Chapter 7 Structure and Synthesis of Alkenes Introduction ydrocarbon with carbon-carbon double bonds Sometimes called olefins, oil-forming gas Planar Pi bond is the functional group. More reactive than sigma bond (more exposed). Bond dissociation energies: C=C BDE 611 kj/mol C-C BDE -347 kj/mol Pi bond 264 kj/mol Chapter 7-Alkenes 1 Slide 7-2 1
Orbital Description Sigma bonds around C 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. end A B side side C D end top face A B D bottom face C Chapter 7-Alkenes 1 Slide 7-3 Bond Lengths and Angles ybrid orbitals have more s character. Pi overlap brings carbon atoms closer. Bond angle with pi orbital increases. Angle C=C- is 121.7 Angle -C- is 116. 6 Chapter 7-Alkenes 1 Slide 7-4 2
Pi Bond Sideways overlap of parallel p orbitals. No rotation is possible without breaking the pi bond (264 kj/mole). Cis isomer cannot become trans without a chemical reaction occurring. Chapter 7-Alkenes 1 Slide 7-5 Elements of Unsaturation A saturated hydrocarbon: C 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. Chapter 7-Alkenes 1 Slide 7-6 3
Propose a Structure: for C 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. Chapter 7-Alkenes 1 Slide 7-7 eteroatoms alogens take the place of hydrogens, so add their number to the number of s. Oxygen doesn t change the C: ratio, so ignore oxygen in the formula. Nitrogen is trivalent, so it acts like half a carbon. C C N C Chapter 7-Alkenes 1 Slide 7-8 4
Structure for C 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. Chapter 7-Alkenes 1 Slide 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 index number. In a ring, the double bond is assumed to be between carbon 1 and carbon 2. Place numbers in front of suffix to indicate where double bond is. Chapter 7-Alkenes 1 Slide 7-10 5
Name These Alkenes C 2 C C 2 C 3 C 3 1-butene but-1-ene C C 3 C C 3 2-methyl-2-butene 2-methylbut-2-ene CC 2 C 3 3 C 2-sec-butyl-1,3-cyclohexadiene 2-sec-butylcyclohexa-1,3-diene C 3 3-methylcyclopentene 3-n-propyl-1-heptene 3-n-propylhept-1-ene Chapter 7-Alkenes 1 Slide 7-11 Alkene Substituents = C 2 methylene (methylidene) - C = C 2 vinyl (ethenyl) - C 2 - C = C 2 allyl (2-propenyl) Name: Chapter 7-Alkenes 1 Slide 7-12 6
Common Names Usually used for small molecules. Examples: C 2 C 2 ethylene C 2 C C 3 propylene C 3 C 2 C C 3 isobutylene Chapter 7-Alkenes 1 Slide 7-13 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 for rings less than 8. Trans cycloalkenes are not stable unless the ring has at least 8 carbons (and even here trans is rare). Chapter 7-Alkenes 1 Slide 7-14 7
Name these: C 3 Br C C C C C 3 C 2 Br trans-2-pentene trans-pent-2-ene cis-1,2-dibromoethene Br Cl C C Br What about this? Chapter 7-Alkenes 1 Slide 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-Alkenes 1 Slide 7-16 8
Example, E-Z 1 3 C C C 2 2Z 1 2 Cl Cl 1 C C 3 C C C 2 2 1 2 5E 3,7-dichloro-(2Z, 5E)-2,5-octadiene 3,7-dichloro-(2Z, 5E)-octa-2,5-diene Chapter 7-Alkenes 1 Slide 7-17 Commercial Uses: Ethylene Chapter 7-Alkenes 1 Slide 7-18 9
Commercial Uses: Propylene Chapter 7-Alkenes 1 Slide 7-19 Other Polymers Chapter 7-Alkenes 1 Slide 7-20 10
Stability of Alkenes Measured by heat of hydrogenation: Alkene + 2 Alkane + energy More heat released, higher energy alkene. Chapter 7-Alkenes 1 Slide 7-21 Substituent Effects More substituted alkenes are more stable. 2 C=C 2 < R-C=C 2 < R-C=C-R < R-C=CR 2 < R 2 C=CR 2 unsub. < monosub. < disub. < trisub. < tetra sub. Cis vs Trans vs terminal Alkyl group stabilizes the double bond. Alkene less sterically hindered. Chapter 7-Alkenes 1 Slide 7-22 11
