Key Notes on Addition Reaction of Alkynes

 A ten-membered carbon ring is the smallest structure that may support this function without causing significant strain due to its linear configuration (the bond angle of a sp-hybridised carbon is 180°). The addition reaction is the most common chemical transformation of a carbon-carbon double bond, and therefore, we might expect the same to be true for carbon-carbon triple bonds. The truth is that the vast majority of alkene addition reactions also occur with alkynes, and they do so with identical regio- and stereoselectivity.

Electrophilic reagents

When we look at the additional reactions of electrophilic reagents to alkynes, such as strong Brnsted acids and halogens, we discover a remarkable paradox that has to be explained. It should be noted that the additions to alkynes are much more exothermic than additions to alkenes, although the rate of addition to alkynes is much slower than that of addition to equivalently substituted alkenes by a ratio of 100 to 1000. For example, the reaction of one equivalent of bromine with 1-penten-4-yne resulted in the formation of 4,5-dibromo-1-penten-4-yne as the primary product.

In the presence of Br2, the HC-CH2-CH=CH2 is transformed into the HC-CH2-CHBrCh2Br.

Despite the fact that these electrophilic additions to alkynes are sluggish, they do occur and are characterised by Markovnikov Rule regioselectivity and anti-stereoselectivity in the majority of cases. However, there is a complication in that the products of these additions are themselves substituted alkenes, which means they can be subjected to more addition. The high electronegativity of halogen substituents on double bonds has the effect of reducing the nucleophilicity of the bond, which in turn has the effect of slowing down the pace of electrophilic addition processes. As a result, there is a delicate balance between whether the product of an initial addition to an alkyne will suffer future addition to a saturated product and whether it will not. Although the initial alkene products may often be isolated and identified, they are frequently found in mixes of products and may not be recovered in high yields in many instances. Many of these characteristics are illustrated in the reactions that follow. In the last example, 1,2-diiodoethane does not undergo additional addition because vicinal-iodoalkanes are relatively unstable. As a result, the reaction proceeds without further addition.

Addition of Hydrogen Halide to an Alkyne is a chemical reaction.

In summary, the reactivity of hydrogen halides is as follows: HI > HBr> HCl > HF.

The rule of Markovnikov is followed:

The addition of hydrogen occurs on the carbon atoms with the largest amount of hydrogens, while the addition of halogen occurs on the carbon atoms with the fewest hydrogens.

Protonation happens on the carbocation, which is the more stable of the two. Haloalkanes are formed as a result of the addition of HX.

Excess HX results in the formation of anti addition, which results in the formation of a geminal dihaloalkane.

The addition of a HX to an Internal Alkyne is described in detail here.

In the HBr molecule, as shown in Figure 1, the electrons attack the hydrogen and because this is a symmetric molecule, it does not matter which carbon it adds to; however, in an asymmetric molecule, the hydrogen will form a covalent bond with the carbon that contains the greatest number of hydrogens. Upon covalent bonding the hydrogen to one of the carbons, a carbocation intermediate will form on the other carbon. 

Conclusion

Therefore we can finally conclude that it is necessary to add the bromine at the end of the process since it is an excellent nucleophile because it has electrons to donate or share. Consequently, bromine assaults the carbocation intermediate by interacting with it through the strongly substituted carbon. As a result of this reaction, you obtain 2-bromobutene from your 2-butyne reaction product.