Organic Chemistry Lecture 2 PDF

Dr: Haneen Mohamed Lecture two (Chemical bonding and hybridization) Dr: Aya Elmatary  HUE Organic Chemistry-1 1- Valence Bond (VB) Theory and Orbital Hybridization The valence bond theory would predict that forms 2 covalent bonds. However, CH2,...

Dr: Haneen Mohamed Lecture two (Chemical bonding and hybridization) Dr: Aya Elmatary  HUE Organic Chemistry-1 1- Valence Bond (VB) Theory and Orbital Hybridization The valence bond theory would predict that forms 2 covalent bonds. However, CH2, methylene is a very reactive molecule, can’t exist outside of a molecular system. Orbital hybridization Definition: Mixing of 2 or more non equivalent atomic orbitals (e.g. s and p) to form a new set of hybrid orbitals (How?) (The first step in hybridization is the excitation of 1 or more electrons) Number of hybrid orbitals: is equal to number of pure atomic orbitals used in hybridization process 2 ‫ مع‬S ‫واحد من‬ Shape: Hybrid orbitals have totally different shape from original atomic orbitals. ‫مع‬S ‫واحد من‬ P ‫واحد من‬ SP ‫هيكون‬  HUE Organic Chemistry-1 SP3 hybridization CH4&CH3CH3 Structure of Methane sp3 Orbitals on C overlap with 1s orbitals on 4 H atoms to form four identical C- H bonds Each C–H bond has a strength of 436 kJ/mol and length of 109 pm Bond angle: each H–C–H is 109.5°, the tetrahedral angle SP3 = ‫ روابط سيجما‬4 SP3= ‫روابط سيجما‬3 SP=‫ رابطه سيجما‬2 Dr. Haneen Mohamed 01019132155  HUE Organic Chemistry-1 Valance-Shell Electron-Pair Repulsion (VSEPR) Model lone-pair electrons affect shape and bond angle VSEPR model can be used to predict the bond angles in hybridized compounds Outer Shell of valence electrons surrounds an atom NH3 : bond angle is 107.3° not 109.5° Why? The unshared pair of electrons on the nitrogen H2O: bond angle is 104.5° not 109.5°. Distortion is greater, why? The 2 unshared pair of electrons on the oxygen H2S: bond angle is 92° not 109.5°. Why? The 2 unshared pair of electrons on the sulfur…. But bond angle distortion is greater than H2O, why? Difference in electronegativity between O & S atoms (O > S) Structure of Ethane Two C’s bond to each other by σ overlap of an sp3 orbital from each Three sp3 orbitals on each C overlap with H 1s orbitals to form six C–H bonds All bond angles of ethane are tetrahedral Dr. Haneen Mohamed 01019132155  HUE Organic Chemistry-1 SP2 hybridization Ethylene, Ethene, CH2=CH2 sp2 hybrid orbitals: 2s orbital combines with two 2p orbitals, giving 3 orbitals (spp = sp2). This results in a double bond. sp2 orbitals are in a plane with 120°angles, trigonal planar. Remaining p orbital is perpendicular to the plane. Two sp2-hybridized orbitals overlap to form a sigma (σ) bond p orbitals overlap side-to-side to formation a pi (π) bond sp2–sp2 σ bond and 2p–2p π- bond result in sharing four electrons and formation of C-C double bond H atoms form σ bonds with 4 sp2 orbitals C–C double bond in ethylene shorter and stronger than single bond in ethane Dr. Haneen Mohamed 01019132155  HUE Organic Chemistry-1 SP hybridization Acetylene, Ethyne, CHΞCH sp orbitals are linear, 180° apart on x-axis 2 sp hybrid orbitals from each C form sp–sp σ bond pz orbitals from each C form a pz–pz π bond by sideways overlap and py orbitals overlap similarly. Sharing of 6 electrons forms C≡C 2 sp orbitals form σ bonds with 2 H Dr. Haneen Mohamed 01019132155  HUE Organic Chemistry-1 Rule for determining hybridization of any atom lone-pair electrons and hybridization NH3 = 3 bonds + 1 lone-pair electrons So, it’s SP3 H2o = 2 bonds + 2 lone-pair electrons So, it’s SP3 H2S = 2 bonds + 2 lone-pair electrons So, it’s SP3 The rule says that for any atom: N° of hybridized orbitals( steric number) = N° of attached atoms + N° of LPs Dr. Haneen Mohamed 01019132155  HUE Organic Chemistry-1 Differences between the different kinds of hybridization Differences between sigma (σ) and pi (π) Mos Dr. Haneen Mohamed 01019132155  HUE Organic Chemistry-1 Oxygen atomic number is 8 so Bonding electrons are? Nonbonding electrons are? Lone pairs of electrons are?  HUE Organic Chemistry-1 NH3, type of hybridization…molecular geometry… bond… Sp- linear- 120 Sp2- trigonal pyramidal -180 Sp3- trigonal pyramidal -107.3 In sp2 hybridization, number of remaining non hybrid p orbitals is 1 2 3 Molecular geometry of water molecule is... Bent Tetrahedral Trigonal pyramidal