lecture 8 - alkenes & alkynes

General

Organic ChemistryTwo credits

Second Semester 2009

King Saud bin Abdulaziz University for Health Science

Reference Book: Organic Chemistry: A Brief Course, by Robert C. Atkins and Francis A. CareyThird Edition

Instructor: Rabih O. Al-Kaysi, PhD.

Chapter 4Chapter 4

Alkenes & Alkynes

Lecture 8

AlkynesAlkynes

Sources of Sources of AlkynesAlkynes

AcetyleneAcetyleneAcetyleneAcetylene

Industrial preparation of acetylene isIndustrial preparation of acetylene isby dehydrogenation of ethyleneby dehydrogenation of ethylene

CHCH33CHCH33

800°C800°C

1150°C1150°C

cost of energy makes acetylene a morecost of energy makes acetylene a moreexpensive industrial chemical than ethyleneexpensive industrial chemical than ethylene

HH22CC CHCH22

HH22CC CHCH22 HCHC CHCH

HH22++

HH22++

NomenclatureNomenclature

HCHC CHCHAcetylene and ethyne are both acceptableAcetylene and ethyne are both acceptableIUPAC names forIUPAC names for

NomenclatureNomenclatureNomenclatureNomenclature

Higher alkynes are named in much the sameHigher alkynes are named in much the sameway as alkenes except using an -way as alkenes except using an -yneyne suffix suffixinstead of -instead of -eneene..

HCHC CCHCCH33

PropynePropyne

HCHC CCHCCH22CHCH33

1-Butyne1-Butyne

(CH(CH33))33CCCC CCHCCH33

4,4-Dimethyl-2-pentyne4,4-Dimethyl-2-pentyne

Physical Properties of AlkynesPhysical Properties of Alkynes

The physical properties of alkynes are The physical properties of alkynes are similar to those of alkanes and alkenes.similar to those of alkanes and alkenes.

Structure and Bonding in Alkynes:Structure and Bonding in Alkynes:spsp Hybridization Hybridization

StructureStructureStructureStructure

linear geometry for acetylenelinear geometry for acetylene

CC CCHH HH

120 pm120 pm

106 pm106 pm 106 pm106 pm

CC CCCHCH33 HH

121 pm121 pm

146 pm146 pm 106 pm106 pm

Cyclononyne is the Cyclononyne is the smallest cycloalkyne smallest cycloalkyne stable enough to be stable enough to be stored at room temperaturestored at room temperaturefor a reasonable length for a reasonable length of time. of time.

Cyclooctyne polymerizesCyclooctyne polymerizeson standing.on standing.

Just for general knowledge, will not be tested on

22ss

22pp

22spsp

Mix together (hybridize) the 2s orbital Mix together (hybridize) the 2s orbital and and oneone of the three 2p orbitals of the three 2p orbitals

22pp

Bonding in acetylene is based on Bonding in acetylene is based on spsp-hybridization-hybridizationfor each carbonfor each carbon

Bonding in acetylene is based on Bonding in acetylene is based on spsp-hybridization-hybridizationfor each carbonfor each carbon

22spsp

Mix together (hybridize) the 2s orbital Mix together (hybridize) the 2s orbital and and oneone of the three 2p orbitals of the three 2p orbitals

22pp

Bonding in acetylene is based on Bonding in acetylene is based on spsp-hybridization-hybridizationfor each carbonfor each carbon

Bonding in acetylene is based on Bonding in acetylene is based on spsp-hybridization-hybridizationfor each carbonfor each carbon

Each carbon has two Each carbon has two half-filled half-filled spsp orbitals orbitalsavailable to form available to form bonds. bonds.

Bonds in AcetyleneBonds in Acetylene Bonds in AcetyleneBonds in Acetylene

Each carbon isEach carbon is

connected to aconnected to a

hydrogen by ahydrogen by a

bond. The two bond. The two

carbons are connectedcarbons are connected

to each other by ato each other by a

bond and two bond and two bonds. bonds.

Bonds in AcetyleneBonds in Acetylene Bonds in AcetyleneBonds in Acetylene

One of the twoOne of the two

bonds in bonds in

acetylene isacetylene is

shown here.shown here.

The second The second bond is at rightbond is at right

angles to the first.angles to the first.

