台灣大學開放式課程 有機化學乙 蔡蘊明教授 本著作除另有註明, 作者皆為蔡蘊明教授, 所有內容皆採用創用 CC 姓名標示 - 非商業使用 - 相同方式分享 3.0 台灣授權條款釋出

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1 台灣大學開放式課程 有機化學乙 蔡蘊明教授 本著作除另有註明, 作者皆為蔡蘊明教授, 所有內容皆採用創用 姓名標示 - 非商業使用 - 相同方式分享 3.0 台灣授權條款釋出

2 hapter 7 Alkenes and alkynes: synthesis alkenes also called olefins alkynes common name: acetylenes

3 Nomenclature Alkene IUPA: ane ene 3 3 = ,5-dimethyl-2-hexene 5 6 (5,5-dimethylhex-2-ene) methylcyclopentene 5 3,5-dimethylcyclohexene 3 l l l cis-1,2-dichloroethene l trans-1,2-dichloroethene

4 Q: Br l cis or trans? The (E), (Z) designation: 例 1 igher priority at the same side: (Z) at the opposite side: (E) (E)-3-ethyl-4-methyl-2-pentene (Z)-(S)-3,4-dimethyl-2-hexene

5 As substituent: vinylic hydrogen 2 allylic hydrogens 例 a vinyl group l an allyl group Br chloroethene common name: vinyl chloride 3-bromopropene common name: allyl bromide

6 Alkyne IUPA: ane yne IUPA: methyl-1-hexyne 2 = 2 1-penten-4-yne (or pent-1-en-4-yne) parent smaller # lass name: enyne

7 R acetylenic hydrogen A monosubstituted acetylene or 1-alkyne or terminal alkyne A propargyl group 例 O propargylic hydrogen Propargyl alcohol

8 Physical properties, structure and bonding In general: non-polar, water insoluble, more reactive than alkanes Alkenes π bond E σ MO π planar 0 non-bonding energy level sp 2 (for σ bonding) π σ bond p σ

9 Alkynes R two orthogonal p orbitals R' linear shape two orthogonal π systems sp (for σ bonding) End on view: E MO σ cylindrical π 1 π 2 0 π 1 π 2 σ π 1 & π 2 are degenerated (same E)

10 Bon length 1.10 Ǻ 1.09 Ǻ 1.06 Ǻ Ǻ 1.34 Ǻ 1.20 Ǻ 3 - σ bond: sp 3 -sp 3 sp 2 -sp 2 sp-sp higher s character shorter bond - σ bond: same trend

11 Acidity pk a = Recall: A A K a = [ ][A ] Why? The power of s character sp more s character (orbital electronegativity is higher) Rel. basicity: anion is more stabilized 3 2 > > sp 3 sp 2 sp [A]

12 ompare with others: 2 O > RO > R- - > 2 N- > 2 = 2 > 3 3 pk a liq. N 3 R NaN R Na 2 sodium amide When complexed with transition metal ion: a sodium acetylide or alkynide N 3 R M becomes more acidic due to the adjacent positive charge

13 The stability of alkenes eat of hydrogenation 2 o = 127 kj/mol 2 o = 120 kj/mol 2 o = 115 kj/mol kj/mol ~ ~ ~ 5 Rel. stability: > >

14 eat of combustion 6O 2 4O o O = 2729 kj/mol 6O 2 4O O o = 2714 kj/mol More stable by 15 kj/mol Overall relative stability: trans cis R R R R R R R R R > > > > > > R R R R R tetrasubstituted tri- disubstituted mono- un- R: alkyl group

15 Points to remember: More R more stable Reason: R: donates e sp 2 orbital: more e demanding MO view: good match hyperconjugative stabilization Trans is more stable than cis Reason: steric repulsion R R R better R

16 ycloalkenes cyclohexene trans-cyclohexene very strain, very unstable Number of carbon < 6 only cis cis-cyclooctene trans-cyclooctene (chiral)

17 Dehydrohalogenation E2 preffered (E1 more side product) B X E2 B X use a strong base: NaOEt, t-buok, high conc. high temp. nonpolar solvent if possible 例 Br EtONa EtO, 55 o 79% Br EtONa EtO, 55 o 100%

18 Regioselectivity: Zaitsev s rule 位置選擇 3 2 Br 3 b B a b B Br B Br a Zaitsev s rule: the more stable alkene will be major 例 Br EtONa EtO, 70 o 69:31 more stable (trsubstituted)

19 The reason of Zaitsev s rule TS: B δ has appreciable double bond character X δ More stable product More stable TS faster Br EtO EtO Br reaction coordinate

