lecture 8 - alkenes & alkynes
TRANSCRIPT
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