Organic Chemistry: Concepts and Applications - Headley Allan D. 2020
Addition Reactions Involving Alkenes and Alkynes
8.8 The Mechanism for Addition Reactions Involving Alkynes
Owing to the presence of pi (π) bonds in alkynes, addition reactions involving alkynes are similar to those encountered with alkenes. A major difference, however, is that there are two pairs of pi (π) electrons; thus, alkynes can react with two moles of the electrophile and nucleophile as shown in Reactions (8-85) and (8-86).
(8-85)
(8-86)
8.8.1 Addition of Bromine to Alkynes
As mentioned earlier, owing to the polarizability of the very large bromine molecule, it is possible for it to add to the nucleophilic alkyne. Shown in Reaction (8-87) is the first step in the mechanism for addition of bromine to 2-butyne to form a bromonium ion.
(8-87)
Note the stabilization of the alkene bromonium ion due to the large polarizable bromine atom. In the final step of the reaction, the nucleophilic bromide anion adds to the bromonium ion to give the trans-dibromide product alkene as shown in Reaction (8-88).
(8-88)
In the presence of excess bromine, an additional reaction will take place with the resulting product of Reaction (8-88) as shown in Reaction (8-89).
(8-89)
Problem 8.14
Give the major organic product of the reactions shown below.
8.8.2 Addition of Hydrogen Halide to Alkynes
The mechanism for the addition of HBr or HCl to alkynes is similar to the addition reaction of these reagents to alkenes, except that the possibility exists for two moles to be added since alkynes have two sets of pi (π) electrons. Reaction (8-90) shows the addition of hydrobromic acid to propyne.
(8-90)
Note that in the first step of the reaction mechanism, the addition is regiospecific and follows Markovnikov addition, in which the hydrogen adds to the carbon of the triple bond with the greatest number of hydrogens, in this case the terminal carbon, which has only one hydrogen. As expected, in the presence of excess HBr, another addition reaction will take place to the resulting alkene, as shown in Reaction (8-91).
(8-91)
Note that the addition of the second mole of HBr to the alkene also follows the Markovnikov addition. If the same reaction were carried out involving an alkyne with an internal triple bond, there is a mixture of addition products.
Problem 8.15
Give the products that result after the addition of excess HCl to 2-pentyne.
8.8.3 Addition of Water to Alkynes
Once a good understanding of the mechanism of different addition reactions is gained, that knowledge can be applied to understanding of the same type of reactions involving a wide range of different molecules. The addition of H-OH (H2O) to alkynes follows a very similar mechanism as that of the addition of water to alkenes. Recall that the addition of water to alkenes requires a catalyst, such as a proton, which is an extremely good electrophile to get the reaction started. For the addition of water to alkynes, the electrophilic catalyst that is typically used is Hg2+ in the form of the mercury salt HgSO4. In the first step of the mechanism, the nucleophilic alkyne adds to the electrophilic mercury cation, which is then attacked by water to yield the mercury enol as shown in the Reaction (8-92).
(8-92)
In the next step of the mechanism, the nucleophilic double bond reacts with a proton to generate a stabilized carbocation, which is stabilized through resonance by the electrons on the adjacent oxygen, as shown in Reaction (8-93).
(8-93)
In the next step of the reaction mechanism, the mercury ion is lost to form an enol as shown in Reaction (8-94).
(8-94)
Enols are not stable and they quickly undergo a migration of the double bond and hydrogen to form the more stable carbonyl-containing compound. The carbonyl compound is more stable than the enol since the carbon—oxygen double bond is shorter and stronger than the carbon—carbon double bond of the enol. The equilibrium involving these two species is called a keto-enol equilibrium or keto-enol tautomerism. The mechanism for the keto-enol tautomerism in an acidic medium is shown in Reaction (8-95).
(8-95)
Thus, for the addition of water to terminal alkynes in the presence of an electrophilic catalyst, such as Hg2+, a methyl ketone is the major organic product since the reaction is regiospecific. For the addition of water to symmetrical alkynes in the presence of an electrophilic catalyst, such as Hg2+, there is only one product as the major organic product. On the other hand, if the alkyne is unsymmetrical, the possibility exists for the formation of two different organic ketones as shown in Reaction (8-96).
(8-96)
It is possible to have a regiospecific hydration reaction to a terminal alkyne which is the opposite of that discussed above. Consider the reaction of terminal alkyne shown in Reaction (8-97) with a much bulkier borane than the one used earlier for the hydroboration—oxidation of alkenes. For this reaction, the very bulky substituted disiamylborane[bis(1,2-dimethylpropyl)borane], abbreviated as sia2BH, is used and will ensure an addition to the alkyne in an anti-Markovnikov manner for the same reason explained previously in Section 8.6.2.
(8-97)
The final product of the hydroboration reaction (after oxidation with H2O2 in the presence of NaOH) is an enol. As mentioned earlier, the keto-enol equilibrium favors the carbonyl form, which is the final product as shown in Reaction (8-98).
(8-98)
Thus, for a reaction involving a terminal alkyne and this very bulky diborane, disiamylborane [bis(1,2-dimethylpropyl)borane], followed by oxidation, the final product is an aldehyde as shown in Reaction (8-99).
(8-99)
Note that these reactions using disiamylborane, followed by oxidation result in reactions that are regiospecific and anti-Markovnikov.
Problem 8.16
For the reactions given below, provide either the major organic product or an appropriate reactant.
