CHEM 3013
ORGANIC CHEMISTRY I
LECTURE NOTES
CHAPTER 8
1.
The Alkyne Functional Group
Alkynes are compounds with carbon-carbon triple bonds.
Recall that alkanes use sp3 hybrid orbitals and that alkenes use sp2 hybrid orbitals. The
alkynes form bonds using a third type of hybrid orbital, the sp orbital. The consequence of this kind
of bonding are two fold: (1) Because of the extra "electron glue" provided by the two sets of πelectrons, the alkyne bond is the shortest C-C bond known (about 1.2Å, as compared with 1.33Å for
alkenes and 1.54Å for alkanes). and (2) Alkynes are incapable of Cis/Trans geometric isomerism
because of the resulting linear configuration of the substituents.
H C
C H
Acetylene (an alkyne)
E 2p
N
E
R
G
Y
2p
mix 2s and
2p orbitals
2sp
2s
1s
1s
Elemental
Carbon
sp Hybridized
Carbon in Alkynes
π-bonds (two 2p orbitals on carbon
H
H
σ-bond (1s orbital
on hydrogen and sp
orbital on carbon)
σ-bond (two sp orbitals on the carbons
Bonding in Alkenes
2.
Nomenclature of Alkynes
The simplest possible alkyne is C2H2 is known by the common name of acetylene. Another
name for alkynes as a class is acetylenes.
a.
IUPAC Nomenclature
The systematic nomenclature of alkynes is similar to alkenes.
1
2
1. Name the longest chain containing the triple bond
2. The suffix -ane is replaced by -yne and the triple bond is given the lowest possible
number.
3. When double and triple bonds are present in the same molecule, the principal chain is the
carbon chain containing the greatest number of double and triple bonds.
4. The compound still ends with the suffix -yne
5. Precedence is given to the naming scheme which gives the lowest number for the first
multiple bond regardless whether it is double or triple.
6. In naming the compounds, the double bond is named first...dropping the last e of the -ene
suffix.
C C
C
C
The eight membered
ring is the smallest
stable cyclic alkene
2,6-dimethyl-hept-2-yne
3,6,8-trimethyl-4-isopropylcyclooctyne
C C
C C
trans- 5-nonen-3-yne
(Z)-3-ethyl-2-nonen-6-yne
Compounds that contain both double and triple bonds are named as alkynes.
Precedence is given to the naming scheme which gives the lowest number
for the first multiple bond regardless if it is a double or triple. In naming, the
double bond is cited first. If both double and triple bonds are at equivalent
the double bond takes precedence.
Alkyne Nomenclature
b.
Alkynes as Substituents
Side-chain groups containing a triple bond are called alkynyl groups and are named by
replacing the final -e and adding the suffix -yl.. The alkynyl group is numbered outward from the
point of attachment to the principal chain.
4'
H
6
4
5
H3C
3'
2'
3
1
2
C
CH2
CH3
C
Named as a substituent;
Fewer carbons than ring
1'
H
trans-3-(2-butynl)-6-methyl-cyclohexene
Naming Alkynes as substituent groups
c.
Properties of Alkynes
Terminal alkynes (those that have a C-H bond) are less stable than internal alkynes. The
situation is similar to alkenes (recall that alkene stability increases with alkyl substitution), and for the
same reason: Hyperconjugation. Overlap of π-type orbitals with SP3 Hybridized orbitals on adjacent
alkyl groups leads to a stabilizing interaction.
Structural Isomers
H C C CH2CH2CH3
H3C C C CH2CH3
2-Pentyne
Internal Alkyne
1-Pentyne
Terminal Alkyne
E
N
Heats of Hydrogenation
E
R
H C C CH2CH2CH3 + 2 H2
H3C CH2CH2CH2CH3 G
∆H˚ = -44.2 Kcal/mole
Y
H3C C C CH2CH3 + 2 H2
H3C CH2CH2CH2CH3
∆H˚ = -40.9 Kcal/mole
1-pentyne
2-pentyne
3.3 Kcal
mole
pentane
Internal alkynes are more stable than terminal alkynes because of Hyperconjugation.
