Alkynes – Acidic Character

Alkynes are unsaturated hydrocarbons that possess triple bonded carbon atoms in their chemical structure. Alkyne possesses several characteristics, and its acidic nature is one common character. It is acidic, whereas other hydrocarbons don’t have a similar nature.

About the acidic character of Alkynes

Alkynes are slightly acidic in nature. It is observed by the reaction of alkynes with other strong bases to produce salt and release the hydrogen gas, which is the conventional acid-base reaction. Moreover, alkynes are weak acids, and they are even weaker than water and other unsaturated hydrocarbons.

Alkynes’ acidic nature is strongly influenced by their Sp hybridisation state. Alkynes are made up of 50 percent s and 50 percent p characters. These characteristics are determined by acetylene. Moreover, the presence of s character depicts its acidic character. Before moving forward, let’s have a look at what alkynes are first.

What are Alkynes?

Unsaturated hydrocarbons, which have the three known bonds between two carbon molecules, can be termed alkynes. The triple bond is acetylenic bonds, and the general molecular formula is CnH2n-2. Also, alkynes of a particular type are used to produce organic compounds. Ethyne, benzyne, propyne, and hexyne are some of the common alkynes. 

What causes alkynes to be acidic? 

In a nutshell, the reason for the acidic character of alkynes is the presence of s-character in the orbitals of sp hybridised form. Alkynes tend to drop the hydrogen atoms and act as Bronsted-Lowry acids to form alkynide ions. The sp hybridised condition of carbon molecules in alkynes can be used to describe the acidic property of alkynes. Since s-electrons are adjacent to the nucleus, an electron in an s-orbital is gripped more securely than one in a p-orbital.

The electrons in sp hybrid orbitals are held more securely by the nuclei but are quite electronegative owing to the excessive s-character. Owing to this, relative sp2 or sp3 hybrid orbitals possess more prominent s-character instead of p-character in sp hybridisation. As a result, the H-C bond’s electron pair is shifted closer to the carbon atom, which aids in proton release. 

Relativity of acidic character of alkyne

With alkane and alkenes

Alkynes do have a slightly acidic character owing to their hybridisation factor. But, at the same time, alkanes and alkenes have a glaring, acidic character. Alkanes and alkenes have two and one carbon bonds between the terminal and non-terminal sides. Single-bonded carbons are sp3 hybridised in this case. Similarly, alkenes have their carbons sp2 hybridised. 

As discussed earlier, alkynes have an acidic nature due to their s character. Alkane and alkenes have lower levels of s-character, which help them decrease their electronegativity. Hence, they do not release the H+ ions, which are the main indicators of the acidic medium.

With Alcohols

Looking at the main factor of acidic nature, H+ ion is released more steadily from alcohols than alkynes. Further, the released hydrogen ion forms an oxygen atom which carries a lot more electronegativity than alkynes. As a result, alcohols carry a stronger acidic character relative to alkynes. 

With Aldehydes

Aldehydes are also acidic in nature, but they aren’t acidic due to their sp character hybridisations. Instead, they are acidic due to their stable anions. Aldehydes have electronegative carbons with double bonds, which carry the inductive effect with them.

In turn, whenever the C-H bond breaks, the O atom withdraws some of the negative charges from the molecule, and the subsequent anion becomes more stable. The pKa value of aldehydes is less than alkynes, which determines the more acidic nature. 

Conclusion

From all the above, we have learned all the essential aspects and logical reasoning behind the acidic character of alkynes, along with a definition of alkynes. We learned that the alkynes carry weak acidic nature due to their hybridised character contents. Owing to this, they easily release H+ molecules, resulting in increased electronegativity and unstableness. Also, the relativity of their acidic character with alkanes and alkenes is high. At the same time, their acidic character is weak from alcohols and aldehydes.