Before understanding the acidic nature of alkynes, it is vital to have an idea of what alkynes are in the first place. To be very specific, alkynes are unsaturated hydrocarbons. It means that they contain pi and sigma bond connections between hydrogen and carbon. Their general formula is CnH2n-2. They are highly reactive compounds and probably the most reactive of all compounds, especially when compared to alkanes as well as alkenes. They are the simplest hydrocarbons available right now.
A molecule of an alkyne contains a minimum of one triple linkage between a couple of carbon atoms. Take ethyne as an example here. CH=CH or ethyne strongly reacts with bases like sodamide and sodium metal (NaNH2) to make sodium acetylide while liberating dihydrogen gas. The whole procedure where alkynes respond to bases and release dihydrogen gas proves the acidity of alkynes.
HC ≡ CH + Na → HC ≡ C– Na+ + 1/2H2
Alkynes are acidic because of their potential of dropping hydrogen atoms for creating alkynide ions. Hence, alkynes serve in the form of Bronsted-Lowry acids. As has already been pointed out earlier, alkynes contain a triple bonded atom of carbon which is called “sp'' hybridized.
Because of the maximum percentage or around 50% of the “s” character present in alkynes, “sp” hybridized orbitals of the atom of carbon in alkynes display high electronegativity. The orbitals attract C-H linkages of alkynes to a considerable extent. It is one of the most important reasons why the molecules of alkyne can lose hydrogen atoms very easily, thus making way for alkynide ions. Therefore, you can rightly say that the atom of hydrogen attached to the triple bonded atom of carbon is acidic. It proves the presence of acidic hydrogen in alkynes.
Coming to the question of why alkynes are acidic in nature all over again, it is to be noted that the acidity of alkynes happens to be greater in comparison to the acidity of alkenes and alkanes. This is because the atoms of carbon in alkenes and alkanes are “sp2” and “sp3” respectively. Therefore, the molecules have a lesser percentage of the “s” character when compared to alkynes.
Hence, in such cases, the electronegativity of the atom of carbon is lesser when compared to alkynes. It is only because of this reason that alkenes and alkanes do not react with bases for liberating hydrogen gas. Further, it should be noted that only the atom of hydrogen attached to the triple linked atom of carbon is acidic and not the other atoms of hydrogen present within the alkyne series. The general trend of acidity in alkynes is presented like this:
HC≡CH > HC=CH2> CH3–CH3
2HC = CH + 2Na -> 2HC = CNa + H2
Acetylene Sodium acetylide
This equation depicts the acidic character of alkynes.
The acidic character of alkynes is also dependent on the unchanging nature of the formed conjugate base to a considerable extent. When the terminal alkynes happen to lose protons, the process gives way to the formation of acetylide ions that acts in the form of a steady conjugate base. As has already been pointed out, sp-hybridized carbon has an electronegative nature. This is mainly because it contains 50% of the s-character and thus can hold a negative charge most effectively. Therefore, terminal alkynes are acidic.
Coming to the question of what atom causes acidity in alkynes, it can rightly be said that the acidic nature of an alkyne is because of the presence of a high percentage of the s-character in the sp-hybridized orbitals. The s-character connects with the hydrogen atom s-orbital for forming a covalent bond.
It is the high percentage of the s-character in the sp-hybridized atom of carbon that causes the O bond’s overlap area to move very close to the atom of carbon. The whole procedure leads to bond polarization which further causes the atom of hydrogen to become positive but very slightly. However, it is this minuscule positive charge that makes the atom of hydrogen a very weak proton that can easily be removed with the use of a solid base.
On the other hand, s-character in hybridized carbon bonds tends to be less in alkenes and alkanes. This makes way for lesser electronegative carbon atoms corresponding to less of movements towards the atoms present in the overlap area of the O bond. It is the location of the overlap area that makes all the corresponding atoms of hydrogen less deficient in electrons and hence less acidic as well. The reality is that the atoms of hydrogen linked to alkenes and alkanes can easily be removed in the form of protons, provided there is the availability of strong as well as non-aqueous bases.
1. Explain The Acidity Of Alkyne? How Can You Prove The Acidic Nature Of Alkynes?
Alkynes come up as very weak acids mainly because of the more electronegative nature of the sp-hybridized or triple bonded atom of carbon. This character, however, is not found in alkenes and alkanes. Alkyne hydrogen is attached to the triple bonded atom of carbon and has the possibility of losing in the form of H+ ion. This is one of the main reasons behind the acidic nature of alkynes.
2. What Are The Different Varieties Of Alkynes And Their Related Functional Groups?
Functional groups are the atoms bonded and arranged in a specific manner. These help in identifying the reaction and chemistry of all the organic compounds. Alkynes are functional groups referring to the compounds that contain the triple bond of carbon.
There are two varieties of alkynes, and they are terminal alkynes and internal alkynes. The internal alkynes remain right in the middle of a molecule, as is suggested by their name. On the other hand, the terminal alkynes remain right at the end. These alkynes contain hydrogen atoms on any one of the triple bonded atoms of carbon. Then there are E and Z alkynes that can be distinguished based on molecular weight.