H
H
C
C
C H
H
Energy Differences in Alkynes
3.
Synthesis of Alkynes
There are relatively few general methods of alkyne syntheses, the best two are: (1) alkylation
of acetylene (to be discussed somewhat later), and (2) Twofold elimination of HX from dihalides.
Acetylene itself can be made by a simple process of hydrolysis of calcium carbide as described
by the following reaction: CaC2 + 2 H 2O → H-C≡C-H + Ca(OH)2. Acetylene is a gas, and was
one of the first illuminating gases used in gas lights. The reaction described above is also the source
of acetylene used in "carbide cannons" which are often fired off when a touchdown is scored during
a football game. A small amount of CaC2 is added to water, acetylene is generated and then ignited in
an enclosed space. Thus creating the BOOM.
Recall that the elimination of HX from an alkyl halide (upon treatment with a base) will result
in the formation of alkenes. The π-bond systems of alkynes can be prepared in much the same
manner. Since an alkyne has two π-bonds however, we have to eliminate two molecules of HX.
This is usually done by treatment of a vicinal dihalide with an excess of strong base. Vicinal dihalides
are the starting material of choice because they are readily available by the addition of bromine or
chlorine to an alkene. Thus the overall method of halogenation-dehalogentaion provides
an excellent route for going from an alkene to an alkyne.
Although different bases can be used for this double elimination of 2HX from dihalides, salts
of the amide anion (NH2-) are usually preferred because it's strongly basic nature gives higher yields
of products.
The twofold elimination process occurs in two discrete steps, passing through a vinyl halide
intermediate.
3
4
Alkynes via gem-dihalides
H3C
CH3 2 eq. strong base
C
Cl Cl
H3C C C H
Alkynes via vicinal dihalides
H
H3C
Cl
C
CH3 2 eq. strong base
C
H
H3C C C CH3
Cl
NH2- = Amide anion....conjugate base of ammonia (NH3)
pKa (NH3) = 35
Mechanism
H
H
H3C
Cl
C
H
C
CH3
+ NH2
H3C
Cl
C
C
CH3
+ NH2
Cl
Alkynes are formed via two consecutive
elimination reactions.
H3C C C CH3
Alkynes via Elimination Reactions of Dihalides
4.
Electrophilic Reactions of Alkynes
a.
Addition of HX and X2
The chemistry of alkynes is dominated by electrophilic addition reactions similar to that of
alkenes. For example, HX can be added to alkynes to give vinylic halides as products. However,
HX will also react with the alkene product ( alkynes are somewhat less reactive than alkenes to
electrophilic addition) to result in formation of a geminal dihalide (both halogens on the same carbon).
Thus every addition reactions to alkynes will result in some formation of dihalide. The reaction
leading to vinyl halide can be maximized by avoiding an excess of HX .
The initial addition process follows Markovnikov's rule where applicable. That is, when
terminal alkynes react, the vinyl halide produced will be the one in which the H+ (electrophile) has
added to the terminal carbon . The reason for this alkyne regioselectivity is directly analogous to that
described for alkenes. The electrophile will add so as to form the more stable vinyl
cation intermediate. As for simple carbocations, the vinyl cation with the greater amount of alkyl
substitution at the carbon bearing the + charge will be more stabilized through hyperconjugation.
While regioselectivity is not observed in electrophilic addition to internal alkynes ( the two
possible vinyl cations in this case are of equal stability), the reaction often shows stereoselectivity.
The vinyl halide produced by addition of HX to an internal alkyne very often (but
not always) shows a trans relationship of the H and X.
The electrophilic addition of X2 to alkynes leads to vinylic dihalides (alkenes with halogens
on each carbon of the double bond). The stereoselectivity of this reaction again leads to trans
products.
