Organic Chemistry, 5th Edition L.G. Wade. Jr Chapter 7 Structure and Synthesis of alkenes Jo blackburn Richland college, dallas TX Dallas County Community College District c 2003. Prentice hall
Chapter 7 Structure and Synthesis of Alkenes Jo Blackburn Richland College, Dallas, TX Dallas County Community College District © 2003, Prentice Hall Organic Chemistry, 5th Edition L. G. Wade, Jr
Functional Group Pi bond is the functional group More reactive than sigma bond Bond dissociation energies >C=C BDE 146 kcal/mol >C-C BDE -83 kcal/mol >Pi bond 63 kcal/mol Chapter 7
Chapter 7 2 Functional Group • Pi bond is the functional group. • More reactive than sigma bond. • Bond dissociation energies: ➢C=C BDE 146 kcal/mol ➢C-C BDE -83 kcal/mol ➢Pi bond 63 kcal/mol =>
Orbital Description Sigma bonds around c are sp hybridized Angles are approximately 120 degrees No nonbonding electrons Molecule is planar around the double bond Pi bond is formed by the sideways overlap of parallel p orbitals perpendicular to the plane of the molecule Chapter 7
Chapter 7 3 Orbital Description • Sigma bonds around C are sp2 hybridized. • Angles are approximately 120 degrees. • No nonbonding electrons. • Molecule is planar around the double bond. • Pi bond is formed by the sideways overlap of parallel p orbitals perpendicular to the plane of the molecule. =>
Bond Lengths and Angles H 1.33A H 1.54A H HH C)116.6° H 121 H H H ethylene ethane Hybrid orbitals have more s character Pi overlap brings carbon atoms closer Bond angle with pi orbitals increases Angle c=C-His121.7° Angle H-C-H is116.6° => Chapter 7
Chapter 7 4 Bond Lengths and Angles • Hybrid orbitals have more s character. • Pi overlap brings carbon atoms closer. • Bond angle with pi orbitals increases. ➢Angle C=C-H is 121.7 ➢Angle H-C-H is 116. 6 =>
Pi Bond Sideways overlap of parallel p orbitals No rotation is possible without breaking the pi bond 63 kcal/mole) Cis isomer cannot become trans without a chemical reaction occurring CH H H H . CH HoC CH3 H CH3 H3C=C CIS no overlap with the ends perpendicular trans Chapter 7
Chapter 7 5 Pi Bond • Sideways overlap of parallel p orbitals. • No rotation is possible without breaking the pi bond (63 kcal/mole). • Cis isomer cannot become trans without a chemical reaction occurring. =>