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 ©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 2
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 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. => Chapter 7 3
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 1.33A H 1.54A H 116.6° 121.7° 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-His116.6° Chapter 7 4
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. CH3 CH3 CH cis no overlap with the ends perpendicular trans Chapter 7 5
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. =>
Elements of Unsaturation A saturated hydrocarbon:CH2n+2 0 Each pi bond (and each ring)decreases the number of H's by two. Each of these is an element of unsaturation. To calculate:find number of H's if it were saturated,subtract the actual number of H's, then divide by 2. => Chapter 7 6
Chapter 7 6 Elements of Unsaturation • A saturated hydrocarbon: CnH2n+2 • Each pi bond (and each ring) decreases the number of H’s by two. • Each of these is an element of unsaturation. • To calculate: find number of H’s if it were saturated, subtract the actual number of H’s, then divide by 2. =>
Propose a Structure: for CsHs First calculate the number of elements of unsaturation. ·Remember: >A double bond is one element of unsaturation. >A ring is one element of unsaturation. >A triple bond is two elements of unsaturation.= Chapter 7 7
Chapter 7 7 Propose a Structure: • First calculate the number of elements of unsaturation. • Remember: ➢A double bond is one element of unsaturation. ➢A ring is one element of unsaturation. ➢A triple bond is two elements of unsaturation. => for C5H8
Heteroatoms Halogens take the place of hydrogens,so add their number to the number of H's. Oxygen doesn't change the C:H ratio,so ignore oxygen in the formula. Nitrogen is trivalent,so it acts like half a carbon. HH H -C-C-N-C- => HHHH Chapter 7 8
Chapter 7 8 Heteroatoms • Halogens take the place of hydrogens, so add their number to the number of H’s. • Oxygen doesn’t change the C:H ratio, so ignore oxygen in the formula. • Nitrogen is trivalent, so it acts like half a carbon. C H H C H H N C H H H =>
Structure for CH-N? Since nitrogen counts as half a carbon, the number of H's if saturated is 2(6.5)+2=15. Number of missing H's is 15-7=8. ·Elements of unsaturation is8÷2=4. 二> Chapter 7 9
Chapter 7 9 Structure for C6H7N? • Since nitrogen counts as half a carbon, the number of H’s if saturated is 2(6.5) + 2 = 15. • Number of missing H’s is 15 – 7 = 8. • Elements of unsaturation is 8 ÷ 2 = 4. =>
IUPAC Nomenclature Parent is longest chain containing the double bond. -ane changes to -ene.(or -diene,-triene) Number the chain so that the double bond has the lowest possible number. In a ring,the double bond is assumed to be between carbon 1 and carbon 2. 二> Chapter 7 10
Chapter 7 10 IUPAC Nomenclature • Parent is longest chain containing the double bond. • -ane changes to -ene. (or -diene, -triene) • Number the chain so that the double bond has the lowest possible number. • In a ring, the double bond is assumed to be between carbon 1 and carbon 2. =>