Reactions of Alkynes:
Due to the presence of loosely held π electrons, alkynes undergo reactions alike to those of alkenes. We will see in this chapter that several of the chemical characteristics of alkynes are like to those of alkenes. Characteristic reactions of alkynes include electrophilic additions, reduction and oxidation. Several significant reactions of alkynes are summarized in Table.
Table: Reactions of alkynes
Electrophilic addition reactions are characteristic of alkynes. Several common electrophilic addition reactions are discussed below:
Alkynes can add on the halogen acid (HX). The addition of halogen acid cann't occurs in the dark, but is catalysed via light or metallic halides. Similar to alkenes, the addition is in accordance by Markownikoffs rule; for instance, ethyne combines by hydrogen bromide to form first 1-bromoethene and then 1, 1-dibromoethane.
The mechanism of such reactions is similar as in the hydrohalogenation of alkenes, for example,
As we know that if one of the carbon atoms involved in double bond formation (with another carbon atom) carries a positive change, then the species is called alkenyl cation.
Addition of another molecule of hydrogen bromide could give either CH3CHBr (a secondary carbocation) or CH2CH2Br (a primary carbocation). Because the secondary carbocation is more stable than the primary carbocation, the reaction proceeds via the secondary carbocation to form 1, 1-dibromoethane. Therefore:
Because of the electron-withdrawing nature of bromine atom, the availability of π electrons in 1-bromoethene is less than that in ethane. Therefore, the electrophilic addition (of HBr) to 1-bromoethene is much slower than that to ethene,
In the presence of free radical initiators such as peroxides, anti Markownikoff addition of HBr is examined as with alkenes. For example, addition of HBr in the presence of peroxides to 1-butyne gives 1, 2-dibromobutane has given below:
Alkynes react by chlorine and bromine to yield tetrahaloalkanes. 2 molecules of halogen add to the triple bond. A dihaloalkene is an intermediate and can be isolated using proper reaction conditions. Ethyne, for example, on treatment with bromine water provides only 1, 2-dibromoethene whereas through bromine alone, it forms 1, 1, 2, 2-tetrabromoethane.
The addition of halogens to ethyne is stereo selective; the predominant product is the trans isomer.
We have seen in previously that addition of a water molecule to an alkene provides an alcohol. Likewise, addition of a water molecule to an alkyne gives an enol. An enol has the - OH group attached to a double-bonded carbon atom.
In fact, enols are very unbalanced and they isomerise (or tautomerise) to provide aldehydes or ketones. The procedures via those enols are converted into adledhydes or ketone is termed keto-enol isomerism or keto-enol tautomerism. For instance, whenever ethyne undergoes hydration, it gives an aldehyode, for example ethanal, while, propyone gives a ketone, i.e. propanone.
The arrow is longer towards aldehyde or ketone side showing the direction in which the equilibrium is favoured. In case of unsymmetrical alkyne addition of water place in accordance. Through Markowhikoff's rule. The enol is converted into air aldehyde or ketone via a mechanism alike to the hydration of a double bond. The enol double bond is protonated to provide a corbocation. The in the instance following below is a protonated ketone. Instead of adding water, this ion loses a proton to provide ketone.
Now we might ask why carbocation is not attacked via water molecule to give a diol, for instance
This reaction doesn't occur, since it is reversible, and the equilibrium between the ketone and the analogous diol in most cases favours formation of the ketone.
Addition of borane to alkynes provides alkenyl boranes that can be oxidised via basic hydrogen peroxide to ketones via their enol.
The symmetrical internal alkynes provide a single product whereas unsymmetrical internal alkynes provide a mixture of both the possible ketones. For instance, 3-hexyne gives 3-hexanone while 2-bexyne gives a mixture of 2-hexanone and 3-hexanone.
The terminal alkynes on hydroboration give aldehydes. Another reaction of organoboranes is protonolysis. That is, the alkenyl boranes, formed after the addition of borane to alkynes, on treatment by ethanoic acid yield cis-alkenes. This reaction sequence provides another process of converting alkynes to cis-alkenes.
Like alkenes, alkynes undergo catalytic hydrogenation. The addition of hydrogen to an alkyne takes place in 2 steps. First addition consequences in the formation of an alkene; because an alkene can as well undergo catalytic hydrogenation, the 2nd addition provide an alkane. By using a calculated amount of hydrogen and a poisoned catalyst, hydrogenation can be stopped at the alkene stage. A catalyst mixed by a selective inhibiting agent is termed a poisoned catalyst. Such catalysts selectively block the hydrogenation of alkenes.
This is a stereo selective addition reaction giving predominantly cis alkenes. In the absence of a poison, catalytic hydrogenation of an alkyne gives the alkane. Stereoselective reaction is a' reaction which yields predominantly one isomer.
Now we can ask: can we change the reduction of alkynes so as to get only trans alkenes. The answer is yes; we can get only trans products, but with a different reducing agent and through a different mechanism. If we carry out the reduction of an alkyne with sodium metal or lithium metal in liquid ammonia, trans alkene is almost an exclusive product. For example, 3-heptyne is reduced to trans 3-heptene is the following way:
In the first step of this mechanism, the alkyne accepts one electron to give a radical anion. The radical anion is protonated by ammonia solvent to give an alkenyl radical; which gets further reduced by accepting another electron to give an alkenyl anion. This species is again protonated to give the alkene.
A radical anion has one centre with a negative charge and another, with an unpaired electron.
Formation of the trans alkene is due to the rapid equilibration of the intermediate alkenyl radical between the cis-and trans-forms. The equilibrium lies on the side of the more stable trans species.
In other words, we can say reduction of alkyne to double bond can yield either cis-alkene or trans-alkene, depending upon the choice of the reducing agent.
Reagents and reactions that lead to oxidative cleavage of alkenes as well lead to cleavage of alkynes. Addition of ozone to an alkyne produces the ozonide. The ozonides on hydrolysis provide 1, 2-dicarbonyl compounds, which undergo oxidative cleavage to carboxylic acids via hydrogen peroxide shaped in the reaction. For instance, 2-hexyne on ozonolysis gives butanoic and ethanoic acids. Similar products are attained whenever alkynes are oxidized by alkaline permanganate and then hydrolysed using mineral acid. Oxidative cleavage reactions are utilized as a tool in structure determination. The carboxylic acids formed would tell us that of the carbon atoms were linked through the triple bond in the original alkyne.
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