Bonds in AcetyleneBonds in Acetylene Bonds in AcetyleneBonds in Acetylene

This is the secondThis is the second

of the twoof the two

bonds in bonds in

acetylene.acetylene.

C—C distanceC—C distanceC—C distanceC—C distance

C—H distanceC—H distanceC—H distanceC—H distance

H—C—C anglesH—C—C anglesH—C—C anglesH—C—C angles

% % ss character character% % ss character character

153 pm153 pm153 pm153 pm

111 pm111 pm111 pm111 pm

111.0°111.0°111.0°111.0°

spsp33spsp33

25%25%25%25%

134 pm134 pm134 pm134 pm

110 pm110 pm110 pm110 pm

121.4°121.4°121.4°121.4°

spsp22spsp22

33%33%33%33%

120 pm120 pm120 pm120 pm

106 pm106 pm106 pm106 pm

180°180°180°180°

spspspsp

50%50%50%50%

hybridization of Chybridization of Chybridization of Chybridization of C

Comparison of ethane, ethylene, and acetyleneComparison of ethane, ethylene, and acetylene

Ethane Ethylene Ethane Ethylene AcetyleneAcetylene

Acidity of Acetylene and Terminal AlkynesAcidity of Acetylene and Terminal Alkynes

HH

CC CC

Acetylene is a weak acid, but not nearlyAcetylene is a weak acid, but not nearlyas weak as alkanes or alkenes.as weak as alkanes or alkenes.

Acetylene is a weak acid, but not nearlyAcetylene is a weak acid, but not nearlyas weak as alkanes or alkenes.as weak as alkanes or alkenes.

CompoundCompound ppKKaa

HFHF 3.23.2

HH22OO 1616

NHNH33 3636

4545

CHCH44 6060

HH22CC CHCH22

HCHC CHCH 26262626

Just for general knowledge, will not be tested on

Solution: Use a stronger base. Sodium amideSolution: Use a stronger base. Sodium amideis a stronger base than sodium hydroxide.is a stronger base than sodium hydroxide.

Solution: Use a stronger base. Sodium amideSolution: Use a stronger base. Sodium amideis a stronger base than sodium hydroxide.is a stronger base than sodium hydroxide.

NHNH33NaNaNHNH22 ++ HCHC CHCH NaCNaC CHCH ++

––HH22NN

....::

HH CC CHCH HH....

++ ++ CC CHCH::––

stronger acidstronger acidppKKaa = 26 = 26

weaker acidweaker acidppKKaa = 36 = 36

Ammonia is a weaker acid than acetylene.Ammonia is a weaker acid than acetylene.The position of equilibrium lies to the right.The position of equilibrium lies to the right.

HH22NN

Just for general knowledge, will not be tested on

Preparation of Alkynes Preparation of Alkynes byby

Alkylation of Acetylene and Terminal AlkynesAlkylation of Acetylene and Terminal Alkynes

Preparation of AlkynesPreparation of AlkynesPreparation of AlkynesPreparation of Alkynes

Carbon-carbon bond formationCarbon-carbon bond formationalkylation of acetylene and terminal alkynesalkylation of acetylene and terminal alkynes

Functional-group transformationsFunctional-group transformationseliminationelimination

There are two main methods for the preparationThere are two main methods for the preparationof alkynes:of alkynes:

Alkylation of acetylene and terminal alkynesAlkylation of acetylene and terminal alkynesAlkylation of acetylene and terminal alkynesAlkylation of acetylene and terminal alkynes

H—C H—C C—HC—H

RR—C —C C—HC—H

RR—C —C C—C—RR

RR XX

Alkylation of acetylene and terminal alkynesAlkylation of acetylene and terminal alkynesAlkylation of acetylene and terminal alkynesAlkylation of acetylene and terminal alkynes

XX––::++CC––::H—C H—C C—RC—RH—C H—C ++

• The alkylating agent is an alkyl halide, andThe alkylating agent is an alkyl halide, andthe reaction is nucleophilic substitution.the reaction is nucleophilic substitution.

• The nucleophile is sodium acetylide or the The nucleophile is sodium acetylide or the sodium salt of a terminal (monosubstituted) sodium salt of a terminal (monosubstituted) alkyne.alkyne.

