ALKYNE THEORY

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ALKYNE THEORY
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Acetileno o etino

Alkynes are hydrocarbons that contain carbon-carbon triple bonds. The general molecular formula for acyclic alkynes is CnH2n-2 and its degree of unsaturation is two. Acetylene or ethyne is the simplest alkyne, it was discovered by Berthelot in 1862.

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ALKYNE THEORY
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Rule 1. Alkynes respond to the formula $C_nH_{2n-2}$ and are named by replacing the suffix -ane of the alkane with the same number of carbons by -yne.

 
alkyne nomenclature01
 
[1] Ethyne
[2] Propyne
[3] But-2-yne
[4] But-1-yne
 

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ALKYNE THEORY
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Alkynes have similar physical properties to alkanes and alkenes. They are poorly soluble in water, have a low density and have low boiling points. However, alkynes are more polar due to the greater attraction that an sp carbon exerts on electrons, compared to an sp 3 or sp 2 carbon.

propiedades físicas de alquinos

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ALKYNE THEORY
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The carbon-hydrogen bond of terminal alkynes is polarized and shows a slight tendency to ionize.

 
acidez de alquinos
 
Propyne Hydrogen is weakly acid, with a pKa = 25. Using strong bases (NaH, LDA, NH 2 - ) it can be deprotonated, obtaining its conjugate base -propynyl sodium- a very basic and nucleophilic species.
 
acidez de alquinos
 
 

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ALKYNE THEORY
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As we saw in the previous point, terminal alkynes have acidic hydrogens that can be removed by strong bases, forming acetylides (conjugate base of the alkyne). Acetylides are good nucleophiles and give mechanisms of nucleophilic substitution with primary substrates.

 
alquilación de alquinos

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ALKYNE THEORY
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Alkynes can be prepared by double dehydrohalogenation of vicinal or geminal dihaloalkanes.

Double elimination from geminal dihaloalkanes
Sintesis alquinos - Doble eliminación

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ALKYNE THEORY
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Alkynes are hydrogenated with two equivalents of hydrogen, in the presence of a finely divided metal catalyst, to form alkanes. The most commonly used catalysts are: platinum, palladium, rhodium...

hidrogenacion-alquinos

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ALKYNE THEORY
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The hydrogenation of alkynes to alkanes occurs through an intermediate that is the alkene. It is possible to stop the hydrogenation in the alkene using a poisoned palladium catalyst, called Lindlar's catalyst.
The poisons decrease the activity of the catalyst and allow to stop the hydrogenation in the alkene.
 
hidrogenación de alquinos con Lindlar

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ALKYNE THEORY
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An alternative to hydrogenation with Lindlar's catalyst is the reduction of alkynes with sodium or lithium in liquid ammonia, the difference lies in the alkene obtained. Thus, hydrogenation with Lindlar's catalyst produces cis alkenes, while hydrogenation with sodium in liquid ammonia generates trans alkenes.

 
The one-electron reduction occurs through an intermediate called an anion-radical. Protonation of the anion with ammonia produces hydrogenation of the alkene.
 
reduccion-monoelectronica

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ALKYNE THEORY
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Alkynes react with aqueous sulfuric acid in the presence of a mercury catalyst to form enols. The enol isomerizes (tautomerizes) rapidly under the reaction conditions to give aldehydes or ketones.

 
hidratación de alquinos

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ALKYNE THEORY
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Hydroboration is the anti-Markovnikov hydration of an alkyne. A hindered borane (dicyclohexylborane or diisoamylborane) is used as a reagent, obtaining an enol that tautomerizes to aldehyde or ketone. Terminal alkynes give rise to aldehydes, internal ones to ketones.

hydroboration alkynes01
 
[1] Dicyclohexylborane (Cy2BH)
[2] Diisoamylborane
 
hydroboration alkynes01 2
 

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ALKYNE THEORY
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Similar to alkenes, alkynes add hydrogen halides (HBr, HCl, HI) to the triple bond to form alkenyl halides.

 
The reaction mechanism occurs through a carbocation, formed on the most substituted carbon of the triple bond. Therefore, it is a regioselective reaction that follows the Markovnikov rule, adding the halogen to the most substituted carbon of the alkyne.
 
Adición de haluros de hidrógeno a alquinos

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ALKYNE THEORY
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Alkynes react with chlorine and bromine to form tetrahaloalkanes. The triple bond adds two halogen molecules, although it is possible to stop the reaction in the alkene by adding only one halogen equivalent.

Halogenación de alquinos

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ALKYNE THEORY
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Alkynes react with ozone to form carboxylic acids. In this reaction, the triple bond is broken, transforming each carbon of the alkyne into the carboxylic group.

ozonolisis de alquinos