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