Chapter 8: Alkynes - Science Learning Center

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 sp 3 25 75
Alkene sp 2 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 the
ending from –ene to –yne.
Reactions to Know
Preparation of Alkynes
Synthesis of Alkynes:
1. Dehydrohalogenation of vicinal dihalides:
Br
R
H
C
Br
C
H
R
2 Alc. KOH
R
C C R
Note: vic-dihalides are prepared from alkenes. Thus, in effect, this is a conversion of
alkene to alkyne.

Reactions of Alkynes
1. Hydrohalogenation:
X
H
R
C
C
R
2 HX
R
C
C
R
X
H
(Geminal Dihalide)
2. Mercury (II)-Catalyzed Hydration:
R
C
C
H
OH
H 2 O, H 2 SO 4
HgSO 4
R C C
(Ketone)
The enol is an intermediate in the reaction. However, it immediately rearranges to form
the a ketone through a process called keto-enol tautomerism.
3. Hydroboration-Oxidation: (Internal alkynes will give a ketone; terminal alkynes
will give an aldehyde).
Enol
H
H
R
C
H 2
O
C
CH3
OH
O
R C C R
1) BH 3
2) H 2 O 2 , OH
R C C
H
R
R
C
C
H 2
R
Enol
(Ketone)

As in the mercury-catalyzed reaction, the enol is an intermediate which quickly
undergoes keto-enol tautomerism to form the ketone.
H
BR
1) BH 2
3
R C C R C C
2) H 2 O 2 , OH R C C
R
R
H
Enol
OH
R
Examples:
a) Internal alkyne to ketone
OH
1) BH 3
H 3 C C C CH 3
2) H 2 O 2 , OH
H 3 C C C CH 3
b) Terminal alkyne to aldehyde
OH
H
Enol
1) BH 3
H C C CH 3
2) H 2 O 2 , OH
H C C CH 3
4. Hydrogenation:
H
Enol
R
O
C
C
H 2
O
R
H 3 C C C CH 3
H 2
O
H C C CH 3
H 2
R
C
C
R
2 H 2 / Pd
R
H
C
H
C
R
(Alkane)
H
H
5. Hydrogenation Using Lindlar Catalyst:
H
H
R
C
C
R
H 2
C C
Lindlar's cat.
R
R
(Cis Alkene)

6. Hydrogenation Using Lithium and Ammonia (Na can be used instead of Li):
H
R
R
C
C
R
Li, NH 3
C C
(Trans Alkene)
R
H
7. Cleavage of Internal Alkynes:
R
C
C
R'
KMnO 4 or
O 3
O
O
R C OH + R' C OH
(Carboxylic Acid)
8. Cleavage of Terminal Alkynes:
R
C
C
H
KMnO 4 or
O 3
O
R C OH + CO 2
(Carboxylic Acid and CO 2 )
9. Alkylation Using Acetylide Ions:
R
C
C
H
1) NaNH 2 , NH 3
2) R'CH 2 Br
R
C
C
CH 2 R'

Practice Problems:
1) What are the products of the following reaction:
a)
H 3 C C C CH 2 CH 3 ?
Li, NH 3
b)
H 3 C C C H
c)
KMnO 4 or
O 3
1) BH 3
H C C CH 2 CH 3
2) H 2 O 2 , OH
?
Answer
Answer
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 the
stability of the conjugate base. Why are acetylide anions more stable than vinylic or alkyl
anions?
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, the
negative charge is stabilized to a greater extent.
1. Formation of Acetylide Anions
RC C H NH 2 Na + RC C Na + NH 3
Acetylide anion
Acetylide Anions are important to organic synthesis because they can be used to make a
carbon chain longer.

2. Alkylation of Acetylide Anions
RC C + 1° RX ! RC CR
You’ll learn the mechanism for this reaction in a later chapter.
Practice Synthesis Problem:
H 3 CH 2 C
CH 3
H from H 3 CH 2 CC
C
C
2) How could you prepare
needed reagents?
H
CH and any
Answer

Answer
1) What are the products of the following reaction:
a)
H 3 C C C CH 2 CH 3 ?
Li, NH 3
H 3 C C C CH 2 CH 3 C C
Li, NH 3
H
CH 2 CH 3
H 3 C
H
Return to Problem

Answer
1) What are the products of the following reaction:
b)
H 3 C C C H
KMnO 4 or
H 3 C C C H
O 3
KMnO 4 or
O 3
H 3 C C OH
O
CO 2
Return to Problem

Answer
1) What are the products of the following reaction:
c)
1) BH 3
H C C CH 2 CH 3
2) H 2 O 2 , OH
?
H C C CH 2 CH 3
1) BH 3
2) H 2 O 2 , OH
HC CH2 CH 2 CH 3
Return to Problem
O

Answer:
H 3 CH 2 C
CH 3
H from H 3 CH 2 CC
C
C
2) How could you prepare
needed reagents?
H
CH and any
H 3 CH 2 CC
CH
NaNH 2
H 3 CH 2 CC
C
BrCH 3
H 3 CH 2 C
H
C
C
CH 3
H 2
H Lindlar's Catalyst
H 3 CH 2 CC CCH 3
Return to Problem