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Organic Chemistry
CHƯƠNG 7 (t.t)
HYDROCARBON CHƯA NO MẠCH H?
ALKYNES
Alkynes
Alkynes have C-C triple bond.
They are more unsaturated than alkenes.
The general formula is CnH2n-2.
The first member of alkyne family is C2H2.
IUPAC name : ethyne
Common name : acetylene
H-C C-H
Characteristics of alkynes
The family name ends in –yne in the IUPAC nomenclature system.
Geometric isomerism is not possible for alkynes.
Triple bond is due to the formation of sp-hybridized C.
The triple bond is composed of one strong sigma bond and two weak pi bonds.
Chemical reactions are same as alkenes but requires double the reagent as two pi bonds are present in alkynes.
Acetylene is an important industrial product. Acetylene torch used in welding to melt and vaporize steel and iron.
ALKYNES
ALKYNES
I. Nomenclature of Alkynes -- “yne”
Step 1: Name longest continuous chain containing the triple bond; parent is “yne”
Step 2: Number to give the carbon-carbon triple bond the lowest number.
I. Nomenclature of Alkynes -- “yne”
Step 3: If there is more than one triple bond, indicate by numbers where they are and use prefixes:
Nomenclature of Compounds containing both double and triple carbon-carbon bonds: “enyne”
Rule 1: Number from the end to give the multiple bond (double or triple) the lowest number and give each multiple bond a number.
Rule 2: The parent name is
#-alken-#-yne
Nomenclature of Compounds containing both double and triple carbon-carbon bonds: “enyne”
Rule 3: If number is the same the double and the triple bond, give the double bond the lower number.
Nomenclature of Alkynes: Substituents and Cycloalkynes
Substituents: (ethynyl group)
ethynylcyclooctane



Cycloalkyne
cyclooctyne

Nomenclature
IUPAC: use the infix -yn- to show the presence of a carbon-carbon triple bond




Common names: prefix the substituents on the triple bond to the word “acetylene”
Common name:
IUPAC name:
2-Butyne
1-Buten-3-yne
Cycloalkynes
Cyclononyne is the smallest cycloalkyne isolated
it is quite unstable and polymerizes at room temp
the C-C-C bond angle about the triple bond is approximately 155°, indicating high angle strain
Classification of Alkynes
Terminal alkyne: monosubstituted alkyne: has triple bond at the end of the chain.


Internal alkyne: disubstituted alkyne: has triple bond inside chain.
II. Structure
II. Structure
Bond lengths:
H-C bond is 106 pm
C-C bond is 120 pm
both bond lengths are shorter than in alkene and alkane
percent s character is greater in alkynes (50%) vs 33% in alkenes vs 25% in alkanes:
electrons held closer to nucleus; results in shorter bond lengths
ALKYNES
Alkynes
• stretch: weak absorption at 2260-2100 cm–1

- not observed for symmetrical alkynes (v. weak for ‘pseudo’
symmetric alkynes
- terminal alkynes (R-C C-H) absorptions are stronger than
internal (R-C C-R) absorptions

• C C–H stretch:
- 3333–3267 cm–1
- strong, narrow (as compared to OH or NH)

• C C–H bend:
- 700-610 cm–1: broad, strong absorption
- 1400-1220 cm–1, overtone of above
Terminal Alkynes
Alkyne
C-H stretch
3310 cm–1
Alkyne
CC stretch
2119 cm–1
Alkyne
C-H bend
630 cm–1
Alkyne
C-H bend overtone
1260 cm–1
Alkyne Synthesis
Alkyne Synthesis
Alkyne Synthesis
Alkylation of Alkyne Anions
Alkyne anions are both strong bases and good nucleophiles
They participate in nucleophilic substitution reactions with alkyl halides to form new C-C bonds to alkyl groups; they undergo alkylation
because alkyne anions are also strong bases, alkylation is practical only with methyl and 1° halides
with 2° and 3° halides, elimination is the major reaction
Alkylation of Alkyne Anions
alkylation of alkyne anions is the most convenient method for the synthesis of terminal alkynes




alkylation can be repeated and a terminal alkyne can be converted to an internal alkyne
Preparation of Alkynes
By double dehydrohalogenation of either
geminal dihalide (halides on same carbon) or
vicinal dihalide (halides on adjacent carbons)
Alkyne Synthesis
Double Elimination
Preparation of Alkynes: Double Elimination


Vicinal dihalides (Cl or Br on adjacent carbons)
Requires two moles of very very strong base such as NaNH2 in NH3 (NH2-)
Triple bond forms between the carbons that had the halogen
Preparation of Alkynes
Mechanism
Double dehydrohalogenation




Preparation:
alkene-->dihalide-->alkyne
Alkyne Synthesis
Alkyne Synthesis
Alkene to Alkyne
Bromination and two consecutive dehydrohalogenation reactions

