UNIT 6 Theories of Covalent Bonding and Intro to Organic Chemistry VSEPR Theory, Molecular Shapes, and Valence Bond Theory Electron Domains Electron domains are the regions in the molecules where it is most likely to find electrons. • For a bond (single, double, or triple), the electron domain is between the two atoms in the bond and consists of all the electrons involved in the bond. • For nonbonding pairs of electrons, the domain is the nonbonding pair and is centered on a single atom. Valence Electrons • Valence electrons are the electrons in the outermost unfilled shell of the atom or ion. These are usually s and p electrons, but can be d electrons. • For the main group elements, the number of valence electrons is the last digit of the group number. • Knowing the number of valence electrons allows us to draw Lewis symbols for the elements and Lewis structures for compounds. Lewis Structures • Lewis structures are very helpful in studying bonding because they show only the valence electrons of the atoms or ions. The Octet Rule • Atoms tend to gain, lose, or share electrons until they are surrounded by eight valence electrons (the s and p subshells are full). We use this rule to draw Lewis structures for compounds. Lewis Structures of Covalent Compounds Follow these steps in order. 1. Decide which atoms are bonded. 2. Count all valence electrons. 3. Put two electrons in each bond. 4. Complete the octets of the atoms attached to the central atom except H, which takes a duet. 5. Put any remaining electrons on the central atom. 6. If the central atom has less than an octet, form double or triple bonds. Putting Formal Charges on Lewis Structures The formal charge of any atom in a compound or ion may be calculated using the following: FC = # of valence electrons – number of bonds – number of nonbonding electrons FC of O = 6-2-4 = 0 FC of O = 6-1-6 = -1 FC of O = 6-3-2 = +1 Resonance Structures Three completely equivalent Lewis structures can be drawn for the nitrate ion, NO3-. Reality is a blend of the three. There are no double bonds in the nitrate ion, but each bond is more stable than just a single bond. All three structures are resonance structures. From Lewis Structure to Electron Domain Geometry via VSEPR • The Lewis structure shows the covalent bonds (solid lines) and the nonbonding electrons (dots) that are present in a compound. • This allows the identification of the electron domains of the molecule. Domains are the regions in the molecules where it is most likely to find electrons. • Electron domain geometry gives bond angle and hybridization. VSEPR Theory Valence Shell Electron Pair Repulsion – qualitative explanation of molecular shapes • Electrons in a domain are subject to electrostatic repulsion from the electrons in the other domains. The domains will orient themselves so as to minimize this repulsion. • The orientation of these domains is a function of the number of domains around the central atom and is one of several simple geometric figures. Bond Angles when a Nonbonding Electron Pair or Multiple Bond is Present • Nonbonding electron pairs take up more space than bonding pairs and have the effect of squeezing (decreasing) the bond angles among the atoms. • Multiple bonds exert more repulsion than single bonds, and have the same effect on bond angle as the nonbonding electron pair. Bond Angles for Atoms in Organic Molecules: Effect of Nonbonding Electron Pairs Example: C in an alkane 4 bonding domains tetrahedral geometry Bond angles 109.5° Example: O in an ether 4 electron domains tetrahedral geometry Example: N in an amine 4 electron domains tetrahedral geometry Bond angles on N are <109.5° because the epair takes up extra space. Bond angles on O are <109.5° because the two e- pairs take up extra space. Some Electron Domain Geometries for C in Organic Molecules carbocations H-C≡C-H Valence Bond Theory Lewis Structures – Explain bonding as a sharing of electron pairs and geometry through VSEPR. VB Theory - A more quantitative approach to explaining bonding. Here bonds are explained by the overlap of orbitals on the two atoms in the bond. ENERGY Orbital Energy Diagram 4s 4p 3s 3p 2s 1s 2p 3d The periodic table may also be used to determine the electron configuration of the elements. Valence Bond Theory Bonds occur from the overlap of atomic orbitals. Cl: 1s22s22p63s23p5 H: 1s1 The bond in H-H is formed by the overlap of the two H 1s orbitals. The