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Alkyne Reactions


Alkynes are hydrocarbons containing a carbon-carbon triple bond and have the molecular formula CnH2n-2. The first member of the series ethyne, is better known as acetylene. Those alkynes with the triple bond at the end of a chain are called terminall alkynes and are monosubstituted acetylenes. Acetylene was discovered by Davey in 1836. 

Acetylene was readily available to chemist by the end of the nineteenth century and it was prepared by the action of water on calcium carbide. Reactions of acetylene were studied in detail by the German chemist, Walter Reppe in the 1930s and 1940s. Many compounds containing triple bonds exist in nature, including the fatty acid tariric acid, the plant poison cicutox in and the enediyne antibiotics like dynemicin A.

Addition Reaction of Alkynes

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The electrophilic addition of hydrogen halides and halogens is as expected. Because there are two $\pi$ bonds, addition can occur two times. For unsymmetrical situations, Markovnikov's rule is followed. Hydrogenation gives complete saturation by using two equivalents of H2, Reduction can be stopped at the alkene stage using the Lindlar palladium catalysts, Pd-BaCO3 or Pd-CaCO3.

Hydration, with the help of a catalyst initially gives alcohol products which have the hydroxy group directly attached to a C=C. This structure is the enol, and it quickly tautomerizes to the more stable ketone. Thus they undergo many addition reactions just as alkenes do. As a general rule, however alkynes are somewhat less reactive than alkenes, so the various reactions can often be stopped at the monoaddition stage if only one molar equivalent of reagent is used. 

HBr addition

CH3CH2CH2C≡CH $\rightarrow$ CH3CH2CH2CH2C(Br)=CH2 $\rightarrow$ CH3CH2CH2CH2C(Br)2-CH 

HCl addition

CH3CH2C≡CCH2CH $\rightarrow$ CH3CH2C(Cl)=CH-CH2CH3 $\rightarrow$ CH3CH2C(Cl)2CH2CH2CH 

Br2 addition

CH3CH2C≡CH $\rightarrow$ CH3CH2C(Br)=C(Br)-H $\rightarrow$ CH3CH2C(Br)2-CH(Br)2

Oxidation of Alkynes

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Alkynes react smoothly with thallium (III) nitrate to afford products whose structure depends upon the nature of the substituents present on the alkynyl substrate. Thus, whereas diarylacetylenes are efficiently transformed in the presence of excess thallium(III) nitrate, into the corresponding 1,2-diketones, dialkylacetylenes generates the corresponding $\sigma$-hydroxy ketones.

Ph-CC-Ph $\rightarrow$ Ph-C(O)-C(O)-Ph

Et-CC-Et $\rightarrow$ Et-C(O)-C(OH)-Et


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Tautomerism is within a set of constitutional isomers there is a subset of special constitutional isomers which are termed as tautomers. The aldehyde or ketone is called the keto form and the keto enol equilibrium is referred to as keto-enol isomerism or keto-enol tautomerism.

Tautomerism arises because of the proximity of an acidic $\alpha$-hydrogen to a lone pair of electrons on the carbonyl oxygen atom. The enol is known as such because there is an alkene functional group together with a hydroxyl group. The presence of this alkene functional group makes the enol susceptible to oxidative cleavage like what would happen to an alkene molecule.