Chapter 3. Alkena dan Alkuna: Nomenklatur dan Reaksinya

1. Chapter 3. Alkena dan Alkuna: Nomenklatur dan Reaksinya Tutik Dwi Wahyuningsih Jurusan Kimia FMIPA UGM 2011

2. Alkena dan Alkuna • Introduction: kegunaan alkena • Struktur alkena • Nomenklatur Alkena & Alkuna • Nomenklatur E/Z • Jenis/tipe ikatan rangkap dua • Reaksi pada Alkena Adisi Substitusi Diels Alder Pemutusan

3. Example : a mixture of  and  bonds, but no triple bonds

4. Commercial Uses: Ethylene =>

5. Commercial Uses: Propylene =>

6. Other Polymers =>

7. 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 =>

8. Alkenes Geometrical isomers are possible since there is no rotation about a C=C  bond. Cis- and trans- isomers possible.

9. Functional Group • Pi bond is the functional group. • More reactive than sigma bond.

10. Orbital Description • Sigma bonds around C are sp2 hybridized. • Angles are approximately 120 degrees. • No nonbonding electrons. • Molecule is planar around the double bond.

11. => 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.

12. 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.=>

13. Name These Alkenes 1-butene 2-sec-butyl-1,3-cyclohexadiene 2-methyl-2-butene 3-n-propyl-1-heptene => 3-methylcyclopentene

14. = CH2 - CH = CH2 - CH2 - CH = CH2 vinyl (ethenyl) allyl (2-propenyl) methylene (methylidene) => Name: Alkene Substituents

15. => Common Names • Usually used for small molecules. • Examples:

16. 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. =>

17. Name these: cis-1,2-dibromoethene => trans-2-pentene

18. 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). =>

19. 1 2 1 1 2 2 1 2 Example, E-Z 2Z 5E (2Z, 5E)-3,7-dichloro-2,5-octadiene =>

20. Definisi • Ikatan rangkap duaterkonjugasi: dipisahkan oleh satu ikatan tunggal.

21. Ikatan rangkap dua terisolasi : dipisahkan oleh dua atau lebih ikatan tunggal. • Ikatan rangkap dua terakumulasi : ikatan rangkap dua berdekatan. • Contoh : 1,2-pentadiena

22. => 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.

23. Alkenes

24. Cis-2-butene 28.6 kcal Isobutylene (CH3)2C=CH2 28.0 kcal Trans-2-butene 27.6 kcal => Disubstituted Isomers • Stability: cis < geminal < trans isomer • Less stable isomer is higher in energy, has a more exothermic heat of hydrogenation.

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 dipole-dipole interactions occur. • Alkyl groups are electron-donating toward the pi bond, so may have a small dipole moment.=>

26. Polarity Examples  = 0.33 D  = 0 =>

27. ADDITION REACTION An addition reaction is one in which the two reactants add together to make the product A + B AB with no other pieces lost or left over.

28. c o n c ELECTROPHILIC ADDITION TO DOUBLE BONDS X C C C C + EX E electrophilic reagent C l conc. C C EXAMPLES: C C + HCl H explained later O H H2SO4 + H2O C C C C H OSO3H . 0 oC H2SO4 + C C C C H

29. Addition Reactions of Alkenes and Alkynes A common addition reaction is hydrogenation: CH3CH=CHCH3 + H2  CH3CH2CH2CH3 Hydrogenation requires high temperatures and pressures as well as the presence of a catalyst (e.g. Ni). Note: hydrogenation forms alkanes from alkenes.

30. Addition Reactions of Alkenes and Alkynes It is possible to cause hydrogen halides and water to add across  bonds: CH2=CH2 + HBr  CH3CH2Br ( a bromide) CH2=CH2 + H2O  CH3CH2OH (analcohol) The addition of water is usually catalysed by H2SO4.

31. Addition Reactions of Alkenes and Alkynes The most dominant reaction for alkenes and alkynes involves the addition of something to the two atoms which form the double bond: Note that the C-C  bond has been replaced by two C-Br  bonds.

32. => Electrophilic Addition • Step 1: Pi electrons attack the electrophile. • Step 2: Nucleophile attacks the carbocation.

33. X => Addition of HX (1) Protonation of double bond yields the most stable carbocation. Positive charge goes to the carbon that was not protonated.

34. => Addition of HX (2)

35. Reaksi Adisi via Intermediet Karbokation Hidrasi Adisi Hidrogen halida

36. THIS IS A REGIOSELECTIVE REACTION <10% >90% major minor One of the possible products is formed in larger amounts than the other one(s). REGIOSELECTIVE Compare Only one of the possible products is formed (100%). REGIOSPECIFIC

37. Regiospecificity • Markovnikov’s Rule: The proton of an acid adds to the carbon in the double bond that already has the most H’s. “Rich get richer.” • More general Markovnikov’s Rule: In an electrophilic addition to an alkene, the electrophile adds in such a way as to form the most stable intermediate. • HCl, HBr, and HI add to alkenes to form Markovnikov products. =>

38. C H C H 3 2 C l MARKOVNIKOFF RULE PREDICTING THE MAJOR PRODUCT When adding HX to a double bond, the hydrogen of HX goes to the carbon which already has the most hydrogens + HCl major product ..... conversely, the anion X adds to the most highly substituted carbon ( the carbon with most alkyl groups attached).

39. AN “EMPIRICAL” RULE Markovnikoff formulated his rule by observing the results of hundreds of reactions that he performed. EMPIRICAL = DETERMINED BY OBSERVATION He had no idea why the reaction worked this way, only that as a general rule it did give the stated result.

40. SOME ADDITIONAL EXAMPLES Only the major product is shown - all are regioselective. C H C H 3 3 Cl l + HCl C H 3 C H 2 C l + HCl C H C H 3 C H C H 2 + HCl Cl All these reactions follow the Markovnikoff Rule.

41. MARKOVNIKOFF RULE ANOTHER WAY TO STATE THE RULE When the reaction forms the carbocation intermediate, the most highly substituted carbocation is favored : tertiary > secondary > primary. least favored methyl carbocation CH3 + primary carbocation R CH2 + secondary carbocation R CH R + most favored tertiary carbocation R (lowest energy) C R R +

42. Addition Reactions of Alkenes and Alkynes Reactions of alkynes resemble those of alkenes:

43. Addition Reactions of Alkenes and Alkynes 3,3-dichlorohexane

44. Alkene SynthesisOverview • E2 dehydrohalogenation (-HX) • E1 dehydrohalogenation (-HX) • Dehalogenation of vicinal dibromides (-X2) • Dehydration of alcohols (-H2O) =>

45. 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+ =>

46. End of Chapter 3