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 Functional Group • Pi bond is the functional group. • More reactive than sigma bond. • Bond dissociation energies: C=C BDE C-C BDE Pi bond 146 kcal/mol -83 kcal/mol 63 kcal/mol => Chapter 7 2 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 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 Chapter 7 => 4 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. => Chapter 7 5 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. => Chapter 7 6 Propose a Structure: for C5H8 • 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 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. H H H C C N C H H H H Chapter 7 => 8 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. => Chapter 7 9 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. => Chapter 7 10 Name These Alkenes CH2 CH CH2 CH3 1-butene CHCH2CH3 CH3 C CH CH3 CH3 H3C 2-sec-butyl-1,3-cyclohexadiene 2-methyl-2-butene CH3 3-n-propyl-1-heptene 3-methylcyclopentene => Chapter 7 11 Alkene Substituents = CH2 methylene (methylidene) - CH = CH2 vinyl (ethenyl) - CH2 - CH = CH2 allyl (2-propenyl) => Name: Chapter 7 12 Common Names • Usually used for small molecules. • Examples: CH3 CH2 CH2 ethylene CH2 CH CH3 propylene Chapter 7 CH2 C CH3 => isobutylene 13 Cis-trans Isomerism • Similar groups on same side of double bond, alkene is cis. • Similar groups on opposite sides of double bond, alkene is trans. • Cycloalkenes are assumed to be cis. • Trans cycloalkenes are not stable unless the ring has at least 8 carbons. => Chapter 7 14 Name these: H CH3 Br C C CH3CH2 Br C C H H trans-2-pentene H cis-1,2-dibromoethene => Chapter 7 15 E-Z Nomenclature • Use the Cahn-Ingold-Prelog rules to assign priorities to groups attached to each carbon in the double bond. • If high priority groups are on the same side, the name is Z (for zusammen). • If high priority groups are on opposite sides, the name is E (for entgegen). => Chapter 7 16 Example, E-Z 1 H3C 1 Cl C C H 2 2Z Cl 1 CH CH3 2 H CH2 2 1 C C H 2 5E (2Z, 5E)-3,7-dichloro-2,5-octadiene => Chapter 7 17 Commercial Uses: Ethylene => Chapter 7 18 Commercial Uses: Propylene => Chapter 7 19 Other Polymers => Chapter 7 20 Stability of Alkenes • Measured by heat of hydrogenation: Alkene + H2 Alkane + energy • More heat released, higher energy alkene. 30.3 kcal 27.6 kcal => Chapter 7 21 Substituent Effects • More substituted alkenes are more stable. H2C=CH2 < R-CH=CH2 < R-CH=CH-R < R-CH=CR2 < R2C=CR2 unsub. < monosub. < disub. < trisub. < tetra sub. • Alkyl group stabilizes the double bond. • Alkene less sterically hindered. => Chapter 7 22 Disubstituted Isomers • Stability: cis < geminal < trans isomer • Less stable isomer is higher in energy, has a more exothermic heat of hydrogenation. Cis-2-butene CH3 C C H Isobutylene Trans-2-butene CH3 H (CH3)2C=CH2 H 28.6 kcal C C CH3 Chapter 7 CH3 28.0 kcal 27.6 kcal H => 23 Cycloalkene Stability • Cis isomer more stable than trans. • Small rings have additional ring strain. • Must have at least 8 carbons to form a stable trans double bond. • For cyclodecene (and larger) trans double bond is almost as stable as the cis. => Chapter 7 24 Bredt’s Rule • A bridged bicyclic compound cannot have a double bond at a bridgehead position unless one of the rings contains at least eight carbon atoms. • Examples: Stable. Double bond in 8-membered ring. Unstable. Violates Bredt’s rule => Chapter 7 25 Physical Properties • • • • Low boiling points, increasing with mass. Branched alkenes have lower boiling points. Less dense than water. Slightly polar Pi bond is polarizable, so instantaneous dipoledipole interactions occur. Alkyl groups are electron-donating toward the pi bond, so may have a small dipole moment. => Chapter 7 26 Polarity Examples H3C CH3 H C C H CH3 C C H H3C cis-2-butene, bp 4 °C H trans-2-butene, bp 1 °C = 0.33 D =0 => Chapter 7 27 Alkene Synthesis Overview • • • • E2 dehydrohalogenation (-HX) E1 dehydrohalogenation (-HX) Dehalogenation of vicinal dibromides (-X2) Dehydration of alcohols (-H2O) => Chapter 7 28 Removing HX via E2 • Strong base abstracts H+ as X- leaves from the adjacent carbon. • Tertiary and hindered secondary alkyl halides give good yields. • Use a bulky base if the alkyl halide usually forms substitution products. => Chapter 7 29 Some Bulky Bases CH3 H3C _ CH(CH3)2 C O N CH3 H tert-butoxide CH(CH3)2 H3C diisopropylamine N CH3 2,6-dimethylpyridine (CH3CH2)3N : triethylamine => Chapter 7 30 Hofmann Product • Bulky bases abstract the least hindered H+ • Least substituted alkene is major product. H CH3 CH3 C C CH2 H Br H _ CH3CH2O CH3 CH3CH2 H3C C C C C CH3CH2OH CH3 H CH3 C C CH2 H Br H _ (CH 3)3CO CH3CH2OH H H3C 71% H CH3 H 29% CH3 CH3CH2 H3C C C C C H CH3 28% Chapter 7 H => H H3C 72% 31 E2: Diastereomers Ph Br H H CH3 Ph H Br CH3 H H H Ph Ph Ph Ph CH3 H Ph Ph CH3 Br => Stereospecific reaction: (S, R) produces only trans product, (R, R) produces only cis. Chapter 7 32 E2: Cyclohexanes Leaving groups must be trans diaxial. => Chapter 7 33 E2: Vicinal Dibromides • Remove Br2 from adjacent carbons. • Bromines must be anti-coplanar (E2). • Use NaI in acetone, or Zn in acetic acid. - I Br CH3 H CH3 Br H H3C H Chapter 7 C C CH3 H => 34 Removing HX via E1 • • • • Secondary or tertiary halides Formation of carbocation intermediate Weak nucleophile Usually have substitution products too => Chapter 7 35 Dehydration of Alcohols • Reversible reaction • Use concentrated sulfuric or phosphoric acid, remove low-boiling alkene as it forms. • Protonation of OH converts it to a good leaving group, HOH • Carbocation intermediate, like E1 • Protic solvent removes adjacent H+ =>36 Chapter 7 Dehydration Mechanism H H O H C C H O H O H O S C C O H _ HSO4 O H H O H H C C C C Chapter 7 H2O: C C => + H3O 37 Industrial Methods • Catalytic cracking of petroleum Long-chain alkane is heated with a catalyst to produce an alkene and shorter alkane. Complex mixtures are produced. • Dehydrogenation of alkanes Hydrogen (H2) is removed with heat, catalyst. Reaction is endothermic, but entropy-favored. • Neither method is suitable for lab synthesis => Chapter 7 38 End of Chapter 7 Chapter 7 39