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. =>
Elements of Unsaturation A saturated hydrocarbon: C,H2n+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 hs if it were saturated, subtract the actual number of hs then divide by 2 > Chapter 7
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 CsH& 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
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 hs 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 - cCNC > HHH H Chapter 7
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 chan? Since nitrogen counts as half a carbon the number of hs if saturated is 2(6.5)+2=15. Number of missing hs is 15-7=8 ·E| ements of unsaturation is8÷2=4. > Chapter 7
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. =>
UPAC Nomenclature Parent is longest chain containing the double bond Number the chain so that the double e) ane changes to -ene. or-diene, -triene bond has the lowest possible number In a ring, the double bond is assumed to be between carbon 1 and carbon 2 > 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. =>
