Why Are Alkynes More Acidic Than Alkenes and Alkanes?
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 di-hydrogen gas. The whole procedure where alkynes respond to bases and release di-hydrogen gas proves the acidity of alkynes.
HC ≡ CH + Na → HC ≡ C– Na+ + 1/2H2
Understanding the Comparative Acidity of Alkynes
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'' hybridised.
Because of the maximum percentage or around 50% of the “s” character present in alkynes, “sp” hybridised 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
HC≡CH>CH3–C≡CH>>CH3–C≡C–CH3
Understanding the Acidic Character of Alkynes
(-)(+)
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 act in the form of a steady conjugate base. As has already been pointed out, sp-hybridised 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.
What Atom Causes Acidity?
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-hybridised 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-hybridised 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 polarisation 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 hybridised carbon bonds tends to be less in alkenes and alkanes. This makes way for lesser electronegative carbon atoms corresponding to less movement 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.
Relative Acidity of Alkynes
Alkyne's acidity stems from its tendency to lose hydrogen atoms and create alkylidenes. As a result, alkynes function as Bronsted-Lowry acids. In alkynes, the triple bound carbon atom is "sp" hybridised. The "sp" hybridised orbitals of carbon atoms in alkynes have a high electronegativity due to the large percentage of "s" character (50%) in alkynes. The C-H bond of alkynes is strongly attracted by these. Alkyne molecules may easily lose hydrogen atoms and create alkynide ions as a result. As a result, the hydrogen atom connected to the triply bound carbon atom has an acidic character.
Because the carbon atoms in alkanes and alkenes are "sp3" and "sp2" hybridised, the acidity of alkynes is larger than that of alkanes and alkenes. As a result, compared to alkynes, these molecules have a lower fraction of "s" character. As a result, the carbon atom's electronegativity is lower in these situations than in alkynes. As a result, alkanes and alkenes do not display hydrogen gas liberation reactions with bases. It's also worth noting that only hydrogen atoms linked to a triply bonded carbon atom are acidic, not the hydrogen atoms in the rest of the alkyne chain. The following is the overall trend in acidity:
HC≡CH > H2C=CH2> CH3–CH3
HC≡CH>CH3–C≡CH>>CH3–C≡C–CH3
Hybridisation Effect
The stability of the related carbanions generated by deprotonation might explain the significant rise in acidity of terminal alkynes compared to other hydrocarbons. The suffix "-ide" denotes that the molecule is a negatively charged ion in the nomenclature of organic compounds.
The type of the hybridised orbital occupied by the lone pair of electrons determines the carbanion's stability. The lone pair in ethane occupies an sp3 orbital, while it occupies an sp2 orbital in ethene and an sp orbital in acetylene, as indicated above. The “s” character in the sp3, sp2, and sp orbitals is 25 percent, 33 percent, and 50 percent, respectively. A hybrid orbital with a greater “s” character will efficiently stabilise the negative charge since "s" orbitals are closer to the positively charged nucleus. In the presence of a suitable base, the acetylide ions will be the most stable and easily produced.
Conclusion
Alkynes contain pi and sigma bond connections between hydrogen and carbon. They are highly reactive compounds and probably the most reactive of all compounds. The whole procedure where alkynes respond to bases and release di-hydrogen gas proves the acidity of alkynes.
FAQs on Acidity of Alkynes Explained
1. What makes some alkynes acidic while others are not?
Only terminal alkynes, which have a hydrogen atom directly attached to a triple-bonded carbon, are acidic. This is because the carbon atom is sp-hybridised, making it highly electronegative. This pulls the electron density away from the hydrogen atom, allowing it to be released as a proton (H⁺). Internal alkynes lack this specific hydrogen atom and are therefore not acidic.
2. Why are alkynes more acidic than alkanes and alkenes?
The acidity of these hydrocarbons depends on the hybridisation of the carbon atom attached to the hydrogen. The order of acidity is: Alkynes (sp) > Alkenes (sp²) > Alkanes (sp³). This is because the sp hybridised carbon in alkynes has the highest s-character (50%), making it the most electronegative. This increased electronegativity stabilises the negative charge on the resulting conjugate base (acetylide ion) more effectively.
3. How can I test for the acidic nature of a terminal alkyne in a lab?
You can identify the acidic hydrogen of a terminal alkyne by reacting it with a strong base or specific reagents. For example:
- When a terminal alkyne reacts with a strong base like sodamide (NaNH₂), it forms a sodium acetylide precipitate.
- Reacting it with Tollens' reagent or Fehling's solution also produces a characteristic precipitate, confirming its acidic nature.
4. What exactly is an acetylide anion?
An acetylide anion is the conjugate base formed when a terminal alkyne donates its acidic proton. For an alkyne with the formula R-C≡C-H, the corresponding acetylide anion is R-C≡C:⁻. This anion is relatively stable because the negative charge resides on an sp-hybridised carbon atom, which can accommodate the charge due to its high s-character.
5. How does the 's-character' of an orbital influence the acidity of alkynes?
The 's-character' refers to the percentage of the s-orbital's contribution to a hybrid orbital. An s-orbital is closer to the nucleus than a p-orbital. Therefore, an orbital with higher s-character holds its electrons more tightly, making the atom more electronegative. Since an sp-hybridised carbon (in alkynes) has 50% s-character, it strongly attracts the C-H bond electrons, weakening the bond and making the hydrogen easier to remove as a proton.
6. Is an alkyne more acidic than ammonia (NH₃)?
Yes, a terminal alkyne is significantly more acidic than ammonia. The acidity of a substance is often measured by its pKa value, where a lower pKa indicates a stronger acid. The pKa of ethyne (a terminal alkyne) is about 25, while the pKa of ammonia is about 38. This large difference means that the conjugate base of an alkyne (acetylide ion) is much more stable than the conjugate base of ammonia (amide ion).
