Organic hemistry, 5 th Edition L. G. Wade, Jr. hapter 7 Structure and Synthesis of Alkenes Jo Blackburn Richland ollege, Dallas, TX Dallas ounty ommunity ollege District 2003, Prentice all Functional Group Pi bond is the functional group. More reactive than sigma bond. Bond dissociation energies: = BDE 146 kcal/mol - BDE -83 kcal/mol Pi bond 63 kcal/mol hapter 7 2 Orbital Description Bond Lengths and Angles Sigma bonds around are sp 2 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. hapter 7 3 ybrid orbitals have more s character. Pi overlap brings carbon atoms closer. Bond angle with pi orbitals increases. Angle =- is 121.7 Angle -- is 116. 6 hapter 7 4 Pi Bond Sideways overlap of parallel p orbitals. No rotation is possible without breaking the pi bond (63 kcal/mole). is isomer cannot become trans without a chemical reaction occurring. hapter 7 5 Elements of Unsaturation A saturated hydrocarbon: n 2n+2 Each pi bond (and each ring) decreases the number of s by two. Each of these is an element of unsaturation. To calculate: find number of s if it were saturated, subtract the actual number of s, then divide by 2. hapter 7 6 1
Propose a Structure: for 5 8 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. hapter 7 7 eteroatoms alogens take the place of hydrogens, so add their number to the number of s. Oxygen doesn t change the : ratio, so ignore oxygen in the formula. Nitrogen is trivalent, so it acts like half a carbon. N hapter 7 8 Structure for 6 7 N? Since nitrogen counts as half a carbon, the number of s if saturated is 2(6.5) + 2 = 15. Number of missing s is 15 7 = 8. Elements of unsaturation is 8 2 = 4. IUPA 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. hapter 7 9 hapter 7 10 Name These Alkenes Alkene Substituents 2 2 3 1-butene 3 3 3 2-methyl-2-butene 23 3 2-sec-butyl-1,3-cyclohexadiene = 2 methylene (methylidene) - = 2 vinyl (ethenyl) - 2 - = 2 allyl (2-propenyl) 3 3-methylcyclopentene 3-n-propyl-1-heptene hapter 7 11 Name: hapter 7 12 2
ommon Names Usually used for small molecules. Examples: 2 2 ethylene 2 3 propylene 3 2 3 isobutylene is-trans Isomerism Similar groups on same side of double bond, alkene is cis. Similar groups on opposite sides of double bond, alkene is trans. ycloalkenes are assumed to be cis. Trans cycloalkenes are not stable unless the ring has at least 8 carbons. hapter 7 13 hapter 7 14 Name these: 3 3 2 trans-2-pentene cis-1,2-dibromoethene hapter 7 15 E-Z Nomenclature Use the ahn-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). hapter 7 16 Example, E-Z ommercial Uses: Ethylene 1 1 2 l 3 l 1 3 2 2 2 1 2 2Z 5E (2Z, 5E)-3,7-dichloro-2,5-octadiene hapter 7 17 hapter 7 18 3
ommercial Uses: Propylene Other Polymers hapter 7 19 hapter 7 20 Stability of Alkenes Measured by heat of hydrogenation: Alkene + 2 Alkane + energy More heat released, higher energy alkene. Substituent Effects More substituted alkenes are more stable. 2 = 2 < R-= 2 < R-=-R < R-=R 2 < R 2 =R 2 unsub. < monosub. < disub. < trisub. < tetra sub. Alkyl group stabilizes the double bond. Alkene less sterically hindered. 30.3 kcal 27.6 kcal hapter 7 21 hapter 7 22 Disubstituted Isomers Stability: cis < geminal < trans isomer Less stable isomer is higher in energy, has a more exothermic heat of hydrogenation. is-2-butene 3 3 Isobutylene Trans-2-butene ( 3 ) 2 = 2 3 3 28.6 kcal 28.0 kcal 27.6 kcal hapter 7 23 ycloalkene Stability is 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. hapter 7 24 4
edt 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: Unstable. Violates edt s rule Stable. Double bond in 8-membered ring. hapter 7 25 ysical Properties Low boiling points, increasing with mass. anched 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. hapter 7 26 Polarity Examples Alkene Synthesis Overview 3 3 3 3 cis-2-butene, bp 4 trans-2-butene, bp 1 µ = 0.33 D µ = 0 E2 dehydrohalogenation (-X) E1 dehydrohalogenation (-X) Dehalogenation of vicinal dibromides (-X 2 ) Dehydration of alcohols (- 2 O) hapter 7 27 hapter 7 28 Removing X via E2 Some Bulky Bases Strong base abstracts + 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. 3 _ 3 O 3 tert-butoxide ( 3 ) 2 N ( 3 ) 2 diisopropylamine ( 3 2 ) 3 N : triethylamine 3 N 3 2,6-dimethylpyridine hapter 7 29 hapter 7 30 5
ofmann Product Bulky bases abstract the least hindered + Least substituted alkene is major product. 3 3 2 3 3 2 _ 3 2 O 3 2 O 3 3 3 _ 3 ( 3 3 ) 3 O 3 2 O 3 3 2 3 71% 29% 28% 3 2 3 72% hapter 7 31 3 E2: Diastereomers 3 Stereospecific reaction: (S, R) produces only trans product, (R, R) produces only cis. 3 3 hapter 7 32 E2: yclohexanes E2: Vicinal Dibromides Remove 2 from adjacent carbons. omines must be anti-coplanar (E2). Use NaI in acetone, or Zn in acetic acid. Leaving groups must be trans diaxial. I - 3 3 3 3 hapter 7 33 hapter 7 34 Removing X via E1 Secondary or tertiary halides Formation of carbocation intermediate Weak nucleophile Usually have substitution products too hapter 7 35 Dehydration of Alcohols Reversible reaction Use concentrated sulfuric or phosphoric acid, remove low-boiling alkene as it forms. Protonation of O converts it to a good leaving group, O arbocation intermediate, like E1 Protic solvent removes adjacent + hapter 7 36 6
O O Dehydration Mechanism O O S O O 2 O: O SO 4 hapter 7 37 _ 3 O + Industrial Methods atalytic cracking of petroleum Long-chain alkane is heated with a catalyst to produce an alkene and shorter alkane. omplex mixtures are produced. Dehydrogenation of alkanes ydrogen ( 2 ) is removed with heat, catalyst. Reaction is endothermic, but entropy-favored. Neither method is suitable for lab synthesis hapter 7 38 End of hapter 7 hapter 7 39 7