TOPIC 4: ALKYNES:
STRUCTURE, REACTIVITY AND SYNTHESIS
Alkynes are hydrocarbons that contain a carbon-carbon triple
bond, which is the strongest and shortest type of carbon-carbon bond
that exists. Many of the reactions of alkynes are similar in nature to
those of alkenes, but there are important differences too. In this unit,
we will examine the structure and reactivity of alkynes as well as how
many of the reactions you have learned about thus far are used in the
important process of organic synthesis.
I. NAMING ALKYNES
* Alkynes follow the general set of naming rules we saw for
alkanes and alkenes, with the name of the parent chain ending
in –yne.
1. Find the parent hydrocarbon. Find the longest carbon chain that contains
the triple bond. Name accordingly using –yne as the suffix.
2. Number the carbon atoms in the chain. Begin at the end nearest the triple
bond. If the triple bond is equidistant from the two end points, begin at the
end nearest the first branch point.
3. Write the full name.
• Number the substituents according to their positions on the chain, and arrange
alphabetically, just as for alkanes.
• Indicate the position of the triple bond by giving the number of the first carbon
where it begins.
• If more than one triple bond exists, use numbers to indicate the position of
each and use the suffixes –diyne, -triyne, etc.
• Compounds containing both double and triple bonds are called enynes.
Numbering of an enyne chain begins nearer the first multiple bond,
whether double or triple. A number is used to indicate the positions of
both the double and triple bond. Where a tie exists, the double bond
receives the lower number.
7
2
1
8
4
3
5
6
5-ethyl-4-methyl-1-octen-6-yne
II. PREPARATION OF ALKYNES: ELIMINATION REACTIONS
* Alkynes can be prepared by elimination of X2 from a 1,2dihalide by treatment with excess strong base or by elimination
of HX from a vinylic halide by treatment with strong base.
ELIMINATION OF X2 FROM 1,2-DIHALIDES
H
Br
2 KOH
C
C
C
Br
C
H
+ 2 H2O
+ 2 KBr
ELIMINATION OF HX FROM VINYLIC HALIDES
 VINYLIC:
- Refers to a substituent directly attached to a double bond
H
NaNH2
C
C
C
C
Br
+ NH3
+ NaCl
III. REACTIONS OF ALKYNES: ADDITION OF HX AND X2
Based on electronic similarity between alkynes and alkenes,
you may expect the chemical reactivity of the two functional
groups to be similar. While there are indeed many similarities,
significant differences also exist.
ELECTROPHILIC ADDITION OF HX
- Reaction can be stopped after addition of 1 equivalent of HX
- Follows Markovnikov’s Rule (X adds to more sub. carbon)
- trans stereochemistry of H and X typically occurs
- Addition of excess HX forms the dihalide
CH3C ≡ CH
HBr
Br
HBr
CH3C = CH
H
add’n of 1 eq. of
HBr
Br H
CH3C ─ C─H
Br H
add’n of excess
HBr yields dihalide
ELECTROPHILIC ADDITION OF X2
- Reaction can be stopped after addition of 1 equivalent of X2
to give the di-substituted product
- Excess addition of X2 yields the tetra-substituted product
- Trans stereochemistry results
CH3CH2C ≡ CH
Br2
CH3CH2
Br
C=C
Br
H
Br2
CH3CH2CBr2CHBr2
Reaction Mechanism:
The reaction mechanism of electrophilic addition of HX to an alkyne is
similar to that of an alkene. It occurs in two steps with a vinylic
carbocation intermediate forming.
H
Br
H
RC
CH
Br
H
R
C
C
H
Br
C
R
C
H
Vinylic carbocation
intermediate
* Although the mechanism for the addition of HX to an alkyne mirrors
that of the mechanism for the addition of HX to an alkene, most other
alkyne additions occur through more complex mechanistic pathways.
IV. HYDRATION OF ALKYNES: ADDITION OF H2O
* Like alkenes, alkynes can be hydration via two different
methods. Addition of H2O in the presence of acid yields the
Markovnikov product while indirect addition of H2O via
hydroboration yields the anti-Markovnikov product. Unlike in
the case of alkenes however, the product of hydration is not
exactly an alcohol. This is due to the existence of what is
known as tautomerisation.
H
O
Rapid
O
C
C
C
H
C
Enol tautomer
Keto tautomer
(less favored)
(more favored)
 ENOL:
- a vinylic alcohol (ene + ol) (-OH is directly attached to a
double bonded carbon)
H
O
- less stable than its keto counterpart
C
C
 TAUTOMERS:
- describes constitutional isomers that interconvert rapidly
- in keto-enol tautomerism, the keto tautomer is favored
(*see example above)
*Note: tautomerisation is not the same as resonance. In resonance,
atoms do not move, only electrons do. In tautomerisation, atoms switch
places. Also, resonance is a concept of how molecules exist: they
do not actually switch back and forth between resonance structures. In
tautomerisation, molecules actually do shift back and forth between
isomeric structures.
A. Acid- Catalyzed Hydration of Alkynes: Markovnikov Product
- Reaction of an alkyne w/ H2O in the presence of acid to form
an enol at more substituted carbon (Markonikov product)
- Enol undergoes tautomerisation to give the favored ketone
product
* See board for mechanism
Reaction Mechanism:
H
H
CH3C
CH
H
H
H3C
C
H2O
C
O
+
C
H
more substituted
vinyllic carbocation
forms
H3C
H
H2O
C
H
H
HO
C
C
H3 C
H
tautomerisation
O
C
H3 C
*
CH3
*Note: This reaction is most useful when applied to a terminal
alkyne because only one product forms. When the reaction
occurs with an internal alkyne, a mixture of ketone products
form.
