SD Shape of Molecules: VSEPR Valence Shell Electron Pair Repulsion O O O 109.5 109.5 O 109.5 O 109.5 109.5 C4 Methane C Rotate 90 CCW C C C C 1
3D Shape of Molecules: Molecular Orbital (MO) Theory M.O. s are described by mathematical equations Aufbau Principle: Fill lowest energy orbital first Pauli exclusion principle: pair spins of electrons und s Rule: Fill degenerate orbitals Start with Atomic Orbitals (A.O. s) (or hybridized A.O. s) Overlap end-to-end or 1s-to-end gives a sigma (σ) bond Overlap of unhybridized p orbitals gives a pi (π) bond node σ sigma star (antibonding M.O., LUMO) 1s + 1s energy σ sigma (bonding M.O., OMO) e e 2 The Orbital dot diagrams are from: http://www.meta-synthesis.com/webbook/39_diatomics/diatomics.html
Orbitals and Bonding: Methane Chemists have proposed that atoms like carbon do not use pure s and pure p orbitals in forming bonds. Instead, atoms use a set of new orbitals called hybrid orbitals. ybridization is the combination of two or more atomic orbitals to form the same number of hybrid orbitals, each having the same shape and energy. 3
Shape and Orientation of sp 3 ybrid Orbitals The mixing of a spherical 2s orbital and three dumbbell shaped 2p orbitals together produces four hybrid orbitals, each having one large lobe and one small lobe (Figure 1.8). The four hybrid orbitals are oriented towards the corners of a tetrahedron, and form four equivalent bonds. 4
Bonding Using sp 3 ybrid Orbitals Each bond a sigma bond (σ) in C 4 is formed by overlap of an sp 3 hybrid orbital of carbon with a 1s orbital of hydrogen. These four bonds point to the corners of a tetrahedron. σ [1s,C sp 3] 5
ybridization and Bonding in Organic Molecules 6
ybridization and Bonding in Organic Molecules Making a model of ethane illustrates one additional feature about its structure. Rotation occurs around the central C C σ bond. 7
ybridization and Bonding in Double Bonded Molecules Each carbon is trigonal and planar. Each carbon is sp 2 hybridized 8
ybridization and Bonding in Organic Molecules σ [1s,C sp 2] π [Cp,C p ] Z Z σ [Csp 2,C 2 sp ] 9
ybridization and Bonding in Organic Molecules Unlike the C C bond in ethane, rotation about the C C double bond in ethylene is restricted. It can only occur if the π bond first breaks and then reforms, a process that requires considerable energy. 10
ybridization and Bonding in Organic Molecules 11
ybridization and Bonding in Organic Molecules σ [1s,C sp ] σ [Csp,C sp ] π [Cp,C p ] Z Z π [Cp,C p ] y y 12
and Draw 3D Model of C 3 CN 13
Summary of Covalent Bonding Organic Compounds Important Slide: Copy this one down tetrahedral trigonal planer linear Note: groups include pairs of nonbonding electrons 14
Me O O Me O O Me Me O 3 C 3 C O C 3 C 3 Me Me Me O O Me O N 2 O O O Ingenol O (+)-Discodermolide 15
Bond Length and Bond Strength As the number of electrons between two nuclei increases, bonds become shorter and stronger. Thus, triple bonds are shorter and stronger than double bonds, which are shorter and stronger than single bonds. 16
Structure and Bonding: Summary 17
Bond Length and Bond Strength 18
Electronegativity and Bond Polarity Electronegativity is a measure of an atom s attraction for electrons in a bond. 19
Electronegativity and Bond Polarity Electronegativity values are used as a guideline to indicate whether the electrons in a bond are equally shared or unequally shared between two atoms. When electrons are equally shared, the bond is nonpolar. When differences in electronegativity result in unequal sharing of electrons, the bond is polar, and is said to have a separation of charge or a dipole. A carbon carbon bond is nonpolar. The same is true whenever two different atoms having similar electronegativities are bonded together. C bonds are considered to be nonpolar because the 20 electronegativity difference between C and is small.
Electronegativity and Bond Polarity Bonding between atoms of different electronegativity values results in unequal sharing of electrons. Example: In the C O bond, the electrons are pulled away from C (2.5) toward O (3.4), the element of higher electronegativity. The bond is polar, or polar covalent. The bond is said to have dipole; that is, separation of charge. δ + means the indicated atom is electron deficient. δ - means the indicated atom is electron rich. The direction of polarity in a bond is indicated by an arrow with the head of the arrow pointing towards the more electronegative element. The tail of the arrow, with a perpendicular line drawn through it, is drawn at the less electronegative element. 21
Polarity of Molecules Use the following two-step procedure to determine if a molecule has a net dipole: 1. Use electronegativity differences to identify all of the polar bonds and the directions of the bond dipoles. 2. Determine the geometry around individual atoms by counting groups, and decide if individual dipoles cancel or reinforce each other in space. Electrostatic potential plot of C 3 Cl 22
Structure and Bonding Polarity of Molecules A polar molecule has either one polar bond, or two or more bond dipoles that reinforce each other. An example is water: A nonpolar molecule has either no polar bonds, or two or more bond dipoles that cancel. An example is carbon dioxide: 23