Download Notes on Alkynes

Chemistry 212
Clark College
Notes on Alkynes
Compounds with triple bonds afford a variety of features above those of an alkene – a linear geometry,
an acidic C–H proton, C – C bond forming reactions and traditional addition reactions. We will explore
the structure, acidity and reactions of alkynes, after a brief overview of the nomenclature of alkynes.
Nomenclature
Molecules with one triple bond have the suffix –yne for the name of the molecule, with a number
preceding the parent name indicating the position of the start of the triple bond. Molecules with more
than one triple bond are named as diynes, triynes, etc…, with numbers for each triple bond.
6-bromo-5,5-dimethyl-2-heptyne
1,4-hexadiyne
Br
If multiple functional groups are present, the triple bond is often the lower priority. Non-carbon
functional groups, such as alcohols, ketones and aldehydes, take precedence over a triple bond, as do
double bonds.
O
R-3-methylpent-4-yn-2-one
Z-3-methyloct-6-yn-3-ene
Structure and Acidity of the Alkyne
sp-hybridized carbons give rise to two pi bonds
Increased s-character in the C–H bond increases the acidity of that bond, as the anion formed has
extra electron density in a short sp-hybridized orbital, close to the nucleus. As a result, the pKa of an
alkyne H is approximately 25. This proton can then be removed by a strong nitrogen base.
R
H
Na
NH2
H
NH3
Keq = 10+13
The acidity of this proton can be exploited as it generates a carbon nucleophile. We will explore
nucleophilic substitution reactions with the alkynyl ion.
Reactions of Alkynes
Preparation of Alkynes
In general, an alkyne can be prepared from an geminal or vicinal dihalide through a double-elimination
process.
X X
A geminal dihalide:
The two halogens are
attached to the same
carbon.
X
A vicinal dihalide:
X The two halogens are
attached to neighboring
carbons.
Alkynes can be prepared in a two-step process from alkenes:
Br
H
2 NaNH2
Br2/CH2Cl2 -orHBr/H2O2
H or fused KOH
Br
Note that in order to form an alkyne, the starting alkene must have at least one H on either side of the
double bond. If a terminal alkene is used to make a terminal alkyne, the acidity of the alkyne must be
considered as a competing reaction. To compensate for this, additional base is added to completely
Alkyne Notes
Chapter 7
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Chemistry 212
Clark College
form and deprotonate the triple bond, and water is added to reprotonate the alkyne and quench the
reaction.
Br
1) 3 NaNH2
H 2) H O
2
H Br
H
An internal alkyne can be isomerized to a terminal alkyne through the “zipper reaction,” simply by
heating an internal alkyne with excess NaNH2 and quenching the reaction with water.
1) xs NaNH2
2) H2O
H
The "zipper" reaction isomerizes an internal
alkyne into a terminal alkyne. The bonds zip
up and down an unbranched chain until water
is added, which traps the alkyne at the end.
Nucleophilic Substitution Reactions
After deprotonating the alkyne CH, the anion can be used as a nucleophile/Lewis base in a substitution
reaction. The electrophile/Lewis acid for this reaction is a 1° alkyl halide–the partial positive charge on
the carbon, caused by the dipole of the C-X bond, attracts the alkyne anion. As the new carbon-carbon
bond is formed, the carbon-bromine bond breaks, leaving a bromide ion in solution.
1) NaNH2
H
2)
The substrate for this SN2 reaction
must be a 1° bromide.
Br
Addition Reactions
With two pi bonds, alkynes often mimic the behavior of alkenes, but twice as much! The presence of
the pi bond allows alkynes to undergo electrophilic addition reactions, using either one or both sets of pi
electrons. These reactions still follow the rules of addition with respect to carbocation stability – the
more substituted carbocation is formed as the intermediate of the addition reaction.
Halide additions
Halides add to alkynes to result with the Markovnikov product. Sequential additions can be somewhat
controlled with reaction stoichiometry – 1 mole of HX or X2 adds once, 2 or more (excess, or xs) moles
will add twice. If the alkyne is terminal, only one product, the Markovnikov product, will result.
However, an internal alkyne will result in a mixture of products.
excess H-Br
H
a geminal dihalide
Br
Br
excess H-Br
Br Br
Br
excess Br2
Br
Br Br
Br
Br
Br
1 equiv Br2
LiBr, CH3COOH
Alkyne Notes
Br
Chapter 7
This reaction can be controlled
by stoichiometry and the addtion
of LiX and acetic acid.
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Chemistry 212
Clark College
Addition of Water
Additions of water occur only once to the alkyne, diols do not form. Upon the addition of the first –OH,
an enol forms. This enol exists in equilibrium with a keto (carbonyl) form. This equilibrium, called a
tautomerization, heavily favors the carbonyl form, such that it is the only form isolated. At this point,
there is no alkene, so a second addition does not occur.
Once again, products change with reagent, and whether the alkyne is internal or terminal.
OH
O
This transformation of internal alkynes,
with the exact same products can be
done with H2SO4/HgSO4 or with 1) BH3,
2) OH-, H2O2, H2O
O
OH
With terminal alkynes, the lack of steric bulk requires specific reagents, however it will also allow us to
specify a product.
OH
1) HB(sia)2
H 2) NaOH, H2O2,
H 2O
H
HgSO4
H2SO4
O
H
The anti-Markovnikov product
yields an aldehyde. A modified
H boron reagent must be used to
ensure a clean transformation.
Oxymercuration results in a 2-ketone
H
O
OH
Reduction/Addition of Hydrogen
Addition of hydrogen to a triple bond can generate the alkane, the E alkene or the Z alkene, depending
on the reagent.
H2, Pt
Pd or Ni
H2, Lindlar
catalyst
Lindlar catalyst = Pd/CaCO3, Pb2+
1) BH3
2) CH3COOH
Na0, NH3
-78°C
The preceeding reactions show how to apply alkene reactions to alkynes, and what modifications must
be made to deal with the triple bond. However, one of the most useful features of the triple bond rests
in the enhanced acidity of the sp-hybridized C-H bond. Upon deprotonating, we now have a great
carbon nucleophile that we can use in a SN2 reaction to make a new carbon-carbon bond. Alkynes are
very useful in organic synthesis for this reason; they provide a method to make carbon-carbon bonds,
and introduce a new functional group that can be transformed into others.
Alkyne Notes
Chapter 7
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