O
O
R
C
C
R'
H2O
C
R
H2SO4
CH2R'
an internal alkyne
+
C
RCH2
mixture
O
R
C
C
H
H2O
H2SO4
an external alkyne
C
R
CH3
single product
R'
B. Hydroboration of Alkynes: anti- Markovnikov Product
- Reaction of an alkyne w/ BH3 in the presence of H2O2/ OH─
to form an enol at less substituted carbon (Markonikov
product)
- Enol undergoes tautomerisation to give the favored keto
product
* See board for mechanism
*Again, this reaction is most useful when applied to a terminal
alkyne because only one product, in this case an aldehyde,
forms. When the reaction occurs with an internal alkyne, a
mixture of products form.
O
R
C
C
H
1. BH3
2. OH- /
an external alkyne
H2O2
C
R
H
single aldehyde
product
Reaction Mechanism:
CH3C
CH
1. BH3
BH2
H
C
H3 C
BH2
H
C
C
H
Transition State:
BH3 adds to less
substituted carbon
H3 C
2. H2O2
C
OH-
OH replaces
BH2
H
OH
H
C
C
*
H3C
H
tautomerisation
O
H3C
C
CH2 *
H
V. REDUCTION OF ALKYNES
* Alkynes are easily reduced, which means that they form an
increase in bonds to hydrogen, by addition of H2 over a metal
catalyst. As a result, alkynes can be reduced to an alkene or
further to an alkane. Depending on what reagents are used in
the reduction, either product can be selected for.
HC
CH
reduction
H2C
CH2
reduction
H3C
CH3
Ethyne
Ethene
Ethane
alkyne
alkene
alkane
A. Complete Reduction: Alkyne to Alkane
- Reduction to the alkane occurs when the alkyne is reacted w/
H2 in the presence of palladium on carbon (Pd/C) as a catalyst
H2
Pd/C
alkyne
alkane
B. Incomplete Reduction: Alkyne to Alkene
PATH 1: Formation of Cis Alkenes
- Incomplete reduction to the alkene occurs when the alkyne is
reacted with H2 in the presence the less active Lindlar catalyst
- The use of H2/ Lindlar catalyst produces CIS alkenes.
H
H
H2
Lindlar
alkyne
catalyst
cis alkene
PATH 2: Formation of Trans Alkenes
- Incomplete reduction to the alkene also occurs when the alkyne
is reacted w/ Na or Li metal in liquid ammonia (NH3).
- The use of Na or Li/ NH3 produces TRANS alkenes.
H
Li
NH3
alkyne
H
trans alkene
VI. OXIDATIVE CLEAVAGE OF ALKYNES
* Alkynes, like alkenes, can by cleaved by reaction with a
powerful oxidizing agent like ozone. A triple bond is generally
less reactive that a double bond however, and yields of
cleavage products are sometimes low. Like alkenes, the
products of alkyne cleavage produce carbonyls, but as part of
a different functional group.
Oxidative Cleavage of Alkynes:
- The products of cleavage of an internal alkyne (R─C≡C─R’) are
two carboxylic acids
- The products of cleavage of an external alkyne (R─C≡C─H) are a
carboxylic acid and CO2
O
O3
1
2
+
Zn/H3O
O
+
1 OH
HO 2
internal
alkyne
H
1
2
external
alkyne
O3
Zn/H3O+
O
1 OH
+
CO2
2
VII. ALKYNE ACIDITY: FORMATION OF ACETYLIDE ANIONS
* According to the Brønsted- Lowry definition, an acid is any
substance that donates H+. Although we usually think of
oxyacids (H2SO4, H3PO4) or halogen acids (HCl, HBr) in this
context, any compound containing a hydrogen atom can be an
acid under the right circumstances.
The most striking difference between alkynes and alkenes and
alkanes is that terminal alkynes are weakly acidic.
Acidity of Simple Hydrocarbons
Type
Example
Ka
pKa
Alkyne
HC ≡ CH
10─25
25
Alkene
H2C= CH2
10─44
44
Alkane
H3C− CH3
10─60
60
Stronger acid
Weaker acid
When a terminal alkyne is treated with a strong base, such as sodium
amide (Na+NH2─), the terminal hydrogen is removed and an
ACETYLIDE ANION is formed:
R
C
C
external
alkyne
H
NH2 Na
R
C
C Na
NH3
Acetylide
anion
The presence of a negative charge and an unshared electron pair
nucleophilic
on carbon makes an acetylide anion extremely ________________.
 ALKYLATION REACTION:
- Substitution reaction in which a new alkyl group becomes
attached to the starting alkyne
- The nucleophilic acetylide anion attacks a positively
polarized carbon atom in a haloalkane
- This a BIG deal a new carbon – carbon bond is formed!
H
H
R
C
C Na
H
C
Br
 
H
R
C
C
C
H
H
NaBr
The reaction conditions for acetylide anion formation and
alkylation are often shown together as a two step reaction process.
acetylide anion formation
CH3CH2CH2C
CH
1. NaNH2
2. CH3CH2Br
alkylation
CH3CH2CH2C
CCH2CH3
-
Acetylide anion alkylation is limited to primary alkyl bromides
and iodides.
-
In addition to their reactivity as nucleophiles, acetylide anions are
sufficiently strong bases that they can cause dehydrohalogenation
(loss of H + halogen) instead of substitution when they react with
secondary and tertiary alkyl halides.
R
H
+ NaBr
H
substitution product (acetylide
anion undergoes alkylation)
H
Br
H
R
C
C
Na
H
H
H
+ HBr
o
2 alkyl bromide
H
elimination product (acetylide
anion acts as a base)