Question

List alkanes, alkenes and terminal alkynes in order of decreasing acidity of their terminal C—H. Also, account for the order of acidities. Give two reactions to show acidic nature of terminal alkynes.

Answer 1

Alkynes are one of the simplest hydrocarbons known to us. They have a general formula of CnH2n-2. Alkynes belong to the family of unsaturated hydrocarbons that is; they contain both sigma and pi bond linkages between carbon and hydrogen.An alkyne molecule contains at least one triple bond between two carbon atoms. For example: ethyne (CH≡CH). Ethyne reacts with strong bases such as sodiummetal and sodamide (NaNH2)to form sodiumacetylide along with the liberation of dihydrogen gas. This reaction of alkynes with bases to liberate dihydrogen gas indicates the acidity of alkynes.

HC ≡ CH + Na → HC ≡ C– Na+ + 1/2H2

Acidity of alkynes is due to their ability to lose hydrogen atom to form alkynideions. Thus, alkynes act as Bronsted-Lowry acids. The triple bonded carbon atom in alkynes is “sp”hybridized. Due to high percentage of “s” character (50%) in alkynes, the “sp” hybridized orbitals of carbon atom in alkynes exhibit high electronegativity. These attract the C-H bond of alkynes to a great extent. Thus, alkyne molecules can easily loose hydrogen atom forming alkynide ions. Hence, we can say that hydrogen atom attached to the triply bonded carbon atom is acidic in nature.

Click here to know about Physical Property of Alkynes

The acidity of alkynes is greater than the acidity of alkanes and alkenes as the carbon atom in alkanes and alkenes are “sp3” and “sp2” hybridized respectively. Hence, these molecules contain lesser percentage of “s” character in comparison to alkynes.Thus, the electronegativity of carbon atom in these cases is lesser than alkynes. Hence, alkanes and alkenes don‘t show the reactions with bases to liberate hydrogen gas. It can further be noted that only hydrogen atom attached to triply bonded carbon atom are acidic not the other hydrogen atoms in the alkyne chain. General trend in acidity is seen as:

HC≡CH > H2C=CH2> CH3–CH3

HC≡CH>CH3–C≡CH>>CH3–C≡C–CH3

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Answer 2

The order of decreasing acidic strengths of their terminal C—H bond is  

        Alkynes> alkenes> alkanes

  Alkynes are more acidic than the alkenes and alkanes. The acidic nature of Acetylene can be explained by the s-character of the sp hybridised orbitals. The electron in s-orbital is held more tightly than in the p-orbital since the s-orbital electrons are closer to the nucleus.

    And also there are more s-character in the sp orbitals, so the electrons in sp orbitals are more strongly held by the nucleus than in the $sp^2$and $sp^3$ electrons. Because of the sp hybridised carbon has electronegative more than $sp^2$ and $sp^3$ hybridised carbon atoms, hydrogen of acetylenic carbon develops a more positive charge and shows acidic character.

$CH\quad \equiv \quad C-H\quad +\quad NaNH_{ 2 }\longrightarrow CH\quad \equiv \quad C^{ - }Na^{ + }(sodiumacetalyide)+NH_{ 3 }CH\quad \equiv \quad C-H+NaNH_{ 2 }→CH\quad \equiv \quad C^{ - }Na^{ + }+NH_{ 3 }$

$CH\quad \equiv \quad CH\quad +\quad 2AgNO_{ 3 }\longrightarrow Ag^{ + }2NH_{ 4 }NO_{ 3 }\quad +\quad 2H_{ 2 }O.$.