Chap 9B - Chem 210 / Chapter 9B Valence Bond Theory Chapter...

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Unformatted text preview: Chem 210 / Chapter 9B Valence Bond Theory Chapter 9.4 – 9.6 Bonding Theories: Valence Bond Theory Valence Bond Theory (VBT) Orbital overlap and spin pairing in 3 diatomic molecules. and spin in Hydrogen, H2 The bond in H2 is is the overlap of 1s overlap 1s (H#1) and (H#2). Theory to explain covalent bonding. Basic idea: “Covalent bonds are formed by the overlap of atomic orbitals, overlap one from each of the two bonding atoms.” 9.4 Covalent Bonding and Orbital Overlap 9.5 Hybrid orbitals 9.6 Multiple Bonds Hydrogen fluoride, HF The bond in HF is the overlap overlap of the 1s (H) and the 2p (F). 1s 2p Two wave functions are in in phase so the amplitude so increases between the nuclei. The Central Themes of VB Theory overlapping A set of overlapping orbitals has a maximum of set two two electrons that must have opposite spins. The The greater the orbital overlap, the stronger overlap (more stable) the bond. bond The The valence atomic orbitals in a molecule are different different from those in isolated atoms. • A hybridization of atomic orbitals may occur so hybridization that the molecule has the observed shape shape (which is predictable by the VSEPR theory). VSEPR Orbital Hybridization Mixing Mixing of two or more atomic orbitals to form a new set of hybrid orbitals with new shape / orientation. 1. Two or more atomic orbitals (nonequivalent, (nonequivalent, such such as s ↔ p) mix to yield hybrid orbitals of very different shape from the original atomic orbitals. Fluorine, F2 The bond in F2 is the overlap is overlap of 2p(F#1) & 2p(F#2). of 2p(F#1) Hybrid Orbitals ↔ Molecular Shape Key Points The number of hybrid orbitals obtained equals the number numbe equals of atomic orbitals mixed. The type of hybrid orbitals obtained depends on the depends atomic orbitals mixed. The set of hybrid orbitals determines molecular shape. set molecular Types of Hybrid Orbitals sp sp2 sp3 sp3d sp3d2 2. Number of hybrid orbitals is equal to number of original atomic orbitals used in the hybridization process. Movie on hybridization http://www.chem.ufl.edu/~myers/chm2045/shapes.htm The sp sp two hybrid orbitals sp & sp two atomic orbitals: 2s & 2p The sp sp Hybridization of orbitals mix characteristics of each atomic orbital. hybrid orbitals Energy hybrid orbitals in gaseous BeCl2 (continued). sp Hybrid Orbitals in BeCl2 sp BeCl Valence orbitals undergo sp sp hybridization in Be so to make two Be sp orbitals at 180o to overlap with the p orbital in each Cl atom. orbital box diagrams with orbital contours in gaseous BeCl2. two hybrid orbitals sp & sp Each covalent bond: Be-Cl is made by the overlap of: 3p (Cl) & sp (Be). overlap 3p (Cl) F09 / Wang / UM-SJTU JI paired in 2s “promotion” for overlap / bonding slightly higher energy. unpaired in sp This explains why BeCl2 is is linear, with 180o bond angle. 1 Chem 210 / Chapter 9B Valence Bond Theory The sp2 Each Each bond is made by the overlap of sp2 (B) ↔ 2p(F). 3 electrons in 2s & 2p unpaired in 3 equal sp2 orbitals. energy of sp2 lower than 2p, higher than 2s. The sp3 sp sp2 Hybrid Orbitals in BF3 BF hybrid orbitals in BF3. • Valence orbitals undergo sp2 sp hybridization in B so to make 2 orbitals at 120o to three three sp 120 overlap with the p orbital in each F atom. • Molecular shape is planar triangle planar on on the plane defined by the three 2 orbitals. sp sp The sp3 sp hybrid orbitals in CH4. in Four atomic orbitals: One s orbital Three p orbitals Mix (hybridize) to produce Four hybridized orbitals of sp3 Each has the same shape They spread in tetrahedral directions in space. sp3 orbital hybridization The sp3 hybrid orbitals in NH3. in hybrid orbitals in H2O. 2s22p2 one p orbital empty. Promotion All electrons unpaired to occupy 4 equal sp2 orbitals; energy between 2s and 2p. 4 bonds made each by the bonds overlap of a sp3 (C) and 1s (H). sp 1s Accounts Accounts for the tetrahedral tetrahedral geometry of the molecule. sp3 Hybrid Orbitals in water & ammonia 1. Where is the lone pair located? 