All About Acidity Of Alkynes

There is at least one triple bond between two carbon atoms in an alkyne molecule, and there are many more. For example, ethyne (CHCH) is a compound. Ethyne combines with strong bases such as sodium metal and sodamide (NaNH2), resulting in the formation of sodium acetylide and the release of dihydrogen gas as a byproduct. The acidity of alkynes is demonstrated by their reaction with bases, which results in the release of dihydrogen gas.

Acidity of Alkynes

Because of their tendency to lose hydrogen atoms and create alkylidenes, alkynes have a high acidity level. As a result, alkynes function as Brnsted-Lowry acids. In alkynes, the triple-bonded carbon atom undergoes “sp ” hybridization. Consequently, the high percentage of “s’ ‘ character (50 percent) in alkynes results in the “sp’ ‘ hybridised orbitals of the carbon atom in alkynes having a high electronegativity value. A significant amount of attraction is exerted by these on the C-H bond of alkynes. As a result, alkyne molecules can easily lose their hydrogen atoms, resulting in the formation of alkynide ions. Thus, the hydrogen atom that is linked to the carbon atom that has been tripled-bonded can be described as acidic in nature.

Physical properties of alkynes

Because the carbon atoms in alkanes and alkenes are “sp3” hybridised, the acidity of alkynes is more than the acidity of alkanes and alkenes, and the acidity of alkynes is larger than the acidity of alkanes and alkenes. Because of this, these molecules possess a lower fraction of the “s” character when compared to alkynes (see Figure 1). As a result, the electronegativity of the carbon atom is lower in these situations than it is in alkynes. As a result, alkanes and alkenes do not exhibit the reactions with bases that result in the liberation of hydrogen gas. It should also be emphasised that only the hydrogen atoms linked to a triply bonded carbon atom are acidic, not the other hydrogen atoms in the alkyne chain, which is an important distinction. 

Acidity is a property of alkynes.

Terminal alkynes are a type of alkyne that has reached the end of its chain. All of these forms of alkynes are acidic to a minor extent. An acid base reaction occurs when a strong base, such as sodium amide, is exposed to a strong acid.

One reason for the acidity of a terminal alkyne is due to the high amount of s character in the sp hybrid orbital, which forms a covalent connection with the hydrogen atom through its interaction with the s orbital of the hydrogen atom. In a sp hybridised carbon, the presence of a high level of s character results in an overlap region of the bond that is significantly closer to the carbon atom. This polarises the bond, forcing the hydrogen atom to shift slightly to the positive side of the equation. Because of this minor positive charge, the hydrogen atom is classified as a weak proton, which can be eliminated by a strong base.

Compared to alkanes and alkenes, the s character of the hybridised carbon bonds is reduced, resulting in a smaller number of electronegative carbon atoms, as well as a smaller shift toward those atoms that are in the overlap region of the bond. Because of the location of the overlap region, the accompanying hydrogen atoms are less electron deficient and, as a result, are less acidic in nature. In actuality, the hydrogen atoms bound to alkanes and alkenes can be extracted as protons; however, substantially stronger non aqueous bases are required to do this.

The acetylide ion is formed as a result of a reversible process. As a result, either the base cannot generate an acid with a stronger acidity than the beginning alkyne by accepting the proton, or the newly formed conjugate acid will protonate the acetylide ion by accepting the proton. The fact that stronger acids are capable of reprotonating the acetylide ion may be observed in the reaction of the acetylide ion with water can be shown in its reaction with water.

Conclusion

Because of the presence of pi-electrons that are not tightly bound, alkynes undergo addition reactions. The existence of a triple bond in alkynes, halogens, water and other substances means that alkyne molecules are more susceptible to hydrogen atom loss and the formation of alkynide ions. As a result, the acidic character of the hydrogen atom coupled to the triply bonded carbon atom has been established. Alkynes are more electronegative than aryl halides as a result of the existence of greater s-character. As a result, hydrogen atoms can be freed as protons with greater ease in Ethyne.