Chapter 4 Alkynes4.1 Nomenclature4.2 Structure of Alkynes4.3 Reactions of alkynes4.3.1 Acidity of Terminal Alkynes4.3.2 Application of the Acidity of Terminal Alkynes in Organic Synthesis4.3.3 Hydrogenation of Alkynes 1. Syn-addition of H2: Synthesis of cis-Alkyenes 2. Metal-Ammonia Reduction of Alkynes Synthesis of trans-Alkenes
4.3.4 Electrophilic Addition of Alkynesa. Addition of HXb. Hydration of Alkynesc. Addition of X2d. Hydroboration and Oxidation of Alkynes4.4 Oxidation of Alkynes4.5 Preparation of Unsaturated Hydrocarbons4.5.1 Preparation of alkenes1. Dehydrohalogenation of alkyl halides2. Dehydration (脱水 )of alcohols 4.5.1 Preparation of Alkynesa. Dehydrohalogenation of vicinal alkyl dihalidesb. Alkylation of alkynide ions
• Suffix: ane
yne
Alkynes: Hydrocarbons containing a carbon-carbontriple bond.
4.1 Nomenclature
P134,4.13P134,4.13
RC CH Terminal or mono-substituted
R'C CR Internal or disubstitutedalkynes
(内炔 )
CCH2CH3HC1-Butyne
• Alkenyne (烯炔 )
CH3CH CH C CH3-Penten-1-yne(3-戊烯 -1-炔 )
CCH2CHHC CH2
1-penten-4-yne(1-戊烯 -4-炔 )
1. Numbering starts from the end near the first multiple bond.1. Numbering starts from the end near the first multiple bond.
2. When there is a choice, double bonds receive lower numbers than triple bonds.
2. When there is a choice, double bonds receive lower numbers than triple bonds.
C CH
Ethynyl(乙炔基 )
4.2 Structure of AlkynesAcetylene (ethyne):sp Hybrid orbitalsC:
Ground state
2p
2s
1sPromotionof electronPromotionof electron
Exited state
2p
2s
1s
sp-hybridized state
1s
2p
sp
Hybri-dization
Hybri-dization
Each sp hybrid orbital:
50% s character50% p character
P18, 1.10P18, 1.10
The shape of an sp hybrid obital:
The two sp hybride orbitalsare oriented 180°away from each other,perpendicular to the two remaining p orbitals.
180°An sp-hybridized carbon atom:
Geometric structure of sp-hybridized C atoms is linear.In the mole. of acetylene, the formation of C-Cσbond : sp-sp overlap;
The formation C-H σbonds: sp-1s overlap.
The formation of two C-Cπbonds: 2py-2py overlap and 2pz-2pz overlap.
The formation C-H σbonds: sp-1s overlap.
H
H
σbondσbond
πbondπbond
πbondπbond
Carbon-carbontriple bonds
Carbon-carbontriple bonds
4.3 Reactions of alkynes4.3.1 Acidity of Terminal AlkynesAcetylene:
C CH HKa
C CH + HCarbanion
Conjugate base(共轭碱 )
Conjugate base(共轭碱 )
HC CH H2C CH2 CH3 CH3
pKa: 25 44 50
Acidity:
HF > H OH > ORH > C CRH > NH2H
pKa: 3.2 15.8 16-17 26 38
4.3.2 Application of the Acidity of Terminal Alkynes in Organic Synthesis1. Preparation of Metal Alkynides
HC C H + Na NH2
Liq. NH3C CH Na + NH3
NaNH2 / Liq.NH3
C C NaNa + NH3
Acid-base reaction
P138,4.16P138,4.16
Stronger acid
Stronger base
Weakerbase
Weakeracid
2. Formation of C-C Bonds(Synthesis of Alkynes)
Sodium alkynides are useful intermediatesas nucleophile.
C C NaNaR Cl C CNa R
R Cl C C RR
R X : Primary halides RCH2Cl3. Formation of other metal alkynides CH3CH2C CH + Ag(NH3)NO3 CH3CH2C CAg
+ 2NH4NO3 + 2NH3Reagent: NH3 solution of AgNO3 or NH3 solution of CuCl
Ch. P77Ch. P77
Identification of C C
4.3.3 Hydrogenation of AlkynesCh.P52(五 )(1)Ch.P52(五 )(1)
+ H2Pt, Pd, Ni or Rh RCH2 CH2R'RCH CHR'
RC CR' Pt, Pd, Ni or Rh RCH CHR'+ H2
CH3(CH2)3C C(CH2)3CH3H2
Lindlarcatalyst
CH3(CH2)3
C C(CH2)3CH3
H H
1. Syn-addition of H2: Synthesis of cis-Alkyenes
Lindlar catalyst
Lindlar catalyst: Pd/ CaCO3, Pb(Ac)2-quinoline (喹啉 )Quinoline:
N
P-2 catalyst:
(96%)
Ni OCCH3
ONaBH4
C2H5OH2Ni2B
C C C CH H
R'RH2
P-2
R R'Ch: P53Ch: P53
2. Metal-Ammonia Reduction of Alkynes Synthesis of trans-Alkenes
CH3(CH2)3C C(CH2)3CH3 Li or Na
CH3(CH2)3
C C
(CH2)3CH3H
H
Liq. NH3
78%4.3.4 Electrophilic Addition of Alkynes
• Reactivity: Alkynes < Alkenes• Regioselectivit: Follow Markov.’s Rule
a. Addition of HX
R'C CH HX R'C CH2
X
HXR'C CH3
X
X
Alkenyl halide Alkyl dihalide
Excess HX
b. Hydration of Alkynes
1-Hexyne An-enol(烯醇 )
CH3(CH2)3C CH3
O
2-Hexanone
Keto-enol tautomerism(酮式-烯醇式互变异构 ):
CC
OH
CC
O
H
CH3(CH2)3C CH + H2O H2SO4
HgSO4 CH3(CH2)3C CH
OH
H
Transformation of Func-tional groups:
C C C
O
H(R)
c. Addition of X2
d. Hydroboration and Oxidation of Alkynes
Ch: P67Ch: P67
6.4 Oxidation of AlkynesAn internal alkynes:
C C R'RKMnO4 or O3 RCOH +
O
R'COH
O
C C HRKMnO4 or O3 RCOH + CO2
O
Identification of C-C triple bonds
6.5 Preparation of Unsaturated Hydrocarbons
6.5.1 Preparation of alkenes1. Dehydrohalogenation of alkyl halides:
(脱卤化氢 )
H
HBr
H
KOH
CH3CH2OHKBr + H2O+
Bromocyclohexane Cyclohexene (81%)2. Dehydration (脱水 )of alcohols
CCH3
CH3
CH3
OH20% H2SO4 (CH3)2C CH2 + H2O
85 ℃ (84%)
Elimination Elimination
6.5.1 Preparation of Alkynesa. Dehydrohalogenation of vicinal alkyl dihalides
CH
CH Br2
CH2Cl2C
HC
H
Br
Br
Diphenylacetylene(85%)b. Alkylation of metal alkynide
2KOH-Ethanol
C C + 2KBr + 2H2O
Problems to Chapter 4. Alkyne
P146.4.25(b)4.26(b),(c)4.29(c)4.41 Show the reactions.4.424.444.45(c)4.484.524.56
Additional problems:1.Predict the products from reaction of 2-
hexyne with the following reagents:(a) 2 equiv Br2 (b) 1equiv HBr (c) Exces
s HBr(d) Li in NH3 (e) H2O, H2SO4, HgSO42. How would you carry out the following reaction?
C CH ? C CCH3 ? C CH H
CH3
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