Mary J. Bojan Chem 110 1 Covalent Bonding What is covalent bonding? Covalent Bonds: overlap of orbitals σ-bond π-bond Molecular Orbitals Hybrid Orbital Formation Shapes of Hybrid Orbitals Hybrid orbitals and Multiple Bonds resonance structuresMary J. Bojan Chem 110 2 Molecular Orbitals • Lewis structures: accounting for bonding and lone-pair electrons (where are the electrons?) • VSEPR: Electron-pair structure, spatial distribution of electrons (3D) How are bonds made? We know electron distribution in atoms: atomic orbitals: (s, p, d …) What is the electron distribution in molecules? Two models: Valence Bond Theory Valence orbitals on one atom overlap with valence orbitals on another atom: this overlap of orbitals is a covalent bond. Molecular Orbital Theory Covered in Chem 112Mary J. Bojan Chem 110 3 Covalent Bonding H + H → H2 H2 molecule Covalent bonding: H (1s) H (1s)Mary J. Bojan Chem 110 4 H + H → H2 Two forces operating: • • balance of forces bond length (0.74 Å for H2)Mary J. Bojan Chem 110 5 Bond Types σ-bond • results from • electron density is π-bond • results from • electron density is • pp Two p-orbitals Examples: s-s s-pMary J. Bojan Chem 110 6 Bonding in CH4 Using only unpaired subshell electrons: Expect: The molecule would not have an octet on carbon. Carbon ground-state: (1s2)2s22p2 1s of H 2p of CMary J. Bojan Chem 110 7 Orbital Hybridization 1. Promote electrons on C 2. hybridization Four atomic orbitals (2s + 3 × 2p) mix to form four hybrid orbitals (4 × sp3) 3. Bond formation: _______________________ Form 4 C⎯H bonds by overlapping each hybrid sp3 orbital with an 1s orbital of hydrogen. σ-bond formation The new bonds are 109o apart. One of the four bonds formed by overlap of an sp3 orbital with a hydrogen 1s orbital 1s 2s 2p1s 2s 2p1s1s 2s 2pshake wellsp3Mary J. Bojan Chem 110 8 Orbital Hybridization NOTE: start with four atomic orbitals s px py pz end up with four hybrid orbitals 4 sp3 The notation means that each hybrid is composed of 1/4 s and 3/4 p orbitals. Hybrid orbitals: combinations of atomic orbitals (on one atom). better for bonding (more directed)Mary J. Bojan Chem 110 9 sp3 Hybrid Orbitals Four atomic orbitals mix to form four hybrid orbitals 1 x s + 3 x p = 4 x sp3Mary J. Bojan Chem 110 10 sp and sp2 Hybrid Orbitals 1 x s + 1 x p 2 x sp Three atomic orbitals mix to form three hybrid orbitals Two atomic orbitals mix to form two hybrid orbitals 1 x s + 2 x p 3 x sp2Mary J. Bojan Chem 110 11 Summary Two atomic orbitals mix to form two hybrid orbitals 1 x s + 1 x p 2 x sp Three atomic orbitals mix to form three hybrid orbitals 1 x s + 2 x p 3 x sp2 Four atomic orbitals mix to form four hybrid orbitals 1 x s + 3 x p 4 x sp3 Five atomic orbitals mix to form five hybrid orbitals 1 x s + 3 x p +1 x d 5 x sp3d Six atomic orbitals mix to form six hybrid orbitals 1 x s + 3 x p +2 x d 6 x sp3d2 Problem: Can’t use atomic orbitals to describe bonding in molecules Solution: make molecular orbitals by mixing atomic orbitals (call them hybrid orbitals) Use VSEPR to determine shape of hybrid orbitals: the electron pairs will get as far from each other as possible. Each hybrid orbital can accommodate 1 pair of electrons.Mary J. Bojan Chem 110 12 Summary of hybridization types Number of electron pairs Atomic orbitals used Hybrid type formed Electron-pair geometry Examples 2 s, p two sp linear BeF2, HgCl2 3 s, p, p three sp2 trigonal planar BF3, SO3, CO32− 4 s, p, p, p four sp3 tetrahedral CH4, NH3, H2O, NH4+ 5 s, p, p, p, d five sp3d trigonal bipyramidal PF5, SF4, BrF3 6 s, p, p, p, d, d six sp3d2 octahedral SF6, ClF5, XeF4, PF6− The hybridization scheme can be deduced from the electron-pair geometry of the molecule.Mary J. Bojan Chem 110 13 Multiple Bonds ethylene: shape about C: hybrid orbitals on C _____ bond angles _______ One C⎯C and two C⎯H bonds (on each carbon) are formed using sp2 orbitals on carbons. ( σ- bonds) C CHHHHOne s and two p atomic orbitals combine to form 3 sp2 hybrid orbitals.Mary J. Bojan Chem 110 14 Multiple bonds These p-orbitals can overlap, sideways: π-bondMary J. Bojan Chem 110 15 Orbital Theory of Bonding explains: • Why rotation about double bond does not occur • Why double bonds occur frequently with C, N, and O but not with larger moleculesMary J. Bojan Chem 110 16 Delocalized Orbitals Molecules with resonance structures have delocalized π bonding e- pair geometry: hybrid orbitals on N and O are Difference between localized and delocalized π bonding Delocalized bonding brings added stability to a molecule. N and O have singly occupied p-orbitalsMary J. Bojan Chem 110 Reactivity of Hydrocarbons Same reaction: hydrocarbon + Br2 Colorless red 17 CH3ALKANE Heptane + Br2 ⎯→ ALKENE (and ALKYNES) 2-pentene + Br2 ⎯→ AROMATIC toluene + Br2 ⎯→Mary J. Bojan Chem 110 18 Stability of aromatic hydrocarbons Alkene + Br2 reacts readily Aromatic + Br2: no reaction • π bonds of alkenes are very reactive toward addition. (π bonds in alkynes even more so.) • π-bonds in benzene are NOT reactive due to the extra stability of delocalized π systemMary J. Bojan Chem 110 19 Summary of Covalent bonding 1. Draw Lewis Structure 2. Use VSEPR to determine shape e- pair geometry molecular geometry 3. What hybrid orbitals are involved in bonding? Determined by electron pair geometry. (Know the shapes of the hybrid orbitals.) 4. Is the molecule polar? Determined by molecular geometry. Remember: Each single bond = covalent bond = σ bond A covalent bond forms when orbitals overlap. σ-bond: head-on overlap π-bond: sidewise overlapMary J. Bojan Chem 110 20 Determine the hybrid orbitals on the nitrogen in angle 1 and the C in angle 2. Angle #1 Angle #2 A. sp3 sp2 B. sp sp2 C. sp2 sp3 D. sp2 sp E. sp3
View Full Document
Unlocking...