HC CCH2CH2CH3
HCl
CH3COOH
H2C CClCH2CH2CH3
Additions follow Markovnikov's Rule
HCl
H3C C CCH2CH2CH3 CH COOH
3
H
CH2CH2CH3 Cl
+
H3C
Cl
H3C
CH2CH2CH3
H
Additions usually (not always) give trans
stereochemistry of H and X
HC CCH2CH2CH3
HCl (XS)
CH3COOH
CH3CCl2CH2CH2CH3
Excess of acid leads to a dihalide product
through two consecutive additions.
H3C C CCH2CH2CH3
Br2
Br
CCl4
H3C
CH2CH2CH3
Br
Halogens add to alkynes to give addition
products with trans stereochemistry...
Watch out for further addition when XS
reagent is present!
Addition of HX and X2 to Alkynes
5
6
Br-
Br
H
H C C H + H+ Br-
H C C
H
H
1st Step:
Electrophilic addition
of H+ to Alkyne
π-bond.
H
C C
H
VINYL CATION
2nd Step:
Nucleophilic attack of
Br- to vinyl cation
Markovnikov Addition
H
R C C
R C C H
HBr
Br
Br-
H
H
Br-
H C C
H
C C
R
H
Br
H
Observed
Product
C C
R
H
R
Alkynes will add the electrophile so as to give the most stable vinyl cation
Mechanism of Electrophilic Addition to Alkynes
R
>
R
H
H
R
>
R
~R
H
R
Most
Stable
R
H
H
>
C C
H
STABILITY
~
H
H
H C C
H
H
Least
Stable
Vinyl Cations are less stable than similarly substituted alkyl carbocations.
This accounts for the somewhat lower reactivity of alkynes towards
electrophilic reagents (Hammonds Postulate)
Vinyl Cation Stability
b.
Hydration of Alkynes
Like the acid-catalyzed addition of water to alkenes, water can be added to alkynes. However,
because of the somewhat lower reactivity of the alkyne, the reaction must be carried out with the use
of the extremely electrophilic Hg(II) to initially activate the alkyne (see oxymercuration-demercuation
of alkenes). The reaction follows a Markovnikov process, with initial production of a vinyl cation
which is subsequently trapped by water (acting as a nucleophile) to give an enol (vinyl alcohol). This
enol is unstable and undergoes a TAUTOMERIZATION (rearrangement in which a new structural
isomer is formed by shift of a hydrogen atom and a π-bond) to form a ketone. Since the reaction
follows a Markovnikov regiochemistry, a methyl ketone is produced from the reaction of a
terminal alkyne.
7
R C C H
R1 C C R2
HgSO4
H+/H2O
HgSO4
H+/H2O
O
R C CH3
O
O
R1 C CH2R2 + R1H2C C R2
The acid catalyzed hydration of alkynes results in the formation of
ketones...... Terminal alkynes
methyl ketones
Internal alkynes
ketone mixture
MECHANISM
R C C H + Hg+2SO4-2
Hg
R C C
OH2
H
Markovnikov type addition of mercuric
ion to alkyne, resulting in formation of
most stable vinylic cation
-H+
Rapid
H O H
O
Rearrangement
C C
R C CH3
8
Keq = 10
R
H
KETO
Tautomer
H
H O Hg
C C
R
H
H30+
H O Hg
C C
R
H
Tautomers are special kinds
ENOL
Tautomer of constitutional isomers
which can rapidly interconvert.
Hydration of Alkynes
c.
Hydroboration of Alkynes
Hydroboration of terminal alkynes is an important reaction for the synthesis of aldehydes.
It should be noted that this reaction is complimentary to the mercury catalyzed hydration described just
previously. The reagent based specificity allows complete regiochemical (and
product) control.
R C C H
HBR2
THF
R1 C C R2 HBR2
THF
O
R C H
O
O
R1 C CH2R2 + R1H2C C R2
H2O2
NaOH,H2O
H2O2
NaOH,H2O
The hydroboration of alkynes results in the formation of:
Terminal alkynes
aldehydes
Internal alkynes
ketone mixture
MECHANISM
H BR2
R C C H
R C C H + HBR2
HBR2 = disiamylborane
A sterically large hydroborating agent; can stop
at the mono adduct
H2O2
NaOH,H2O
H OH
R C C H
ENOL
Tautomer
Keq = 106
H O
R C C H
H KETO
Tautomer
B H
Hydroboration of Alkynes
O
R C CH3
HgSO4
H3O+
Methyl Ketone
R C C H
HBR2
THF
H2O2
NaOH,H2O
H O
R C C H
Aldehyde
H
Complimentary Reactions of Terminal Alkynes
5.