Preparation from Alkenes
a side product may be an allene, a compound containing adjacent carbon-carbon double bonds, C=C=C
An allene
An alkyne
R
H
R
H
X
H
C
C
C
R
R
R
H
Preparation from Alkenes
for a terminal alkene to a terminal alkyne, 3 moles of base are required
Physical Properties
Similar to alkanes and alkenes of comparable molecular weight and carbon skeleton
Acidity of Alkynes
Acidity of Alkynes
ALKYNES
A Look to Why Alkynes are Less Reactive Than Alkenes
Predicting direction of equilibrium:
Equilibrium lies toward weaker species
Reduction of Alkynes
Hydrogenation reaction
Heterogeneous catalysts
Complete reduction



Partial reduction
Lindlar’s catalyst – syn addition
Alkyne Reductions
Lindlar is a special catalyst that allows
the hydrogenation of an alkyne to stop
after one mole of hydrogen is added.
quinoline
syn addition
Most amines, and compounds
containing sulfur, reduce the
activity of catalysts or “poison”
them.
Lindlar Catalyst: syn addition
Reduction of Alkynes
Reduction of Alkynes
Dissolving metal reduction
anti addition



Mechanism
Dissolving Metal Reduction
anti
This reaction proceeds with
anti addition (trans compound).
Catalytic reduction proceeds with
syn addition, hence we have
a choice of methods.
MECHANISM OF Na-LIQUID-NH3 REDUCTIONS
( from (NH3)n )
.
-
anti addition
electron transfer 1
electron transfer 2
+2e
+2H+
:NH2-
All intermediates prefer
the trans geometry.
radical-
anion
radical
anion
SODIUM IN LIQUID AMMONIA
SODIUM AMIDE IN LIQUID AMMONIA
TWO DIFFERENT REAGENTS !
Reducing Agent
Strong Base
=
=
NaNH2 / NH3(l)
Na / NH3(l)
Addition of Hydrogen Halides and Halogens
Addition of hydrogen halide to alkynes
Addition of hydrogen halide to alkynes
Addition of HX
Alkynes undergo regioselective addition of either 1 or 2 moles of HX, depending on the ratios in which the alkyne and halogen acid are mixed
2,2-Dibromopropane
2-Bromopropene
Propyne
Addition of HX
the intermediate in addition of HX is a 2° vinylic carbocation




reaction of the vinylic cation (an electrophile) with halide ion (a nucleophile) gives the product
Addition of HX
in the addition of the second mole of HX, Step 1 is reaction of the electron pair of the remaining pi bond with HBr to form a carbocation
of the two possible carbocations, the favored one is the resonance-stabilized 2° carbocation
Hydration of alkynes
Hydration of alkynes
Hydration of alkynes
Addition of Water
Something to Watch For !
Addition of H2O: hydration
In the presence of sulfuric acid and Hg(II) salts, alkynes undergo addition of water
+
Propyne
O
Addition of X2
Alkynes add one mole of bromine to give a dibromoalkene
addition shows anti stereoselectivity
Addition halogen to alkynes
Addition of X2
the intermediate in bromination of an alkyne is a bridged bromonium ion
Ozonlysis of alkynes
Addition of Borane:
Hydroboration-Oxidation
Good For Terminal Alkyne
Hydroboration
Addition of borane to an internal alkyne gives a trialkenylborane
addition is syn stereoselective
Hydroboration
to prevent dihydroboration with terminal alkynes, it is necessary to use a sterically hindered dialkylborane, such as (sia)2BH





treatment of a terminal alkyne with (sia)2BH results in stereoselective and regioselective hydroboration
Hydroboration
Treating an alkenylborane with H2O2 in aqueous NaOH gives an enol




enol: a compound containing an OH group on one carbon of a carbon-carbon double bond
an enol is in equilibrium with a keto form by migration of a hydrogen from oxygen to carbon and the double bond from C=C to C=O
keto forms generally predominate at equilibrium
keto and enol forms are tautomers and their interconversion is called tautomerism
Hydroboration
hydroboration/oxidation of an internal alkyne gives a ketone




hydroboration/oxidation of a terminal alkyne gives an aldehyde
3-Hexanone
3-Hexyne
O
ALKYNES
Diels-Alder Reaction
Six-membered Ring formation (4+2)
Diene + Dienophile
Cleavage Reactions of Alkynes
Ozonolysis
Hot Potassium Permanganate
ALKYNES
Organic Synthesis
A successful synthesis must
provide the desired product in maximum yield
have the maximum control of stereochemistry and regiochemistry
do minimum damage to the environment (it must be a “green” synthesis)
Our strategy will be to work backwards from the target molecule
Organic Synthesis
We analyze a target molecule in the following ways
the carbon skeleton: how can we put it together. Our only method to date for forming new a C-C bond is the alkylation of alkyne anions (Section 7.5)
the functional groups: what are they, how can they be used in forming the carbon-skeleton of the target molecule, and how can they be changed to give the functional groups of the target molecule
Organic Synthesis
We use a method called a retrosynthesis and use an open arrow to symbolize a step in a retrosynthesis


Retrosynthesis: a process of reasoning backwards from a target molecule to a set of suitable starting materials
Organic Synthesis
Target molecule: cis-3-hexene
Organic Synthesis
starting materials are acetylene and bromoethane
Organic Synthesis
Target molecule: 2-heptanone
Organic Synthesis
starting materials are acetylene and 1-bromopentane
Alkynes
ALKYNES
Some biologically important alkynes:
Interesting properties:
antibiotics
birth control agent
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