Alkynes are hydrocarbons that contain a carbon–carbon triple bond. A triple bond can be thought of as a cylinder of electrons wrapped around the bond. The functional group suffix of an alkyne is “yne.”
There are internal and terminal akynes, internal alkynes, with two alkyl substituents bonded to the sp carbons, are more stable than terminal alkynes.
We now have seen that alkyl groups stabilize alkenes, alkynes, carbocations, and alkyl radicals.
An alkyne is less reactive than an alkene because a vinylic cation is less stable than a similarly substituted alkyl cation. Like alkenes, alkynes undergo electrophilic addition reactions. The same reagents that add to alkenes add to alkynes. Electrophilic addition to a terminal alkyne is regioselective; in all electrophilic addition reactions to terminal alkynes, the electrophile adds to the sp carbon that is bonded to the hydrogen because the intermediate formed—a secondary vinylic cation—is more stable than a primary vinylic cation. If excess reagent is available, alkynes can undergo a second addition reaction with hydrogen halides and halogens because the product of the first reaction is an alkene.
An alkyl peroxide has the same effect on the addition of HBr to an alkyne that it has on the addition of HBr to an alkene—it reverses the order of addition because the peroxide causes to become the electrophile.
When an alkyne undergoes the acid-catalyzed addition of water, the product of the reaction is an enol. The enol immediately rearranges to a ketone. A ketone is a compound that has two alkyl groups bonded to a carbonyl group. An aldehyde is a compound that has at least one hydrogen bonded to a carbonyl group. The ketone and enol are called keto–enol tautomers; they differ in the location of a double bond and a hydrogen. Interconversion of the tautomers is called tautomerization.
The keto tautomer predominates at equilibrium. Terminal alkynes add water if mercuric ion is added to the acidic mixture.
In hydroboration–oxidation, H+ is not the electrophile, H:- is the nucleophile. Consequently, mercuric-ion-catalyzed addition of water to a terminal alkyne produces a ketone, whereas hydroboration–oxidation of a terminal alkyne produces an aldehyde.
Hydrogen adds to an alkyne in the presence of a metal catalyst (Pd, Pt, or Ni) to form an alkane. Addition of hydrogen to an internal alkyne in the presence of Lindlar catalyst forms a cis alkene.
Sodium in liquid ammonia converts an internal alkyne to a trans alkene.
Electronegativity decreases with decreasing percentage of s character in the orbital. Thus the electronegativities of carbon atoms decrease in the order: Ethyne is therefore a stronger acid than ethene,and ethene is a stronger acid than ethane.
An amide ion can remove a hydrogen bonded to an sp carbon of a terminal alkyne because it is a stronger base than the acetylide ion that is formed. The acetylide ion can undergo an alkylation reaction with a methyl halide or a primary alkyl halide to form an internal alkyne. An alkylation reaction attaches an alkyl group to the starting material.
Most ethyne produced commercially is for the synthesis of monomers used in the synthesis of polymers. Polymers are large molecules that are made by linking together many small molecules called monomers.
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