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Depending on the amount of reagent used, the reaction can be repeated once or twice more. Alkynes are less reactive than alkenes because the intermediate formed (a vinylic carbocation) is more unstable than the corresponding carbocation formed by electrophilic addition to an alkene. Alkynes are less reactive than alkenes because the intermediate formed (a vinylic carbocation) is more unstable than the corresponding carbocation formed by electrophilic addition to an alkene. It is produced by the reaction of an alkyne with aqueous acid and mercuric acid, which is then rearranged by keto–enol tautomerism to generate the product of the reaction, a ketone.
Additions to terminal alkynes are a type of addition.
A terminal alkyne is treated by adding hydrogen halide, which results in the hydrogen(s) attaching to the terminal carbon and the halogen(s) attaching to the more substituted carbon, as shown in the diagram. This is yet another illustration of Markovnikov’s rule in action. Similarly, the reaction of a terminal alkyne with aqueous acid and mercuric sulphate results in the formation of a ketone rather than an aldehyde with the addition of one molecule of water, which is followed by the formation of a keto–enol tautomer.
Additions to symmetrical alkynes are a type of addition.
Alkynes perform electrophilic addition reactions with the same reagents as alkenes, which is why they are called alkynes (e.g. halogens and hydrogen halides). Due to the presence of two bonds in alkynes, it is possible for the reaction to proceed once or twice, depending on the amount of reagent that is applied. The reaction of 2-butyne with one equivalent of bromine, for example, results in the formation of a (E)-dibromoalkene. When treated with two equivalents of bromine, the (E)-dibromoalkene that was initially generated reacts further to form a tetrabromoalkane.
The treatment of an alkyne with one equivalent of HBr results in the formation of a bromoalkene. If there are two equivalents of hydrogen bromide present, the reaction can proceed twice as far as producing a geminal dibromoalkane, which is formed when both bromine atoms are joined to the same carbon.
The addition reactions described above are analogous to the addition reactions of alkenes. However, because alkynes are less reactive than benzene, the reaction is substantially slower in the case of an alkyne. Alkynes are expected to be more nucleophilic due to the fact that they have more electrons in the region of the multiple bond, i.e., six electrons in a triple bond as opposed to four electrons in a double bond. The production of a vinylic carbocation occurs as a result of electrophilic addition to an alkyne, on the other hand. This carbocation intermediate, formed by electrophilic addition to an alkene, is significantly less stable than the carbocation intermediate formed by electrophilic addition to an alkene.
The low reactivity of alkynes means that they react slowly with aqueous acid and that mercuric sulphate must be used as a catalyst to speed up the reaction. The result that could be predicted from this reaction would be a diol, according to the literature.
In actuality, no diol is generated during the reaction. The intermediate (an enol) is subjected to acid-catalysed rearrangement, resulting in the formation of a ketone instead. A keto–enol tautomerism is the term used to describe this phenomenon.
When two different isomeric forms (tautomers) of a ketone react rapidly with one another, this is referred to as tautomerism. The ketone tautomers in this example are the keto and enol forms of the same ketone. It is usually only in trace levels that the keto tautomer is present in ketone compounds, while the enol tautomer is typically present in large numbers (typically 0.0001 percent ). In order to avoid further electrophilic addition, as soon as the enol is generated in the preceding reaction, it tautomerizes to the keto form and no more electrophilic addition occurs.
Additions to terminal alkynes are a type of addition.
Whenever a terminal alkyne is subjected to an excessive amount of hydrogen halide, the halogens are forced to coexist on the more substituted carbon.
Markovnikov’s rule, which asserts that more hydrogens will be deposited on the carbon atoms that already contain the greatest number of hydrogens, is illustrated once more in this example. It is the same law that applies to the reaction with acid and mercuric sulphate, which implies that instead of an aldehyde is created following keto–enol tautomerism, a ketone is formed.
Conclusion
The primary reaction of alkynes is addition across the triple bond, which results in the formation of alkanes. Similar to the addition reactions of alkenes, these addition reactions occur in the presence of alkenes. Hydrogenation. Alkynes are hydrogenated catalytically using the same catalysts as alkenes: platinum, palladium, nickel, and rhodium. Platinum, palladium, nickel, and rhodium are the catalysts used in alkene hydrogenation. Hydrogenation occurs in a stepwise fashion, beginning with the formation of an alkene, which then undergoes further hydrogenation to form an alkane.
