Alkynes Acidity: Understanding the Comparative, Properties, Relative

Alkynes are one of the simplest known hydrocarbons. Their general formula is CnH2n-2. Alkynes belong to the family of unsaturated hydrocarbons. It contains both sigma and pi bonds between carbon and hydrogen. Alkynes are unsaturated hydrocarbons. That is, it contains pi and sigma bonds between hydrogen and carbon. Their general formula is CnH2n-2. They are highly reactive compounds, perhaps the most reactive of all compounds, especially when compared to alkanes and alkenes. They are the simplest hydrocarbons currently available.

Alkyne molecules contain at least one triple bond between several carbon atoms. You can take ethyne as an example here. CH=CH or ethyne reacts strongly with bases such as sodium amide and sodium metal (NaNH2) to form sodium acetylide while releasing dihydrogen gas. The whole process of alkynes reacting with bases and releasing dihydrogen gas proves the acidity of alkynes.

Understanding The Comparative Acidity Of Alkynes

Alkynes are acidic because they can donate hydrogen atoms to form alkyne ions. Thus, alkynes work in the form of Bronsted-Lowry acids. As pointed out earlier, alkynes contain triple-bonded carbon atoms, which is called “sp” hybridization.

Due to the maximum percentage of 's' letters present in alkynes, or about 50%, the 'sp' hybridised orbitals of carbon atoms in alkynes exhibit high

electronegativity. The orbital attracts C-H bonds in alkynes considerably. This is one of the most important reasons why alkyne molecules lose hydrogen atoms very easily and are replaced by alkyne ions. Therefore, we can say that a hydrogen atom bonded to a triple-bonded carbon atom is acidic. This proves the presence of acidic hydrogen in alkynes.

As to why alkynes are acidic, it should be noted that the acidity of alkynes is greater compared to that of alkenes and alkanes. This is because the carbon atoms of alkenes and alkanes are 'sp2' and 'sp3' respectively. Hence, the molecule has a lower percentage of 's' letters compared to alkynes.

Therefore, in such cases, the electronegativity of the carbon atom is lower than that of the alkyne. For this reason alone, alkenes and alkanes do not react with bases and do not release hydrogen gas. Furthermore, it should be noted that only hydrogen atoms bonded to triple-bonded carbon atoms are acidic; other hydrogen atoms present in the alkyne series.

Understanding The Acidic Properties Of Alkynes

The acidic properties of alkynes are also highly dependent on the constancy of the conjugate bases formed. When the terminal alkyne loses a proton, the process gives way to the formation of an acetylide ion, acting in the form of a stable conjugate base. As mentioned earlier, sp-hybridised carbons are inherently electronegative. Because it has 50% character, it is most effective at retaining a negative charge. Therefore, terminal alkynes are acidic.

The presence of a high proportion of sp-hybridised orbitals with an s character causes alkynes to be acidic. The s symbol combines with the s orbital of a hydrogen atom to form a covalent bond.

The high proportion of s-character in sp-hybridised carbon atoms moves the overlapping region of O-bonds to the immediate vicinity of the carbon atoms. Bond polarization occurs throughout the process, making more hydrogen atoms positive, but only slightly. However, it is this small positive charge that makes the hydrogen atom such a weak proton that it can be easily removed with solid bases.

In contrast, alkene and alkane hybrid carbon bonds have a lower s-character. This results in less electronegativity for carbon atoms and less transfer to the atoms present in the O-bond overlap region. It is the position of the overlapping region that alleviates the electron deficit of all corresponding hydrogen atoms, thus reducing their acidity. In reality, hydrogens attached to alkenes and alkanes can be easily removed in the form of protons if both strong and non-aqueous bases are available.

Chemical reaction to show the acidic nature of acetylene is given below-

Relative Acidity Of Alkynes

The acidity of alkynes results from their tendency to donate hydrogen atoms to form alkylidenes. As a result, alkynes act as Bronsted-Lowry acids. In alkynes, the triple bond carbon atoms are “sp” hybridised. The 'sp' hybridised orbitals of carbon atoms in alkynes are highly electronegative due to the large proportion (50%) of 's' character in alkynes. The C–H bonds of alkynes are strongly attracted to them.

Alkyne molecules easily lose a hydrogen atom thereby forming an alkyne ion. As a result, hydrogen atoms attached to triple-bonded carbon atoms have acidic properties.

The carbon atoms of alkanes and alkenes are "sp3" and "sp2" hybridised, so the acidity of alkynes is greater than that of alkanes and alkenes. As a result, these molecules have a lower percentage of 's' letters compared to alkynes. As a result, the electronegativity of carbon atoms is lower than that of alkynes in these situations. As a result, alkanes and alkenes do not exhibit hydrogen outgassing reactions with bases. It is also worth noting that only the hydrogens attached to the triple-bonded carbon atoms are acidic, the remaining alkyne chain hydrogens are not.