2. Point out differences between these diagrams and those from CH4. The sp3d Oxygen undergoes sp3 hybridization sp to yield 4 sp3 orbitals. 2 of the 4 is each occupied by a lone lone pair. 2 covalent bonds are made by the covalent overlap of sp3 (O) and 1s (H). sp 1s (H). hybrid orbitals in PCl5 (and SF4 & ClF3.) in The sp3d2 hybrid orbitals in SF6. in PCl5 promotion Lone pairs exert stronger repulsion than bonding pairs. Bond angles in H2O and NH3 are Bond less less than the normal tetrahedral angles: 109.5o. F09 / Wang / UM-SJTU JI promotion SF4 SF4 ClF3 2 Chem 210 / Chapter 9B Valence Bond Theory VBT: orbital hybridization Hybrid Orbitals When s and one p orbital mix sp sp hybridization 2 orbitals at 180° (linear) When s and two p orbitals mix sp2 hybridization sp 3 orbitals at 120° (trigonal planar) When s and three p orbitals mix sp3 hybridization sp 4 orbitals at 109.5° (tetrahedral) When s, three p orbitals and one d orbital mix sp3d hybridization sp 5 orbitals at 120° and 90° (trigonal bipyramidal) bipyramidal) When s, three p and two d orbitals mix sp3d2 hybridization sp 6 orbitals at 90° (octahedral) The conceptual steps from molecular formula to the molecular to hybrid hybrid orbitals used in bonding. used Step 1 Molecular formula Step 2 Lewis structure Step 3 Molecular shape and e- group arrangement Hybrid orbitals Hybrid Orbitals Ethane, CH3CH3 Ethane, CH3-CH3 • Four single bonds around each carbon •Single bonds are called σ (sigma) bonds. • Tetrahedral geometry around each carbon C linear 2 orbitals orbitals sp sp hybrid trigonal planar 3 orbitals sp2 hybrid orbitals sp tetrahedral 4 orbitals sp3 hybrid orbitals sp trigonal bipyramidal 5 orbitals orbitals sp3d hybrid sp octahedral 6 orbitals sp3d2 hybrid orbitals sp • Each Carbon has sp3 hybridized orbitals! sp The σ bonds in ethane(C2H6). both C are sp3 hybridized s-sp3 overlaps to σ bonds SAMPLE PROBLEM 1(a) Use partial orbital diagrams to describe mixing of the atomic orbitals of the central atom leads to hybrid orbitals in each of the following: (a) Methanol, CH3OH The groups around C are arranged as a tetrahedron. H H CO HH sp3-sp3 overlap to form a σ bond 1s orbital of H sp3 hybrids Each C-H bond is made by the overlap of sp3 (C) and 1s (H). C The C-C bond in the middle is made by the overlap of sp3 (1st C) and sp3 (2nd C) . All overlaps are of the σ (sigma) type, in which Overlapping occurs between the two nuclei. between SAMPLE PROBLEM 1(b) Use partial orbital diagrams to describe mixing of the atomic orbitals of the central atom leads to hybrid orbitals: (b) Sulfur tetrafluoride, SF4 tetrafluoride, SF4 has a seesaw shape with 4 bonding & 1 nonbonding epairs. F FS F F O also has a tetrahedral arrangement with 2 nonbonding e- pairs. 3d relatively even distribution of electron density over all σ bonds. 2p single C atom 2s sp3 hybridized C atom 2p sp3 hybridized O atom 3p sp3d 2s single O atom 3s F09 / Wang / UM-SJTU JI 3d S atom hybridized S atom 3 Chem 210 / Chapter 9B Valence Bond Theory Multiple Bonds = 1σ + 1 or 2π All Single bonds ← σ bonds → σ-Bond: electron density lies on the axis between Two types of Overlap: pi (π) bond vs. vs. Sigma (σ) bond Multiple Bonds = 1σ + 1 or 2π π-Bonds: electron density lies above and below the plane of the nuclei. SideSide-ways overlap the nuclei. Pi overlap: zero electron density on the nuclear axis Sigma overlap: right on the axis endend-on overlap Compare and contrast the two types of orbital overlaps in VBT: The sigma (σ) type The Pi (π) type 1st bond formed occurs after sigma bond The 2nd in a double bond The 2nd and 3rd in a triple bond Ethene (ethylene), CH2=CH2 • trigonal planer geometry • 3 single bonds Overlap region: cross the line between 2 nuclei and π bonds in ethylene (C2H4). in 2 σ (sigma) bonds around each C Need 3 hybrid orbitals sp2 hybridization 1 C-to-C π (pi) bond to- weaker