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 =>

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

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

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

20. Definisi
Ikatan rangkap dua terkonjugasi : dipisahkan oleh satu ikatan tunggal.

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

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 A + B AB
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. ELECTROPHILIC ADDITION TO DOUBLE BONDSX C C + EX C C
electrophilic
reagent E C l
conc.
EXAMPLES: C C C C +
HCl H
explained
later O H
H2SO4 C C +
H2O C C H
OSO3H c o n .
0 oC C C +
H2SO4 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 (an alcohol)
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. X
=>

34. Addition of HX (2)
=>

35. Reaksi Adisi via Intermediet Karbokation
Hidrasi
Adisi Hidrogen halida

36. A REGIOSELECTIVE REACTION
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. MARKOVNIKOFF RULE C H C H C l + HCl 3 2 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 C H 3 C H 2 C l
+ 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 C H 3 2 C l
+ HCl C H C H C H C H 3 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 Synthesis Overview
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