Alkynes

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Alkynes Chapters 7,8
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Outline 1. Bonding in alkynes.
sp-Hybridization 2. Physical properties and
nomenclature of alkynes 3. Preparation and use
of alkynes 4. Addition to triple bonds.
Hydrohalogenation 5. Addition to triple bonds.
Hydration 6. Addition to triple bonds.
Hydroboration-oxidation 7. Addition to triple
bonds. Halogenation 8. Addition to triple bonds.
Hydrogenation 9. Reduction of triple bonds.
Trimerization of triple bonds 10. Acidity of
alkynes
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1. Bonding in alkynes. sp-Hybridization 2.
Physical properties and nomenclature of
alkynes 3. Preparation and use of alkynes 4.
Addition to triple bonds. Hydrohalogenation 5.
Addition to triple bonds. Hydration 6. Addition
to triple bonds. Hydroboration-oxidation 7.
Addition to triple bonds. Halogenation 8.
Addition to triple bonds. Hydrogenation 9.
Reduction of triple bonds. Trimerization of
triple bonds 10. Acidity of alkynes
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In alkynes, one s- and two p- bonds form a triple
bond. Now we need to choose a hybridization to
describe systems, containing a triple bond. The
valence shell of the atom of carbon has one
s-orbital and three p-orbitals. When the carbon
is bonded by two p-bonds, each of two p-orbitals
participates in the formation of this p- bond,
and the remaining one s-orbital and one p-orbital
are equally involved in the formation of s-bonds,
giving rise to the sp-hybridization state. The
presence of sp-hybridized carbons is
characteristic for alkynes.
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s- and p-bonds in alkynes
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1. Bonding in alkynes. sp-Hybridization 2.
Physical properties and nomenclature of alkynes
3. Preparation and use of alkynes 4. Addition to
triple bonds. Hydrohalogenation 5. Addition to
triple bonds. Hydration 6. Addition to triple
bonds. Hydroboration-oxidation 7. Addition to
triple bonds. Halogenation 8. Addition to triple
bonds. Hydrogenation 9. Reduction of triple
bonds. Trimerization of triple bonds 10. Acidity
of alkynes
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Boiling points of alkynes are close to the
boiling points of alkenes and alkanes. Alkyne
have lower densities, than water and they are
insoluble in water.
Common nomenclature The compound is treated as a
derivative of acetylene (C2H2)
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Substitutive nomenclature Similar to alkenes,
but with the following differences
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1. Bonding in alkynes. sp-Hybridization 2.
Physical properties and nomenclature of alkynes
3. Preparation and use of alkynes 4. Addition to
triple bonds. Hydrohalogenation 5. Addition to
triple bonds. Hydration 6. Addition to triple
bonds. Hydroboration-oxidation 7. Addition to
triple bonds. Halogenation 8. Addition to triple
bonds. Hydrogenation 9. Reduction of triple
bonds. Trimerization of triple bonds 10. Acidity
of alkynes
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Combustion of acetylene produces very high
temperatures, which makes it useful for
oxygen-acetylene welding 2C2H2 5O2 4CO2
2H2O Alkynes are valuable starting materials for
organic synthesis due to their reactivity.
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1. Bonding in alkynes. sp-Hybridization 2.
Physical properties and nomenclature of alkynes
3. Preparation and use of alkynes 4. Addition to
triple bonds. Hydrohalogenation 5. Addition to
triple bonds. Hydration 6. Addition to triple
bonds. Hydroboration-oxidation 7. Addition to
triple bonds. Halogenation 8. Addition to triple
bonds. Hydrogenation 9. Reduction of triple
bonds. Trimerization of triple bonds 10. Acidity
of alkynes
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Electrophilic addition to triple bonds proceeds
slower, than addition to double bonds and often
requires a catalyst. The Markovnikovs rule is as
valid as for the addition to double bonds. HBr in
the presence of peroxides adds against the rule.
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In the presence of peroxides regioselectivity is
reversed.
The product of hydrohalogenation reacts with
excess of hydrogen halide.
