What is the Alkynes Acidity?
Alkynes are one of the simplest hydrocarbons that we are familiar with. The general formula for these compounds is CnH2n-2.
Alkynes belong to the family of unsaturated hydrocarbons, as they possess both sigma and pi carbon-hydrogen bonding.
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Alkynes Acidity Explanation
There is at least one triple bond between two carbon atoms in an alkyne molecule. For example ethyne (CH≡CH). Ethyne reacts with strong bases, including sodium metal and sodamide (NaNH2), to produce sodium acetylide and dihydrogen gas. This reaction between alkynes and bases, which releases dihydrogen gas, demonstrates the acidity of alkynes.
HC ≡ CH + Na → HC ≡ C– Na+ + 1/2H2
Relative Acidity of Alkynes
Due to their ability to lose hydrogen atoms and form alkynideions, alkynes are acidic. Therefore, alkynes are Brnsted-Lowry acids. In alkynes, the triple-bonded carbon atom is “sp” hybridized. Due to the high percentage of “s” character (50%) in alkynes, the “sp” hybridized carbon orbitals exhibit high electronegativity. These significantly attract the C-H bond of alkynes. Thus, alkyne molecules can readily lose a hydrogen atom, resulting in the formation of alkynide ions. The hydrogen atom attached to the triple-bonded carbon atom is therefore acidic in nature.
Learn more about the physical properties of alkynes.
Alkynes are more acidic than alkanes and alkenes due to the “sp3” and “sp2” hybridization of the carbon atoms in alkanes and alkenes, respectively. Consequently, these molecules contain a lower proportion of “s” character than alkynes. In these cases, the electronegativity of the carbon atom is less than that of alkynes. Therefore, alkanes and alkenes do not react with bases to release hydrogen gas. Further, only the hydrogen atoms attached to carbon atoms with triple bonds are acidic, not the other hydrogen atoms in the alkyne chain. This is the general trend in acidity:
HC≡CH > H2C=CH2> CH3–CH3
HC≡CH>CH3–C≡CH>>CH3–C≡C–CH3
Visit Utopper.com for a detailed discussion on the acidity of alkynes and other acid and base-related topics, such as di and polybasic acids and bases.
Frequently Asked Questions – FAQs
Q.1 Are alkynes strongly acidic?
Since 1-alkynes are very weak acids, their conjugate bases, RCC, are quite robust bases. These anions are also reactive carbon nucleophiles, and it is primarily due to this property that they are useful for organic synthesis.
Q.2 Are all alkynes acidic?
Alkynes have a mild acidity. Strong bases, such as sodium amide, can cause acid-base reactions when exposed to acids. Due to the high s characteristic of the sp hybrid orbital, which combines with the s orbital of the hydrogen atom to form a simple covalent bond, the terminal alkynes are acidic.
Q.3 What is the cause of alkynes’ acidity?
Because alkynes can release hydrogen atoms to form alkyne ions, they are acidic. The alkyne is therefore utilized in the form of Bronsted-Lowry acid. As stated previously, alkynes contain a triple-bonded carbon atom designated as sp. Hybrid.
Q.4 Are alcohols more acidic than alkynes?
More acidic than alkynes is alcohol. H+ is removed from alcohols to produce O. O is more stable than C when H+ is removed from alkynes. Since oxygen has a higher electronegativity than carbon.
Q.5 What hydrocarbon has the highest acidity?
Cyclopentadiene is one of the most acidic hydrocarbons in reality. The pKa of the hydrogen atoms above is 15.
