Why Are Alkynes More Acidic Than Alkenes and Alkanes Explained with pKa Values and Reactions
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 and Factors Affecting Their Acidic Strength
1. What is the acidity of alkynes?
The acidity of alkynes refers to the ability of terminal alkynes to donate a proton (H+) from the carbon–hydrogen bond attached to a triple-bonded carbon. In particular, terminal alkynes (RC≡CH) are weakly acidic because the hydrogen attached to the sp-hybridized carbon can be removed by strong bases. The acidity arises due to:
- High s-character (50%) of the sp-hybridized carbon
- Greater electronegativity of sp carbon
- Stabilization of the conjugate base called the acetylide ion (RC≡C-)
2. Why are alkynes more acidic than alkenes and alkanes?
Alkynes are more acidic than alkenes and alkanes because the sp-hybridized carbon stabilizes the conjugate base better than sp2 or sp3 carbons. The order of acidity is:
- Alkyne (sp) > Alkene (sp2) > Alkane (sp3)
- sp carbon has 50% s-character, making it more electronegative
- Greater s-character pulls electron density closer to the nucleus
- The negative charge in the acetylide ion is more stable
3. Which alkynes are acidic: terminal or internal?
Only terminal alkynes are acidic because they contain a hydrogen directly bonded to the sp-hybridized carbon of the triple bond. Terminal alkynes have the structure RC≡CH and can lose H+ to form an acetylide ion. In contrast:
- Internal alkynes (RC≡CR′) do not have a hydrogen attached to the triple-bonded carbon
- Therefore, internal alkynes do not show acidic behavior
4. What is the pKa value of terminal alkynes?
The typical pKa value of terminal alkynes is approximately 25. This indicates that alkynes are weak acids but significantly more acidic than alkenes and alkanes. For comparison:
- Terminal alkyne: pKa ≈ 25
- Alcohol (ROH): pKa ≈ 16–18
- Alkene: pKa ≈ 44
- Alkane: pKa ≈ 50
5. How do alkynes react with strong bases?
Terminal alkynes react with strong bases to form acetylide salts by deprotonation of the acidic hydrogen. A strong base such as sodium amide removes the proton as follows:
- RC≡CH + NaNH2(l) → RC≡C-Na+ + NH3(g)
6. Why does sp hybridization increase the acidity of alkynes?
sp hybridization increases the acidity of alkynes because sp orbitals contain 50% s-character, which stabilizes the negative charge in the conjugate base. Greater s-character means:
- Electrons are held closer to the nucleus
- The carbon atom is more electronegative
- The acetylide ion (RC≡C-) is more stable
7. What is an acetylide ion?
An acetylide ion is the conjugate base formed when a terminal alkyne loses its acidic proton. Its general formula is RC≡C-, where the negative charge resides on the sp-hybridized carbon. Key features include:
- Formed by deprotonation of RC≡CH
- Strong nucleophile in organic reactions
- Participates in carbon–carbon bond formation
8. Can alkynes react with sodium metal to show acidity?
Yes, terminal alkynes react with sodium metal to form sodium acetylide and hydrogen gas, demonstrating their acidic nature. The balanced reaction is:
- 2RC≡CH + 2Na(s) → 2RC≡CNa(s) + H2(g)
9. What is the difference between the acidity of alcohols and alkynes?
Alcohols are more acidic than alkynes because the negative charge in alkoxide ions is stabilized by the electronegative oxygen atom. Comparison:
- Alcohol (ROH): pKa ≈ 16–18
- Terminal alkyne (RC≡CH): pKa ≈ 25
- Oxygen is more electronegative than carbon
- Alkoxide ion (RO-) is more stable than acetylide ion
10. How is the acidity of alkynes used in organic synthesis?
The acidity of alkynes is used in organic synthesis to generate acetylide ions for carbon–carbon bond formation. After deprotonation with a strong base like NaNH2, the acetylide ion reacts with primary alkyl halides:
- RC≡C-Na+ + R′–Br → RC≡CR′ + NaBr