Catalytic Hydrogenation of Alkynes
Alkynes can be catalytically hydrogenated (reduced) to yield alkenes or alkynes. Because the
reductions of alkenes and alkynes occur with almost equal facility (actually, alkynes can be reduced
slightly easier), it is often difficult to stop the reduction of an alkyne at the alkene stage when using
standard catalysts. Complete hydrogenation of the triple bond to yield an alkane is done through the
use of two equivalents of H2 and a standard catalyst. Partial reduction of the alkyne
(stopping at the alkene stage) can be carried out through the use of a Lindlar
Catalyst. In the Lindlar catalyst, a "poison" (usually an aromatic amine such as pyridine or
quinoline) has been added to the catalyst to lower its activity, further reduction is prevented.
The stereoselectivity of this reaction is determined by SYN addition of the complexed
hydrogen to the sme face of the alkyne π-bond. Cis alkenes are formed.
8
9
H2, Pd/C
EtOH
R C C H
RH2C CH2 + R CH2 CH3 + UNREACTED ALKYNE
The catalytic reduction of alkynes, using standard hydrogenation catalysts gives a mixture of
products: alkenes (from one equivalent of H2 uptake), alkanes (from two equivalents of H2
uptake) and unreacted alkyne (all H2 used up).
The use of two equivalents of H2 will give only alkanes
2 H2, Pd/C
EtOH
R C C H
R CH2 CH3
Catalytic hydrogenation of alkynes can be stoped at the alkene stage through the
use of Lindlar Catalysts (Deactivated Catalyst)
H2
Lindlar
catalyst
R C C H
Lindlar catalyst :
Pd/Caso4 deactivated
by addition of poison
(quinoline)
RH2C CH2
Catalytic Hydrogenation of Alkynes
R1
C C R2
H2
Lndlar
catalyst
R1
R2
C C
H
H
SYN addition leads to cis stereochemistry
R1
H2
Metal surface
attraction weakens
H-H bond
C
H
C
R2
H
Activated H2
adds to alkyne
π-system
R1
R2
C C
H
H
+
Catalyst
regenerated
SYN Addition of Alkynes
6.
Dissolving Metal Reductions of Alkynes
In contrast to the catalytic hydrogenation method, which yields cis-alkenes from alkynes, the
reaction of alkynes with sodium or lithium metal in liquid ammonia produces the
complimentary trans-alkene.
The mechanism of this reaction involves the addition of electrons (given up by the
electropositive metal) to the π-system of the alkyne to from an intermediate radical-anion which reacts
with the ammonia solvent by pulling off a proton. The resulting radical, in turn, undergoes further
reduction to form a vinyl anion. Which again reacts with ammonia, by pulling off a proton, to give
the final trans-alkene.
The trans nature of the reaction occurs because in the radical anion intermediate, the orbital 10
with the lone-pair and the orbital with the single electron want to be as far apart as possible (in order
to minimize repulsions). Thus they take on a trans arrangement.
R1
H
1) Li, NH3
C C R2
2) H2O
R2
R1
Trans only
H
"Dissolving Metal"
Li + NH3
Li+ + e- (NH3)n
Electropositive metals such as Li, Na will ionize in a liquid ammonia solution,
giving up an electron which becomes "solvated " by ammonia molecules
Mechanism
R1
solvated
R1 C C R2 electrons
NH3
R2
R1
as an acid
R1
H
H
R2
NH2 +
+ NH2-
R2
Radical
Anion
The radical and
lone-pair get as
far apart as possible
-
H
solvated
electrons
reduction of
radical to
anion
NH3
as an acid
R1
H
R2
Dissolving Metal Reduction of Alkynes to trans Alkenes
7.
