Chemistry 18B Ch 13 alkyne PDF

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118 B Ch13

13-1

Naming Alkynes IUPAC rules for naming alkenes also apply to alkynes. The suffix -yne is used and and the position of triple bond is indicated by giving the number of the first alkyne carbon. 3

CH3CHCHC CH 1

4-Bromo-3-methyl-1-pentyne 4-bromo-3-methylpent-1-yne

Br CH3

Substituent bearing a triple bond are alkynyl groups.

2-Propynylcyclohexane

In IUPAC name a hydrocarbon containing both double and triple bonds is called ALKENYNE. However the chain is numbered starting from the end closest to either of the functional groups. If the two functional groups are equidistant the double bond is given the lower number. OH 4

CH3CH2CH CHC

CH 1

CH2

3

CHCH2CH2C

hex-1-en-5-yne

(E)-hex-3-en-1-yne

CH (R,E)-non-7-en-2-yn-4-ol

Structure and Bonding Alkynes have high heat of combustion. 5 O2 2H C C H 4 CO2 + 2 H2O

= -622 kcal/mol

Acidity CH3 pKa

H C C H 25

CH3 50

CH2

CH2 44

118 B Ch13

13-2

Preparation of Alkynes: Elimination of HX from vicinal dihalide H H R C C R Br Br

2 NaNH2, NH3(l)

H H R C C H Br Br

RC CR

1. 3 NaNH2, NH3(l) 2. H2O

Mechanism of the preparation of terminal alkynes H3C

H Br Br

CH3C C

NH2

H

NH2 CH3C CH + NH 3

NH3 +

H

H

H CH3

NH2

Br H

NH3 + CH2C C

OH

pKa ~ 16

CH3C CH + OH

Preparation of Alkynes from Alnynyl Anions RC CH 1) n-BuLi, THF 2) R'X SN2 RC CH

n-BuLi, THF

RC C(CH2)3CH3 O -

RC CLi

+

1. CH 3 2. H2O

OH RC CCH2CHCH3

RC CH

118 B Ch13

13-3 O -

Mechanism

+

CH3

RC CLi

RC C CH2

H OH O Li OH CHCH3 RC CCH2CHCH3 + LiOH O

RC CH

C2H5MgBr, THF

1.

H

RC CMgBr 2. H2O

OH RC CCHCH2CH3

Reactions of alkynes Complete Hydrogenation of Alkynes RC CH

2 H2

RCH2CH2R

Pt-C

Hydrogenation using Lindlar Catalyst RC CR

R

H2

R

Lindlar cat.

H

C

C

Lindlar cat. )

CaCO3, Pb(C2H3O2)2, Quinoline (

N

H

cis-alkene Sequential one-electron reduction of alkynes

1) Na or Li NH3 (l) RC CR 2) H2O

H

R C

C

R H trans-alkene

13-4

118B Ch.13 Mechanism R H Na +

RC CR + Na

NH2

H NH2 +

R

Na

H

Na + R C

R

R H R

R C

C

H

NH2

H NH2 +

R

R C

C

H

Electrophilic Addition Reactions of Alkynes Addition of H-X (X = Cl, Br, I) to Internal Alkynes

C2H5 HBr

C2H5C CC2H5

H

H

Br C C

+

C2H5 Z-alkene (60%)

1 Equivalent

C2H5

Br C C C2H5

E-alkene (40%)

Mechanism H C2H5C CC2H5

Br

C2H5 C H

Br

C2H5 C

+ C2H5

Br

C H

C C2H5

R C

13-5

118B Ch13 E to Z isomerization C2H5 H

Br H

C C

Br

C2H5

C2H5C CC2H5

H Br C C H C2H5

H

C2H5 Br +

HBr 2 Equivalent

C2H5 H

C2H5

H Br -H C C C2H5 + H

H C2H5

Br C C C2H5

Br C

C

H

Br C2H5

Addition of H-X (X = Cl, Br, I) to terminal Alkynes H C2H5 HI, - 70oC C C C2H5C CH I 1 Equivalent 35% H

I H C C I H H 65%

C2H5 +

Anti Markornikov addition H-Br to Terminal Alkynes C2H5C CH

HBr, R2O2, hu 1 Equivalent

C2H5 C

C2H5C CC 2H5

Br2 CH3CO2H

C2H5C CC 2H5

2 Br2 CH3CO2H

C

H Major Br

Halogenations of Alkynes C2H5 Br

H

H

Br C C C2H5

C2H5CBr2CBr2C2H5

+

C2H5 H

C

Br H C Br

13-6

118B Ch13 C2H5C CH

Br2 CH3CO2H

C2H5 Br

Br C C H

Addition of H2O to Internal Alkynes C2H5C CC 2H5

H2O H2SO4, HgSO4

O C2H5CCH2C2H5

O O C2H5CCH2CH3 C2H5C CCH3 + C2H5CH2CCH3 H2SO4, HgSO4 50% 50% Acid- catalyzed hydration of the alkynes containing three or more carbons produces ketones as the major organic product. H2O

Mecanism of the Acid Catalyzed Hydration of Alkynes C C

OH

H2O, H H

C C

H2O H +

H C C

+ H O H C C

H

H C H

O C

H2O

H

O tautomerism H H O C C H O C C H

H

H2O

H H2O

O + H3O

118B Ch13

13-7

Anti-Markovnikov addition of H2O to Terminal Alkynes

R C C H

1. BH3, THF 2. H2O, H2O2, NaOH

R

OH

H

H

O tautomerism

RCH2

H

Mecanism of the tautomerism in basic medium R H

O H

+

OH

H

O

R

O

H OH RCH2

H

H

H

H

O

R

Chemistry of alkenyl (vinyl) halides. Br

NaCN

Br

Mg, Et2O

No SN1 or SN2 reaction MgBr

1.

O

OH C2H5

C2H5, Et2O 2. H3O+

Heck Reaction RCH

CHCl

R Pd

RCH

CHPdCl

RCH

CR2

R

R R

H +

OH...