bond in H-Cl is formed by the overlap of the H 1s orbital with the Cl 3p orbital. How would you describe the Cl-Cl bond? Bonds formed by end-to-end overlap are called sigma (σ)bonds. Valence Bond Theory – Hybrid Orbitals Describing bonds as the overlap of s and p orbitals explains some geometries, but certainly not the tetrahedral geometries (e.g. H2O). What orbitals are overlapping here? Valence Bond Theory – Hybrid Orbitals Since the orbitals (s, p, d, f, etc.) are all mathematical solutions to the Schrödinger wave equation, it is true that linear combinations of these orbitals are also solutions to the Schrödinger wave equation. In other words, we may mix s, p, and d orbitals to make new, hybrid orbitals that are also valid. sp Hybrid Orbitals Orbital energy Consider HC≡CH. VSEPR says the molecule is linear. How can hybrid orbitals explain the geometry? 2p 2s C ground state (the orbitals of C are what determine the geometry of HC≡CH.) 1s Two e- are available for bonding, but the geometry is wrong. sp Hybrid Orbitals Orbital energy Consider HC≡CH. VSEPR says the molecule is linear. How can hybrid orbitals explain the geometry? 2p Energy is used to promote one 2s e- to a 2p orbital. 2s A 2s orbital and a 2p orbital mix to make two new orbitals. The other two 2p orbitals are unchanged. 1s sp Hybrid Orbitals Orbital energy Consider HC≡CH. VSEPR says the molecule is linear. How can hybrid orbitals explain the geometry? sp 2p The 2s orbital and one 2p orbital mix to form two sp hybrid orbitals. The energy of mixing is more than paid back when the C-H and C≡C bonds are formed. 1s sp Hybrid Orbitals C C C C C Hybrid orbitals have a small lobe and a large one. The large lobe allows more overlap and, therefore, the formation stronger bonds. The energy needed to make the hybrid orbital is paid back, with interest, in the formation of stronger bonds. sp2 Hybrid Orbitals Consider H2C=CH2. VSEPR says the molecule is linear. How can hybrid orbitals explain the geometry? One 2p orbital remains unhybridized. Orbital energy 2p sp2 Mixing the three orbitals gives three sp2 hybrid orbitals with the same energy. 1s What orbitals overlap to form each of the C-H bonds? sp2 Hybrid Orbitals sp3 Hybrid Orbitals Consider CH4. How can hybrid orbitals explain this tetrahedral geometry? Orbital energy sp3 Mixing the four orbitals gives four sp3 hybrid orbitals with the same energy. 1s The VSEPR geometry tells you the hybridization: Tetrahedral VSEPR geometry sp3 hybrid orbitals. sp3 Hybrid Orbitals What orbitals overlap to form each of the C-H bonds in methane? sp3d Hybrid Orbitals PCl5 has the shape of a trigonal bipyramid. P ground state: 3s 3p 3d Energy is used to promote 3s e- to 3d: Hybridize: sp3d 3d Trigonal bipyramid VSEPR geometry sp3d hybrid orbitals. sp3d2 Hybrid Orbitals SF6 has the shape of an octahedron. S ground state: 3s 3p 3d Energy is used to promote 3s and a 3p e- to 3d: Hybridize: sp3d2 3d Octahedral VSEPR geometry sp3d2 hybrid orbitals. Use Lewis structures to get the electron domain geometry, and that will lead to the bond angles and hybridization. # of edomains 2 3 4 5 6 e- domain geometry linear trigonal planar tetrahedral trigonal bipyramidal octahedral bond angle hybridization 180° 120° sp sp2 109.5° 90°,120°,180° sp3 sp3d 90°,180° sp3d2 Hybridization and Bond Angles in Larger Molecules Just identify the geometry/hybridization around each atom in succession. sp2, trigonal planar geometry, bond angle is >120° (due to double bond) H :O: .. | || H—C—C—O—H acetic acid | ¨ 3, bent geometry, bond angle is sp H <109.5° sp3, tetrahedral geometry, bond angle is 109.5° What orbitals overlap to form each of the bonds in acetic acid? Hybridization and Bond Angles in Larger Molecules H | H—C—C≡C—H propyne | sp, linear geometry, 180° H sp3, tetrahedral geometry, 109.5° Valence Bond Descriptions H | H—C—C≡C—H | H What orbitals overlap to form each of the bonds in propyne? • The three C-H bonds shown at the left are each formed by the overlap of a H 1s orbital with one of the four C sp3 hybrid orbitals. • The C-C bond is formed by the overlap of the remaining sp3 orbital on the first C with one of the sp orbitals on the second C. • The C≡C triple bond is formed by: 1) the overlap of C sp orbitals (sigma bond), 2) the overlap of C 2py orbitals (pi bond), and 3) the overlap of C 2pz orbitals (pi bond). • The final C-H bond is formed by the overlap of the C sp orbital with a H 1s orbital.