31 CH203 Fall 2014 Lecture 31.pdf

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Chapter 7

Alkynes

Lecture 31

November 17th, 2014


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Outline

IUPAC name: Ethyne

Common name:

Nomenclature Physical properties Acidity Preparation Reactions Organic retrosynthesis

Outline

Bond Dissociation Enthalpy (BED)

H 3 C CH 3

H

H H

H

H H966 kJ/mol

376 kJ/mol

727 kJ/mol

351

590


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Common names: prefix the substituents on the triple bond to the word acetylene.

2-Butyne 1-Buten-3-yneIUPAC name:

Common name: Dimethylacetylene Vinylacetylene

Nomenclature


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Cycloalkynes Cyclooctyne is the smallest cycloalkyne isolated.

It is quite unstable and polymerizes at room tem. The C-C-C bond angle about the triple bond is

approximately 155 (rather than the optimal angle of 180), indicating high angle strain.

The C-C-C bond angle about the triple bond isapproximately 160, which still represents a considerabledistortion from the optimal angle of 180, indicating high

angle strain.

Cyclononyne is stable at room temp.

Cyclononyne

cyclopropane

cyclopropene

Cyclooctyne


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Acidity The major differences between the chemistry of

akynes and that of alkenes or alkanes is that ahydrogen bonded to a triply bonded carbon atom of aterminal alkyne is sufficiently acidic that it can beremoved by a strong base to give an acetylide anion.

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The p K a of acetylene and terminal alkynes is approximately25, which makes it stronger acid than ammonia.Terminalalkynes react with sodium amide (NaNH 2) to form alkyne

anions.

Terminal alkynes can also be converted to alkyne anionsby reaction with sodium hydride (NaH) or LDA .

Strong base commonly used to form acetylide anions

C CH H NH2- C C-H NH3

sp hybridization

+ +

pKa 25 pKa 38

(Strongeracid)

(Weakeracid)

Keq = 10 13

(Stronger base)

(Weaker base)

Amide anion Acetylide anion

N- Li+Lithium diisopropylamide(LDA)

[(CH 3)2CH]2N-Li+Na +H-

Sodium hydride

Acidity


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Acidity

Because water is a stronger acid than terminal alkynes,hydroxide ion is not a strong enough base toconvert a terminal alkyne to an alkyne anion.

C CH H OH - C C-H H2O

sp hybridization+ +

pKa 25 pKa 15.7

(Weakeracid)

(Strongeracid)

Keq = 10 -9.3

(Weaker

base)

(Stronger

base)

Base that can not deprotonate terminal alkyne


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Alkyne anions are both strong bases and alsogood nucleophiles ; it has unshared pair of electrons that it can donate to the carbon of amethyl or 1 alkyl halides to form new C-C bonds.They undergo alkylationalkylation . This is the mostconvenient method for the synthesis of alkynes.

In this instance, an alkyn anion nucleophilereplaces the halogen atom of alkyl halide. This typeof reaction is called a nucleophilicnucleophilic substitutionsubstitution .

Alkylation of Alkyne Anions

1o halide

nucleophile electrophile


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HHO

1)

N Li

2 I

(1.0 equiv.)

pka = 10 pka = 25

Whats the reaction product?

HO

Question QuestionProblem


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Preparation from Alkenes Treatment of a vicinal dibromoalkane with two

moles of base , most commonly sodium amide,results in two successive dehydrohalogenationdehydrohalogenationreactions (removal of H and X from adjacentcarbons) and formation of an alkyne.

Formation of vicinaldibromoalkane:

Preparation ofalkyne:


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Preparation from Alkenes

Mechanism of dehydrohalogenationdehydrohalogenation reactions.

With a strong base such as NaNH 2, bothdehydrohalogenations occur readily; However,

with weaker bases such as NaOH or KOH inethanol, it is often possible to stop the reactionafter the first dehydhalogenation and isolatethe haloalkene product.


Na +NH2-

Br

HH

BrH3C

CH 3CH 3

Br

CH 3

H

Na +NH2-

H3CC CCH 3

haloalkene alkyne

(Meso)


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For a terminal alkene to a terminal alkyne , 3 moles of base are required. As soon as any 1-hexyne (a weakacid) forms, it react with sodium amide (a strong base) togive an alkyne salt.


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In the second step dehydrohalogenation of a haloalkene,a side product may be an alleneallene , a compound containingadjacent carbon-carbon double bonds, C=C=C.


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Allene

Allene Allene :: A compound containing a C=C=C group the simplest allene is 1,2-propadiene, commonly

named allene


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Allene

Most allenes are less stable than their isomeric alkynes(they are constitutional isomers), and are generally only

minor products in alkyne-forming dehydrohalogenationreactions.


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ReactionsAlkene Reactions: Alkyne Reactions:

Reactions


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ReactionsOther Alkyne Reactions:Alkylation of Acetylide Anions

Reducing by Hydroboration-Protonolysis to syn-Alkene

Reducing using Dissolving- Metal (Na or Li) to trans-Alkene

Reducing with Lindlar Catalyst to syn-Alkene

Reactions

31 CH203 Fall 2014 Lecture 31.pdf

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