Química Orgânica I Ciências Farmacêuticas Bioquímica Química AFB QO I 2007/08 1 idrocarbonetos insaturados Terpenes: antiviral, antiseptic, anti-inflammatory. OLEFINS oil-forming gas AFB QO I 2007/08 2 1
first algal pheromone (Müller DG et al., Science 1971, 171, 815). 117 y AFB QO I 2007/08 3 Adaptado de: Organic Chemistry, 6th Edition; L. G. Wade, Jr. Organic Chemistry, 6 th edition; McMurry s AFB QO I 2007/08 4 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 AFB QO I 2007/08 5 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 AFB QO I 2007/08 6 3
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. AFB QO I 2007/08 7 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. AFB QO I 2007/08 8 4
6.2 Degree of Unsaturation AFB QO I 2007/08 9 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 also one element of unsaturation. A triple bond is two elements of unsaturation. AFB QO I 2007/08 10 5
Propose a Structure: for C 5 8 Isopreno n saturado, C n 2n+2 ; neste caso é 12 12-8 = 4 elementos de insaturação 4/2 =2 http://physics.nist.gov/physrefdata/molspec/ydro/tml/tables/c58.html AFB QO I 2007/08 11 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 AFB QO I 2007/08 12 6
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. Element of unsaturation is 8 2 = 4. e.g. anilina AFB QO I 2007/08 13 Cycloalkene nomenclature 1 C 3 3 C 3 C 4 3 2 2 1 1 2 1-methylcyclohexene 3-methylcyclohexene 4-methylcyclohexene AFB QO I 2007/08 14 7
Name These Alkenes C 2 C C 2 C 3 C 3 1-butene but-1-ene C C C 3 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 AFB QO I 2007/08 15 6-vinyltridecane 4-isopropyldecane 4-vinyloctane AFB QO I 2007/08 16 8
Alkene Substituents = C 2 methylene (methylidene) - C = C 2 vinyl (ethenyl) - C 2 - C = C 2 allyl (2-propenyl) methylenecyclohexane ethenylbenzene Name: AFB QO I 2007/08 17 In a ring, the double bond is assumed to be between carbon 1 and carbon 2. Br (Z)-7-bromo-4-ethyl-3-methylcyclohept-1-ene AFB QO I 2007/08 18 9
Common Names Usually used for small molecules. Examples: C 3 C 2 C 2 ethylene C 2 C C 3 propylene C 2 C C 3 isobutylene AFB QO I 2007/08 19 AFB QO I 2007/08 20 10
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 AFB QO I 2007/08 21 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 AFB QO I 2007/08 22 11
Commercial Uses: Ethylene AFB QO I 2007/08 23 Commercial Uses: Propylene AFB QO I 2007/08 24 12
Other Polymers AFB QO I 2007/08 25 Stability of Alkenes Measured by heat of hydrogenation: Alkene + 2 Alkane + energy More heat released, higher energy alkene. AFB QO I 2007/08 26 13
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. < tetrasub. Alkyl group stabilizes the double bond. Alkene less sterically hindered. AFB QO I 2007/08 27 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 3 C Isobutylene (C 3 ) 2 C=C 2-120 kj -117 kj Trans-2- butene C 3 C C C 3-116 kj AFB QO I 2007/08 28 14
Relative Stabilities AFB QO I 2007/08 29 yperconjugation Electrons in neighboring filled σ orbital stabilize vacant antibonding π orbital net positive interaction Alkyl groups are more stabilizing than AFB QO I 2007/08 30 15
Bond strengths/hybridization effects sp 3 -sp 3 bond is weaker than sp 3 -sp 2, sp 2 - sp 2 AFB QO I 2007/08 31 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. AFB QO I 2007/08 32 16
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: Unstable. Violates Bredt s rule Stable. Double bond in 8-membered ring. AFB QO I 2007/08 33 AFB QO I 2007/08 34 17
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. AFB QO I 2007/08 35 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 AFB QO I 2007/08 36 18
Alkene Synthesis Overview E2 dehydrohalogenation (-X) E1 dehydrohalogenation (-X) Dehalogenation of vicinal dibromides (- X 2 ) Dehydration of alcohols (- 2 O) AFB QO I 2007/08 37 Removing X via E2 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. AFB QO I 2007/08 38 19
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 AFB QO I 2007/08 39 ofmann Product Bulky bases abstract the least hindered + Least substituted alkene is major product. AFB QO I 2007/08 40 20
E2: Diastereomers Ph Br C 3 Ph Br C 3 Ph Ph Ph Ph C 3 Br Ph Ph C 3 Stereospecific reaction: (S, R) produces only trans product, (R, R) produces only cis. AFB QO I 2007/08 41 E2: Cyclohexanes Leaving groups must be trans diaxial. AFB QO I 2007/08 42 21
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 - Br C 3 Br C 3 3 C C C C 3 AFB QO I 2007/08 43 Removing X via E1 Secondary or tertiary halides Formation of carbocation intermediate May get rearrangement Weak nucleophile Usually have substitution products too AFB QO I 2007/08 44 22
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 Carbocation intermediate, like E1 Protic solvent removes adjacent + AFB QO I 2007/08 45 Dehydration Mechanism AFB QO I 2007/08 46 23
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 AFB QO I 2007/08 47 24