less stable p overlap -π Not on the line Stronger More stable • The σ overlap in one position - σ Explain why “π electrons” are more reactive than “σ electrons.” π electron density Restricted rotation of π-bonded molecules in C2H2Cl2 (as well as C2H4). in “ “ “ “ “ more π electrons” are more reactive than “σ electrons” because they are more “exposed” to other chemical species, which may approach from the top or bottom of the top plane of ethene molecule. “ “ “ “ F09 / Wang / UM-SJTU JI Ethyne (acetylene), CHΞCH • Linear geometry π electron cloud” in this diagram is found both above and below the plane of the atoms. 2 sigma bonds around each carbon sp hybridization • 2 C-C pi bonds CIS TRANS These are two different compounds – They can interconvert only by first breaking the pi bond! breaking pi bond! 4 Chem 210 / Chapter 9B Valence Bond Theory The σ and π bonds in acetylene (C2H2). in Electron density and bond order. VBT Diagram of Acetylene, H-CΞC-H overlap in one position - σ p overlap - π Each carbon Each carbon in C2H2 makes two sp sp hybrid orbitals by mixing one s and by The orbitals one p orbital. The sp orbitals are then used to make two sigma bonds. bonds Two Two 2p orbitals are left alone in each are carbon, which overlaps to make two which pi bonds. pi bonds SAMPLE PROBLEM 2 ANSWER VBT explanation of ethyne (acetylene) 1. There are: 2 sigma bonds made by sp (C) overlap with 1s(H), and 1 sigma bond made by sp(1st C) overlap with sp(2nd C) 2.Each carbon in ethyne has two p orbitals left over (not hybridized). These p orbitals overlap to make two pi (π) bonds between the two two carbon atoms. more 3.The pi electrons are more reactive than the sigma electrons, because they are more “exposed”. A View of the π bonds in benzene, C6H6 H H H All 6 p orbitals overlap to orbitals make 3 pi bonds shared pi by all 6 Carbon atoms. Carbon The pi electrons are delocalized, resulting delocalized, resulting in a very stable structure! H σ bonds Best represented by benzene (C6H6) • a six carbon ring • three alternating single & double bonds • electrons in π bonds spread over the entire molecule • exists as average, resonance structure resonance O C H3 C CH3 π bond Another example of Resonance structures & delocalized electrons: O3. H H sp3 hybridized sp2 sp2 sp3 O hybridized O sp2 H H C H H sp2 sp3 C C sp3 2 2 HHHH sp C sp C sp3 H C 3 sp2 sp 3 H sp3 sp3 H hybridized sp H sp3 Aromatic hydrocarbons 6e in the 3 pi bonds are shared equally among all 6 Carbon atoms. H H SAMPLE PROBLEM 2 1. Identify the sigma bonds in ethyne by specifying the atomic orbitals that are overlapping to form each one. 2. Identify the pi bonds in ethyne. How many are there? 3. Are the pi electrons in ethyne more or less reactive than reactive the sigma electrons? Why? SAMPLE PROBLEM 3 Describe the types of bonds and orbitals in acetone, (CH3)2CO. Delocalized π Bonding in C6H6 All C atoms have sp2 hybridization H Electron density between the two nuclei (related increases bond to bond strength) increases with bond order. Why then is it easier for the C≡C bond in alkynes and the C=C bond in alkenes to react chemically than react the C-C bond in in alkanes? The reactivity of C=C and C≡C bonds is based on the fact that pi bonds are more exposed and weaker than sigma bonds, not on the overall bond strengths, as shown in the above graphs. H H H F09 / Wang / UM-SJTU JI 5 Chem 210 / Chapter 9B Valence Bond Theory A Third Example of Delocalized π Bonding in CO3-2 SAMPLE PROBLEM 4 SAMPLE PROBLEM 5 Explain the bonding in C3H4 by the VBT theory. 1. Draw Lewis structure for the polyatomic ion. 2. How many sigma bonds and pi bonds are present in this ion? 3. What orbitals overlap to make the sigma bonds? F09 / Wang / UM-SJTU JI 6 ...
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This note was uploaded on 07/30/2011 for the course CHEM 210 taught by Professor Zhang during the Spring '09 term at Shanghai Jiao Tong University.

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