Disubstituted Isomers Stability: cis < geminal < trans isomer Less stable isomer is higher in energy, has a more exothermic heat of hydrogenation. Cis-2-butene C 3 C C C 3-120 kj Isobutylene (C 3 ) 2 C=C 2-117 kj Trans-2-butene C C C 3-116 kj C 3 Chapter 7-Alkenes 1 Slide 7-23 Relative Stabilities Chapter 7-Alkenes 1 Slide 7-24 12
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 ( floppy rings). Chapter 7-Alkenes 1 Slide 7-25 Bredt s Rule (Bridgehead Alkenes) 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 Bredt s rule Stable. Double bond in 8-membered ring. Chapter 7-Alkenes 1 Slide 7-26 13
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 dipole-dipole interactions occur. Alkyl groups are electron-donating toward the pi bond, so may have a small dipole moment. Chapter 7-Alkenes 1 Slide 7-27 Polarity Examples 3 C C 3 C C cis-2-butene, bp 4 C C 3 C C 3 C trans-2-butene, bp 1 C µ = 0.33 D µ = 0 Chapter 7-Alkenes 1 Slide 7-28 14
Alkene Synthesis: Overview Simple preparation involves Elimination reactions: E2 dehydrohalogenation (-X) E1 dehydrohalogenation (-X) Dehalogenation of vicinal dibromides (-X 2 ) Dehydration of alcohols (- 2 O) Chapter 7-Alkenes 1 Slide 7-29 Removing X via E2 Strong base abstracts + as X - leaves from the adjacent carbon. Tertiary and hindered secondary alkyl halides give good yields (S N 2 is retarded). Use a bulky base if the alkyl halide usually forms substitution products (such as isopropoxide or t-butoxide). Zaitsev product preferred if possible Chapter 7-Alkenes 1 Slide 7-30 15
E2 Mechanism Lewis base approaches molecule; removal of is more facile than substitution: Nu 3 C Br C 3 Rate bond making = rate bond breaking being removed and X leaving must be periplanar; Anti periplanar preferred to syn periplanar Chapter 7-Alkenes 1 Slide 7-31 Some Bulky Bases 3 C C 3 C O C 3 _ tert-butoxide C(C 3 ) 2 N C(C 3 ) 2 diisopropylamine 3 C N C 3 2,6-dimethylpyridine (C 3 C 2 ) 3 N : triethylamine Chapter 7-Alkenes 1 Slide 7-32 16
ofmann Product Bulky bases abstract the least hindered + Least substituted alkene is major product. Chapter 7-Alkenes 1 Slide 7-33 E2: Diastereomers Ph Br C 3 Br C 3 Ph Ph Ph Ph Ph C 3 Br Ph Ph C 3 => Stereospecific reaction: (S, R) produces only trans product, (R, R) produces only cis. Chapter 7-Alkenes 1 Slide 7-34 17
E2: Cyclohexanes Leaving groups must be trans diaxial. => Chapter 7-Alkenes 1 Slide 7-35 E2: Vicinal Dibromides Remove Br 2 from adjacent carbons. Bromines must be anti-coplanar (E2). Use NaI in acetone, or Zn in acetic acid. I - C 3 Br Br C 3 3 C C C C 3 => Chapter 7-Alkenes 1 Slide 7-36 18
Removing X via E1 Secondary or tertiary halides. Formation of carbocation intermediate. May get rearrangement. Weak nucleophile. Usually have substitution products, too. => Chapter 7-Alkenes 1 Slide 7-37 Dehydration of Alcohols Reversible reaction. Use concentrated sulfuric or phosphoric acid, remove lowboiling alkene as it forms. Protonation of O converts it to a good leaving group, O. Carbocation intermediate, like E1. Protic solvent removes adjacent +. => Chapter 7-Alkenes 1 Slide 7-38 19
Dehydration Mechanism => Chapter 7-Alkenes 1 Slide 7-39 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 ydrogen ( 2 ) is removed with heat, catalyst. Reaction is endothermic, but entropy-favored. Neither method is suitable for lab synthesis. => Chapter 7-Alkenes 1 Slide 7-40 20
End of Chapter 7 Chapter 7-Alkenes 1 Slide 7-41 21