c203ch8
TRANSCRIPT
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Alkynes
Alkynes-hydrocarbons with a carbon-carbon triple bond.
The carbon-carbon triple bond results from the interaction of two sp hybridized carbon
atoms. 180 degree angle. Linear.The carbon-carbon triple bond is the shortest and strongest known
carbon-carbon bond. Why? It has the highest percentage of s character.
Type Hybridization % s character % p character
Alkane sp3 25 75
Alkene sp2 33 67
Alkyne sp 50 50
Nomenclature
Alkynes are named like alkenes with the following adjustments: 1) Number the carbons
on the chain so that the triple bonded carbons receive the lowest possible numbers, 2)Indicate the location of the triple bond by writing the number of the lowest numbered
carbon making up the triple bond in front of the name of the parent chain. 3) Change theending from –ene to –yne.
Reactions to KnowPreparation of Alkynes
Synthesis of Alkynes:
1. Dehydrohalogenation of vicinal dihalides:
C C
R
H
Br
R
HBr
2 Alc. KOHC C RR
Note: vic-dihalides are prepared from alkenes. Thus, in effect, this is a conversion ofalkene to alkyne.
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Reactions of Alkynes
1. Hydrohalogenation:
C CR R2 HX
C C
H
H
R
X
X
R
(Geminal Dihalide)
2. Mercury (II)-Catalyzed Hydration:
C CR H
H2O, H2SO4
HgSO4C CR H
OH
H
C
H2
CR CH3
O
Enol (Ketone)
The enol is an intermediate in the reaction. However, it immediately rearranges to formthe a ketone through a process called keto-enol tautomerism.
3. Hydroboration-Oxidation: (Internal alkynes will give a ketone; terminal alkyneswill give an aldehyde).
C CR R C CR R
OH
H
Enol
C C
H2
R R
1) BH3
2) H2O2, OH
O
(Ketone)
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As in the mercury-catalyzed reaction, the enol is an intermediate which quickly
undergoes keto-enol tautomerism to form the ketone.
C CR R C C
H
R
BR2
R
C C
H2
R R
O
C CR R
OH
H
Enol
1) BH3 2) H2O2, OH
Examples:
a) Internal alkyne to ketone
C CH3C CH3 C CH3C CH3
OH
H
Enol
C C
H2
H3C CH3
1) BH3
2) H2O2, OH
O
b) Terminal alkyne to aldehyde
C CH CH3 C CH CH3
OH
H
Enol
C C
H2
H CH3
1) BH3
2) H2O2, OH
O
4. Hydrogenation:
C CR R 2 H2/ Pd
C CR R
H
H
H
H
(Alkane)
5. Hydrogenation Using Lindlar Catalyst:
C CR R H2
C C
HH
R R
(Cis Alkene)Lindlar's cat.
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6. Hydrogenation Using Lithium and Ammonia (Na can be used instead of Li):
C CR R Li, NH3
C C
RH
R H
(Trans Alkene)
7. Cleavage of Internal Alkynes:
C CR R'
O3C
O
R OH C
O
R' OH
KMnO4 or +
(Carboxylic Acid)
8. Cleavage of Terminal Alkynes:
C CR H
O3C
O
R OH CO2
KMnO4 or +
(Carboxylic Acid and CO2)
9. Alkylation Using Acetylide Ions:
C CR H1) NaNH2, NH3
2) R'CH2Br C CR CH2R'
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Practice Problems:
1) What are the products of the following reaction:a)
C CH3C CH2CH3 Li, NH
3
?
Answer
b)
C CH3C H
O3
KMnO4 or
Answer
c)
C CH CH2CH3
1) BH3
2) H2O2, OH
?
Answer
Alkyne Acidity: Forming Acetylide Anions
Terminal alkynes are weakly acidic.Acidity of simple hydrocarbons:
Alkynes > Alkenes> Alkanes
Why are terminal alkynes more acidic than alkenes or alkanes? Acidity depends upon thestability of the conjugate base. Why are acetylide anions more stable than vinylic or alkylanions?
1) Hybridization of the negatively charged carbon atom:a. Acetylide anion has an sp hybridized carbon. Therefore,the negative charge resides in an orbital that has 50% s
characteristic.
b. Vinylic anion ' sp2 hybridized. 33% s character.c. Alkyl anion ' sp3 hybridized. 25% s character.
Since s orbitals are nearer the positive nucleus and are lower in energy than p orbitals, thenegative charge is stabilized to a greater extent.
1. Formation of Acetylide Anions
RC C H NH2 Na+
RC C Na+ NH3Acetylide anion
Acetylide Anions are important to organic synthesis because they can be used to make a
carbon chain longer.
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2. Alkylation of Acetylide Anions
RC C + 1° R X ! RC CR
You’ll learn the mechanism for this reaction in a later chapter.
Practice Synthesis Problem:
2) How could you prepare
C C
H3CH2C
H
CH3
H from H3CH2CC CH and any
needed reagents?
Answer
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Answer
1) What are the products of the following reaction:a)
C CH3C CH2CH3 Li, NH
3
?
C CH3C CH2CH3 Li, NH3
C C
CH2CH3
HH3C
H
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Answer
1) What are the products of the following reaction:
b)
C CH3C H
O3
KMnO4 or
C CH3C H
O3
H3C C OH
O
CO2KMnO4 or
Return to Problem
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Answer
1) What are the products of the following reaction:
c)
C CH CH2CH3 1) BH3
2) H2O2, OH?
C CH CH2CH3 HC CH2CH2CH3
O
1) BH3
2) H2O2, OH
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Answer:
2) How could you prepare
C C
H3CH2C
H
CH3
H from H3CH2CC CH and any
needed reagents?
BrCH3
H3CH2CC CCH3
H3CH2CC C
H2
NaNH2H3CH2CC CH
C C
H3CH2C
H
CH3
H Lindlar's Catalyst
Return to Problem