Example: Example: Alkylation of acetylene Alkylation of acetylene Example: Example: Alkylation of acetylene Alkylation of acetylene

NaNHNaNH22

NHNH33

CHCH33CHCH22CHCH22CHCH22BrBr

(70-77%)(70-77%)

HCHC CHCH HCHC CCNaNa

HCHC CC CHCH22CHCH22CHCH22CHCH33

NaNHNaNH22, NH, NH33

CHCH33BrBr

Example: Example: Alkylation of a terminal alkyneAlkylation of a terminal alkyne Example: Example: Alkylation of a terminal alkyneAlkylation of a terminal alkyne

CCHH(CH(CH33))22CHCHCHCH22CC

CCNaNa(CH(CH33))22CHCHCHCH22CC

(81%)(81%)

C—CHC—CH33(CH(CH33))22CHCHCHCH22CC

Preparation of Alkynes byPreparation of Alkynes byElimination ReactionsElimination Reactions

Preparation of Alkynes by Preparation of Alkynes by "Double" Dehydrohalogenation"Double" Dehydrohalogenation

Preparation of Alkynes by Preparation of Alkynes by "Double" Dehydrohalogenation"Double" Dehydrohalogenation

Vicinal dihalideVicinal dihalide

XX XX

CC CC

HHHH

The most frequent applications are in preparation The most frequent applications are in preparation of terminal alkynes.of terminal alkynes.

CHCH33(CH(CH22))77CCHH—C—CHH22BrBr

BrBr

Vicinal dihalide Vicinal dihalide Alkyne AlkyneVicinal dihalide Vicinal dihalide Alkyne Alkyne

1. 3NaNH1. 3NaNH22, NH, NH33

2. H2. H22OO

(54%)(54%)

CHCH33(CH(CH22))77CC CHCH

Hydrogenation of AlkynesHydrogenation of Alkynes

Hydrogenation of AlkynesHydrogenation of AlkynesHydrogenation of AlkynesHydrogenation of Alkynes

RCRCHH22CCHH22R'R'catcat

catalyst = Pt, Pd, Ni, or Rhcatalyst = Pt, Pd, Ni, or Rh

alkene is an intermediatealkene is an intermediate

RCRC CR'CR' ++ 22HH22

ExampleExampleExampleExample

+ + H H22

Lindlar PdLindlar Pd

CHCH33(CH(CH22))33 (CH(CH22))33CHCH33

HH HH

(87%)(87%)

CHCH33(CH(CH22))33CC C(CHC(CH22))33CHCH33

CCCC

Addition of Hydrogen Halides Addition of Hydrogen Halides to Alkynesto Alkynes

Follows Markovnikov's RuleFollows Markovnikov's RuleFollows Markovnikov's RuleFollows Markovnikov's Rule

HBrHBr

BrBr

(60%)(60%)

Alkynes are slightly less reactive than alkenesAlkynes are slightly less reactive than alkenes

CHCH33(CH(CH22))33CC CHCH CHCH33(CH(CH22))33CC CHCH22

CHCH

Termolecular transition stateTermolecular transition stateTermolecular transition stateTermolecular transition state

....BrBrHH ::....

RCRC

....BrBrHH ::....

Reaction with two moles of a hydrogenReaction with two moles of a hydrogenhalide yields a geminal dihalidehalide yields a geminal dihalide

Reaction with two moles of a hydrogenReaction with two moles of a hydrogenhalide yields a geminal dihalidehalide yields a geminal dihalide

(76%)(76%)

CHCH33CHCH22CC CCHCCH22CHCH33

2 H2 HFF

FF

FF

CC CC

HH

HH

CHCH33CHCH22 CHCH22CHCH33

Hydration of AlkynesHydration of Alkynes

Hydration of AlkynesHydration of AlkynesHydration of AlkynesHydration of Alkynes

expected reaction:expected reaction:

enolenol

HH++

RCRC CR'CR' HH22OO++

OHOH

RCHRCH CR'CR'

Addition of Halogens to Addition of Halogens to

AlkynesAlkynes

ExampleExampleExampleExample

+ 2 + 2 ClCl22

ClCl

ClCl

(63%)(63%)

CCClCl22CHCH CHCH33HCHC CCHCCH33

Addition is antiAddition is antiAddition is antiAddition is anti

BrBr22

CHCH33CHCH22

CHCH22CHCH33BrBr

BrBr

(90%)(90%)

CHCH33CHCH22CC CCHCCH22CHCH33 CC CC