20 Exception: Br O 75 o O 27.5:72.5 Reason: steric the base is very hindered abstracts the less crowded 1 o hydrogen Formation of the less substituted alkene offmann orientation

21 Stereochemical requirement B B L anti-periplanar preferred L syn-periplanar higher energy

22 例 EtONa EtO l neomenthyl chloride fast 78:22 Ans: l menthyl chloride l EtONa EtO slow l 0:100 preferred conformation adopts anti-periplanar TS easily Fast and follows Zaitsev s rule l Why? l preferred conformation no can obtain anti-periplanar relationship with l l l only this is anti to l offmann orientation

23 Dehydration of alcohols O 2 O Acid catalyzed Ease of dehydration 3 o > 2 o >1 o requires high conc. & high T 例 3 2 O conc. 2 SO o 2 O O 85% 3 PO o 80% 2 O O 20% 2 SO 4 85 o 2 O 84%

24 Mechanism: E1 process for 2 o and 3 o alcohols O O hydronium ion fast O a protonated alcohol or alkyloxonium ion O slow 2 O arbocation formation in RDS More stable cation more stable TS faster reaction 2 O or RO fast O atalyst is regenerated *Equilibrium: remove water drives the reaction to the right (the reverse reaction is the addition of water to olefins)

25 E2 mechanism for 1 o alcohols O O O A (or 3 O ) A or 2 O Weak base Requires harsh condition

26 Rearrangement occurs 例 85% 3 PO 4 O 80 o 20:80 rearranged skeleton

27 Rearrangement of carbocation O 3 O O 2 O O 2 O 1,2-methyl shift Driving force: formation of a more stable carbocation (2 o 3 o )

28 δ δ a b a b major minor follows Zaitsev's rule

29 例 hydride ( ) shift 2 o 3 o methyl shift 1 o 3 o O 2 O,

30 Primary alcohols rearrangement through olefin products O 2 O rearrangement of carbocation

31 Preparation of alkynes By elimination R=R Br 2 R R 2NaN 2 R R Br Br 2N 3 2NaBr Mechanism: N 2 N 2 R R R R R R Br Br Br t-buok can be used in this step a strong base is required in this step (vinyl - bond is stronger)

32 例 Br 2 l 4 Br Br NaN 2 mineral oil o Br Br NaN 2 N 4 l (work-up) Na NaN 2 O l l Pl 5 1) 3NaN 2 0 o 2) 2 N- N 2

33 From acetylide NaN 2 liq. N 3 Na R 2 Br 2 R NaN 2 R' 2 2 R R' 2 Br Na 2 R Only work with 1 o halide 2 o, 3 o halides elimination (acetylides are strong base) S N 2 with inversion of configuration

34 ydrogenation ( 氫化 ) Alkenes Ni or Pd or Pt 25 o catalyst A catalytic hydrogenation (an addition reaction) unsaturated saturated break: - π form: two - σ - σ Overall: exothermic ~ 120 kj/mol owever, E a is high catalyst is required Pd/: palladium on charcoal is often used Other metals: Pt (PtO 2 ), Rh, Ru, Ni

35 Mechanism: forms a π complex 2 metal metal surface is regenerated Stereochemistry: syn addition of the two hydrogens syn anti 例 Pd/ 3 3

36 Alkynes [ 3 = 3 ] Pt Stop at alkene: The Lindlar s catalyst: 2, Pd/aO 3, Pb 2 or quinoline used as catalyst poison (bind the active site or change the surface structure) N R R 2 Lindlar's catalyst R R syn addition cis alkene

37 Dissolving metal reduction 3 ( 2 ) 2 ( 2 ) 2 3 1) Li, 2 5 N 2, 78 o 2) N 4 l Pr Pr 52% Other reagents: Li/N 3(l), Na/N 3(l) Note: normal double bond is not reduced Mechanism: R R The two Rs anti to each other to avoid steric interaction e R R R R NR RN NR R R R e R NR A vinylic anion prefers sp 2 instead of p

38 Summary alides Alcohols dehydrohalogenation acid catalyzed dehydration Alkenes Alkynes catalytic hydrogenation (cis) dissolving metal reduction (trans) alkylation of acetylides 1) halogenation 2) dehydrohalogenation Acetylene Take good cares about mech., regio- and stereoselectivity

39 Planning organic synthesis: retrosynthetic analysis Retrosynthetic analysis: planning backward through bond disconnections reate a flowchart: Target molecule 1st precursor Analyze: Efficiency Number of steps Overall yields Economics Safety Waste issues 2nd precursor Starting material

40 ase study: synthesis of : 3 Br Br 3 Br 3 3 : : 3 Br Br 3 3 : : 2 3 : 3 Br : 3 Br 2 3 : 3 : 3 2 Br Br 3