Oxidative Cleavage of Alkynes
Powerful oxidizing agents such as ozone or acidic KMnO4 will cleave the triple bonds of
alkynes. The triple bond is generally less reactive than is the double bond so yields of this reaction
may be low. If a terminal alkyne is utilized in this reaction, a carboxylic acid and CO2 is produced.
R C C H
R1
O3 or
KMnO4
O3 or
C C R2 KMnO
4
O
R C OH + CO2
R1
O
C OH + R2
O
C OH
Alkynes undergo oxidativecleavage just like alkenes...
However, they are somewhat
less reactive and the yields of
cleavage products can be low.
Oxidative Cleavage of Alkynes
8.
Alkylation of Alkynes
a.
Acidity of Alkynes
One of the striking differences in the chemical behavior of alkenes and alkynes is their
differences in acidities. Terminal alkynes have hydrogens which are many times more acidic than the
hydrogens of alkanes or alkenes. This is a result of the increased s character in the sp bond between
the carbon and hydrogen of terminal alkynes. The anion ( which is the conjugate base of a terminal 11
alkyne) is more stable because the electron pair is closer to the positive charged carbon nucleus.
Thus, the acidic hydrogen of terminal alkynes can be removed by a strong base (such as NH2-) to
form the acetylide anion : R-C≡C:Hydrocarbon
Ka
pKa
HC C H
10-25
25
H2C CH H
10-44
44
H3CH2C H
10-60
60
Conjugate Base
Stronger Acid
HC C
More Stable
H2C CH
Weaker Acid
H3C H2C
Less Stable
Carbon centers with increasing s orbital character will best stabilize carbanions
because the negative anion is closer to the positive nucleus.
Acetylide Formation: A strong base is able to remove a terminal hydrogen from
an alkyne resulting in formation of the conjugate base called
acetylide anion.
R C C H
NaNH2
NH3
R C C Na+ + NH3
NH2- = Amide Anion... A very strong base (about 1015 x stronger than HO-)
NH3 pKa = 35
Acidity of Alkynes
b.
Alkylation of Acetylides
By using the parent acetylene or a terminal alkyne, it is possible to introduce
one, or two alkyl groups and thus form an entirely new alkyne.
The presence of an unshaired pair of electrons on an acetylide anion makes the carbon strongly
nucleophilic. Thus acetylide anions ( R-C≡C:-) can react with alkyl halides such a bromomethane
(CH3-Br) to substitute for the halogen a yield a new alkyne product ( R-C≡C-CH3).
H C C H
KNH2
NH3
Amide anion abstracts
proton...forms the
acetylide anion.
H3C Br
H C C
Acetylide acts as a
nucleophile...forms
a new carbon-carbon
bond (Alkylation)
A second alkylation
step leads to a new H C C C CH
3
carbon carbon bond. 3
Alkylation of Alkynes
∂+ ∂-
KNH2
NH3
Amide again
abstracts
a proton
Br- CH
+3
∂
H C C CH3 + Br-
∂
C C CH3
12
O
CH3CCH3
Cl
PCl5
CH3CCH3
NaNH 2
Cl
∆
Br Br
CHCCH3
Br Br
CO2
HgSO4
H2 O
H2 SO4
(1) O3
(2) Zn / H2 O
HC CCH3
NaNH 2
liq NH3
2 HCl
2 Br2
O
CCH3
HO
+
1-propyne
Cu(NH 3 )4 OH
+ -
Cu C CCH3
+ -
Na
CH3 CH2 Br
Ag(NH3 )2 OH
NaNH 2
∆
CH3CH2C CCH3
+ -
Br Br
CH2CHCH3
Ag C CCH3
CH3
Li in liq NH3
CH3C CCH3
H
C C
H
CH3
H
H
H2 , Ni 2 B
C C
or
CH
CH3
3
Pd/CaCO3
H2 , Quinoline
Alkyne Reaction Summary
C CCH3