2. Introduction
Alkynes are hydrocarbons that have at
least one triple bond, C≡C with the
formula CnH2n-2.
Disubstituted alkynes, R-C≡C-R', are
described as "internal" alkynes;
Monosubstituted alkynes, R-C≡C-H,
are described as "terminal" alkynes.
3. Because of its linear configuration, ten-
membered carbon ring is the smallest that
can contain the alkyne function without
excessive strain.
ethyne (or acetylene) propyne 1-butyne
H-C C-H H-C C- CH3 H-C C-CH2-CH3
2-butyne
Terminal
Internal
CH3-C C-CH3
4. Quizz
Classify each of the following as an internal or
a terminal alkyne:
a) 1-hexyne b) 3-octyne
c) cyclooctyne d) propyne
5. Nomenclature
Akynes are named by general rules similar
to those used for alkanes and alkenes.
The suffix -yne is used in the paren
hydrocarbon name to denote an alkyne,
and the position of the triple bond is
indicated by its number in the chain.
6. Numbering begin at the chain end near the
triple bond so that the triple bond as low a
number as possible.
Compounds containing both double and
triple bonds are called enynes.
CH3CH2CHCH2C CCH2CH3
CH3
Begin numbering at
the end nearer the
triple bond
6-Methyl-3-octyne
7. Numbering of the hydrocarbon chain starts
from the end nearer the first multiple
bond, whether double or triple.
If there is a choice in numbering, double
bonds receive lower numbers than triple
bonds. For example,
HC CCH 2CH2CH2CH CH2
CH3
1-Hepten-6-yne
1
6
7
HC CCH 2CHCH2CH2CH CHCH3
4-Methyl-7-nonen-1-yne
1 6 7 9
8. Preparation of Alkynes
Alkynes are generally prepared by
* Dehydrohalogenation of either geminal
(1,1-) or vicinal (1,2-)alkyl dihalides
(usually Br or Cl), using a strong base,
usually NaNH2.
These reactions are typically E2 reactions
and occur via an alkenyl halide.
9. * The reaction of metal acetylides with
primary alkyl halides.
C C Na + R- X C C R
10. Quizz
What is the alkyne product from the reactions
of the following with NaNH2:
a) 2,2-dibromopropane
b) 1,2-dibromohexane
c) 1,1-dibromooctane
d) 2,3-dibromohexane
11. Structure and Reactivity
The alkyne functional group consists of two
sp hybridised C atoms bonded to each other
via one σ and two bonds.
The 2 bonds are produced by the side-to-
side overlap of the two pairs of p-orbitals
not utilised in the hybrids.
The diagram shows the heats of hydrogenation
of 2-butyne (in the slide: blue in units of kcal/mole)
12. It indicates that alkynes are thermodynamically
less stable than alkenes
to a greater degree than
alkenes are less stable
than alkanes.
The "extra" bond in an alkyne is weaker that an
alkene bond:
2 x 28.3 (C=C) – 65 (C≡C)= 8.4 kcal
13. The 2 C of the C≡C and the 2 atoms
attached directly to the C≡C are linear, so
they cannot exist as cis-/trans- isomers.
Two separate perpendicular p molecular orbitals
σ
σ
14. The bonds are a region of high electron
density; so it allows an acetylinic carbon
to have a greater amount of
electronegative character.
As a result, alkynes are typically
nucleophiles.
Terminal alkynes, R-CC-H, are quite
acidic (pKa = 26).
15. Reacting with a strong base such as
sodium, sodium amide, n-butyllithium or a
Grignard reagent, a terminal alkyne gives
the anion of the terminal alkyne (a metal
acetylide):
2 RC ≡ CH + 2 Na → 2 RC ≡ CNa + H2
16. The acetylide ion is a good nucleophile and
can be alkylated to give higher alkynes.
Like alkenes (C=C), the alkyne C≡C
undergoes a variety of addition reactions in
which one or both of the bonds are
converted to new σ bonds.
17. Alkylation of Alkynes
The acetylide carbanion, RC≡C-, is a good
C-nucleophile and can undergo
nucleophilic substitution reactions (usually
SN2) with 1o or 2o alkyl halides (Cl, Br or I)
to produce an internal alkyne.
18. One or both of the terminal H atoms in
ethyne (acetylene) H-C≡C-H can be
susbtituted providing monosubstituted (R-
C≡C-H) and symmetrical (R=R') or
unsymmetrical (R ≠ R') disubstituted
alkynes R-C ≡ C-R'
19. Mechanism for Alkylation of alkynes
Step 1. The amide ion acts as a
base removing the acidic terminal
H to generate the acetylide ion,
a carbon nucleophile.