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1. Bonding in alkynes. sp-Hybridization 2.
Physical properties and nomenclature of alkynes
3. Preparation and use of alkynes 4. Addition to
triple bonds. Hydrohalogenation 5. Addition to
triple bonds. Hydration 6. Addition to triple
bonds. Hydroboration-oxidation 7. Addition to
triple bonds. Halogenation 8. Addition to triple
bonds. Hydrogenation 9. Reduction of triple
bonds. Trimerization of triple bonds 10. Acidity
of alkynes
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Differences from hydration of alkenes 1. A Hg2
catalyst is required 2. The product of addition
is unstable and isomerizes.
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1. Bonding in alkynes. sp-Hybridization 2.
Physical properties and nomenclature of
alkynes 3. Preparation and use of alkynes 4.
Addition to triple bonds. Hydrohalogenation 5.
Addition to triple bonds. Hydration 6. Addition
to triple bonds. Hydroboration-oxidation 7.
Addition to triple bonds. Halogenation 8.
Addition to triple bonds. Hydrogenation 9.
Reduction of triple bonds. Trimerization of
triple bonds 10. Acidity of alkynes
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Summary of hydration of alkynes
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1. Bonding in alkynes. sp-Hybridization 2.
Physical properties and nomenclature of alkynes
3. Preparation and use of alkynes 4. Addition to
triple bonds. Hydrohalogenation 5. Addition to
triple bonds. Hydration 6. Addition to triple
bonds. Hydroboration-oxidation 7. Addition to
triple bonds. Halogenation 8. Addition to triple
bonds. Hydrogenation 9. Reduction of triple
bonds. Trimerization of triple bonds 10. Acidity
of alkynes
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The unstable bromonium cation equilibrates with
an open carbocation, which can be attacked by
Br- from either side. It makes
stereoselectively of addition unpredictable.
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1. Bonding in alkynes. sp-Hybridization 2.
Physical properties and nomenclature of alkynes
3. Preparation and use of alkynes 4. Addition to
triple bonds. Hydrohalogenation 5. Addition to
triple bonds. Hydration 6. Addition to triple
bonds. Hydroboration-oxidation 7. Addition to
triple bonds. Halogenation 8. Addition to triple
bonds. Hydrogenation 9. Reduction of triple
bonds. Trimerization of triple bonds 10. Acidity
of alkynes
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As opposed to electrophilic addition, alkynes
are more reactive toward hydrogenation, than
alkenes. The triple bond is shorter, than the
double bond and fits the molecule of hydrogen
better.
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1. Bonding in alkynes. sp-Hybridization 2.
Physical properties and nomenclature of alkynes
3. Preparation and use of alkynes 4. Addition to
triple bonds. Hydrohalogenation 5. Addition to
triple bonds. Hydration 6. Addition to triple
bonds. Hydroboration-oxidation 7. Addition to
triple bonds. Halogenation 8. Addition to triple
bonds. Hydrogenation 9. Reduction of triple
bonds. Trimerization of triple bonds 10. Acidity
of alkynes
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1. Bonding in alkynes. sp-Hybridization 2.
Physical properties and nomenclature of
alkynes 3. Preparation and use of alkynes 4.
Addition to triple bonds. Hydrohalogenation 5.
Addition to triple bonds. Hydration 6. Addition
to triple bonds. Hydroboration-oxidation 7.
Addition to triple bonds. Halogenation 8.
Addition to triple bonds. Hydrogenation 9.
Reduction of triple bonds. Trimerization of
triple bonds 10. Acidity of alkynes
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The relatively high acidity of alkynes
significantly affects their chemical properties.
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Formation of acetylenides is specific for
1-alkynes, because a terminal atom of hydrogen
must be present. Using acidity of alkynes and
nucleophilicity of acetylenides, any alkyne can
be synthesized from acetylene
Alkynes are convenient starting materials to
synthesize pheromones natural compounds, used
by animals and insects to exchange information
such as sex attraction, threat, warning etc.