orbitals do not point in the correct direction for bonding . (iv) For example , CH4 is tetrahedral with H-C-H bond angles of 109.5°, but the p orbitals (Px, py, p,) are perpendicular (at 90°) to one another . (v) To explain the four C-H bonds in methane with H-C-H angles of 109.5°, the hypothetical concept of hybridisation is used.

1.1 sp3 hybridisation of carbon in methane, CH4 • Structural studies have shown that CH4 is tetrahedral with all the C-H bonds being of equal bond lengths and the four H-C-H angles are all 109.5°. This implies that carbon must provide four equivalent orbitals for head-on overlap with the 1 s orbital from each H atom to form four identical C-H cr bonds. The hybrid orbitals are oriented in a tetrahedral geometry, containing a single unpaired electron each. Four equivalent hybrid orbitals for carbon are generated by mixing one 2s orbital with the three 2p orbitals, resulting in the formation of four sp3 hybrid orbitals. s-orbital . ,,, , I' ,' I ' , Px-Orbital Py-orbital hybridisation to fom1 four sp3 hybrid orbitals . ,,, 'I' ,' I \ pz-orbital . ,,, ' I ' ,' I \ : Practice : Draw the hybrid orbitals around , an sp3 hybridised atom. I I I I I I , I ' ' ' I \ I ' I ' L------------------• ' ' ' ' ' ' ' ' ' ' ' ' ' ' ~'- -- -----~ ... ... '... .. .. ............ - ' ' ~------·-------~ ... ... .. : ... ,,. ,,, ', I .,,"' .... ....... The large lobes of the hybrid orbitals point towards the corners of a tetrahedron. Notation: The superscripts for hybrid orbitals correspond to the number of atomic orbitals used to form them. The number "1" is not written as it is understood to be "1" For example: sp3 = s 1 p3 one 2s and three 2p orbitals used to form four sp3 hybrid orbitals. atom in ground state 6C* atom in excited state sC* atom in excited state after energy 1 1 2p---- H 2s H 1s In the ground state of C, there are only 2 unpaired electrons . thus it is able to form only 2 cov alent bonds . before hybridisation hybridisation energy energy 1 1 2p-- ----,,, l------------ 1 1 1 1 sp3------1 ~/' ___ _ _____ _ __ J 2s 1 7s er in the excited state, with 4 unpaired electrons , cannot form 4 equivalent C-H bonds to give a symmetrical tetrahedral molecule. _jL 1s Four equivalent sp3 hybrid orbitals are formed from one 2s and three 2p orbitals . 2

Each of the four sp3 hybrid orbitals of carbon overlaps with th e 1 s orbital of a hydrogen atom to give a tetrahedral methane molecule. H . '~J°g_50 H,,· c"' i H H lone pair in sp3 hybrid orbital sp3 hybridisation results in tetrahedral electron pair geometry. Other examples of molecules where central atom is sp3 hybridised : _£H3CH3, NH3, H20 1.2 sp2 hybridisation in ethene, C2H4 H H • Structural studies have shown that C2H4 is planar with all the C-H bonds being of equal bond lengths and the H-C-H and C-C-H angles are all 120°. '\ /..) C=C 1200 • /v " H 120° H This implies that carbon must provide three equivalent orbitals for head-on overlap with the 1s orbital from each H atom, and with the hybrid orbital of another C atom. The hybrid orbitals are oriented in a trigonal planar geometry, each containing a single unpaired electron. The C-H bonds in C2H4 are single covalent bonds, while the carbon-carbon bond is a double bond, C=C. Each carbon forms three cr bonds (two with two hydrogen atoms and one with a carbon atom) and one n bond (with the carbon atom). Each carbon atom undergoes sp2 hybridisation to generate three equivalent sp2 hybrid orbitals (for forming three cr bonds). An unhybridised 2pz orbital remains (for forming one n bond). z z z z Note: Any two of the _k _h k three p orbitals can be .:,y-y ~ y :~y xV Y.___use_dfor_sp 2 _hybrid_isat___,ion s-orbital p)(Orbital Py-orbital p2-orbital hybridisation to form three equivalent sp2 hybrid orbitals z Pract ice . V . ¾ I \ I\ I\ I \ I \ I \ I \ I \ f \ I • I • I \ / \ ,' ·, / \ y I \ I \ I \ a,' '. ,' \ ,'~\ \ I \ I \ , , , , . X \ I \ I , \ I \ I \ f \ I \ \ . \ , . \ / \ / \ unhybridised ------.1 '---------------------.1 '-------------- ----- The large lobes of the hybrid orbitals point towards the corners of an equilateral triangle. Pz-Orbital Draw the hybrid orbitals around an sp2 hybridised atom. L _____ _ ___ __ _____ _ 3

,c atom in ground state ,c· atom in excited state ,c• atom in excited state after energy 2p-- -- H 2s H 1s In the ground state of C, there are only 2 unpaired electrons, thus it is able to form only 2 covalent bonds. before hybridisation hybridisation energy energy 1 1 1 2p-- ---.,, 2p \____________ 1 1 1 ___ __________ .,,,' sp2 2s H 1s When one 2s electron is promoted to vacant 2p orbital to form C" in excited state that has four singly occupied orbitals to form four covalent bonds. -1L 1s Three equivalent sp2 orbitals are formed from one 2s and two 2p orbi1aJs_ There is also one unhybridised 2p orbital_ Two of the sp2 hybrid orbitals overlap with the 1 s orbitals of two H atoms while the other sp2 hybrid orbital overlaps with the sp2 hybrid orbital of the other C atom. The unhybridised 2pz orbitals of each carbon atom contains an unpaired elect