Step 2. The carbanion reacts
with the electrophilic carbon in the
alkyl halide with loss of the leaving
group, forming a new C-C bond.
20. Practice Problems
What is the product of the reactions of CH3-C≡C- with each of the following:
a) 2-bromopropane (d) ethanol
b) 1-iodooctane e) ethyl tosylate
c) (R)-2-bromohexane f) bromobenzene
21. Addition Reactions of Alkynes
Alkynes undergo addition reactions in an
analogous fashion to those of alkenes.
Two factors influence the relative
reactivity of alkynes compared to alkenes:
increased nucleophilicity of the starting
system (C≡C vs C=C), and
22. stability of any intermediates
(for example, carbocations).
Since alkynes are thermodynamically less
stable than alkenes, we might expect
addition reactions of alkynes to be more
exothermic and relatively faster than
equivalent reactions of the alkenes.
23. 1. Hydrogenation of Alkynes
Alkynes can be partially reduced to cis-
alkenes with H2 in the presence of poisoned
catalysts,
eg. Pd / CaCO3/ quinoline which is also
known as Lindlar's catalyst.
Alkynes can be reduced to alkanes with H2
in the presence of catalysts (Pt, Pd, Ni etc.)
24. The reaction is stereospecific
giving only the syn addition
product since the new C-H σ
bonds is formed simultaneously
from H atoms absorbed onto the
metal surface.
25. 2. Dissolving Metal Reduction of Alkynes
The reaction of Na in NH3(l) with alkynes
occurs stereospecificly giving only the
trans-alkene via an anti-addition.
26. The reaction proceeds via single
electron transfer from the Na with
H coming from the NH3.
These reaction conditions do not
reduce alkenes.
27. 3. Reaction of Alkynes with HX
When treated with HX alkynes form vinyl
halides.
Hydrogen halide reactivity order :
HI > HBr > HCl > HF
(paralleling acidity order).
28. In the presence of excess HX, a second
addition can occur to the product alkene
giving a geminal dihalide.
Regioselectivity predicted by
Markovnikov's rule with the H adding to
the C with the most H already present.
29. Reaction proceeds via protonation to give
the more stable carbocation intermediate.
Not stereoselective since reaction proceeds
via planar carbocation (CH3C+=CH2 &
CH3CBr+CH3).
30. Quizz
What would be the product from the
reaction of 2-butyne with excess HBr ?
31. For HBr, in the presence of radicals
(compounds such as peroxides) the
radical addition occurs with opposite
regiochemistry:
Quizz. Why does this reaction have the
opposite regiochemistry ?
33. 4. Hydration of Alkynes
Alkynes can be hydrated to form enols that
immediately tautomerise to ketones.
Reagents: aq. acid, most commonly
H2SO4, with a mercury salt.
34. Regioselectivity predicted by
Markovnikov's rule.
Reaction proceeds via protonation to give
the more stable carbocation intermediate.
CH3-C≡CH + H+ CH3-C+= CH2
Not stereoselective since reactions proceeds
via planar carbocation.
35. 5 . Halogenation of Alkynes
Overall transformation: C≡ C to X-C=C-X
(and potentially to X2C-CX2).
Reagent: normally the halogen (e.g. Br2) in an
inert solvent like methylene chloride, CH2Cl2.
36. Reaction proceeds via cyclic
halonium ion.
Stereoselectivity : anti since
the two C-X bonds form in
separate steps one from X2,
the other X-.
37. 6. Ozonolysis of Alkynes
Overall transformation : C≡C to 2 x CO2
Ozonolysis implies that ozone causes the
alkyne to break (-lysis)
38. Reagents : ozone followed by aqueous
work-up.
Note that each of the CC bonds in the CºC
becomes a C=O bond.
39. Problems at home
Classify each of the following as an internal or a terminal
alkyne (from the slide 4)
a) 1-hexyne b) 3-octyne
c) cyclooctyne d) propyne
What is the alkyne product from the reactions of the
following with NaNH2: (from the slide 10)
a) 2,2-dibromopropane
b) 1,2-dibromohexane
c) 1,1-dibromooctane
d) 2,3-dibromohexane
40. What is the product of the reactions of CH3-C≡C-
with each of the following: (from the slide 20)
a) 2-bromopropane (d) ethanol
b) 1-iodooctane e) ethyl tosylate
c) (R)-2-bromohexane f) bromobenzene
What would be the products of the ozonolysis reactions of:
(a) ethyne ? (b) 1-butyne ?
(c) 2-butyne ? (d) cyclooctyne ?
