Why are terminal alkynes acidic, and how does their acidity compare to other hydrocarbons?

Terminal alkynes are acidic due to the $sp$ hybridization of the carbon atom involved in the C-H bond. The $sp$ orbital has 50% s-character, which means the electrons in the C-H bond are held closer to the carbon nucleus, making the carbon more electronegative. This increased electronegativity polarizes the C-H bond, making the hydrogen atom more easily removable as a proton. The resulting acetylide anion is stabilized by the high s-character of the carbon. Their acidity order is: Terminal Alkynes > Alkenes > Alkanes. However, they are weaker acids than water, alcohols, and carboxylic acids, but stronger than ammonia.

What is the role of Lindlar's catalyst in alkyne reactions?

Lindlar's catalyst is a poisoned palladium catalyst, typically $Pd/CaCO_3$ treated with lead acetate and quinoline. Its role is to selectively hydrogenate alkynes to cis-alkenes. The 'poisoning' reduces the catalyst's activity, preventing further reduction of the alkene to an alkane. The reaction occurs via syn-addition, where both hydrogen atoms add to the same face of the triple bond, leading exclusively to the cis isomer.

How can you distinguish between a terminal alkyne and an internal alkyne using chemical tests?

Terminal alkynes possess an acidic hydrogen atom, which internal alkynes lack. This difference allows for chemical differentiation. Terminal alkynes will react with ammoniacal silver nitrate (Tollens' reagent) to form a white precipitate of silver acetylide, and with ammoniacal cuprous chloride to form a red precipitate of cuprous acetylide. Internal alkynes, having no acidic hydrogen, will not react with these reagents, thus no precipitate will be formed. This makes these reactions specific tests for terminal alkynes.

Explain the application of Markovnikov's rule in the hydration of alkynes.

In the hydration of alkynes (addition of water in the presence of $HgSO_4$ and $H_2SO_4$), Markovnikov's rule dictates the regioselectivity. The hydrogen atom from water adds to the carbon atom of the triple bond that already has more hydrogen atoms (or is less substituted), while the hydroxyl group (-OH) adds to the carbon atom with fewer hydrogen atoms (or is more substituted). This initially forms an enol, which is unstable and rapidly tautomerizes to a more stable keto form. For terminal alkynes (except ethyne), this leads to the formation of a methyl ketone.

What are the products of oxidative cleavage of alkynes with hot, concentrated $KMnO_4$?

Hot, concentrated potassium permanganate ($KMnO_4$) is a strong oxidizing agent that causes oxidative cleavage of the carbon-carbon triple bond. If the alkyne is internal ($R-C equiv C-R'$), the triple bond breaks, and each carbon forms a carboxylic acid ($R-COOH + R'-COOH$). If the alkyne is terminal ($R-C equiv C-H$), the carbon attached to the R group forms a carboxylic acid ($R-COOH$), while the terminal carbon (which was attached to hydrogen) is fully oxidized to carbon dioxide ($CO_2$) and water ($H_2O$). This reaction is useful for determining the position of the triple bond in an unknown alkyne.

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Version 1Updated 22 Mar 2026

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The physical and chemical properties of alkynes are fundamentally dictated by the presence of the carbon-carbon triple bond, which consists of one sigma bond and two pi bonds. This unique structural feature, particularly the $sp$ hybridization of the carbon atoms involved in the triple bond, imparts distinct characteristics. Physically, alkynes exhibit trends in boiling points, melting points, and…

Quick Summary

Alkynes are unsaturated hydrocarbons with a carbon-carbon triple bond ( $C equiv C$ ), having $sp$ hybridized carbons and a linear geometry. Physically, they are nonpolar, insoluble in water but soluble in organic solvents.

Their boiling and melting points increase with molecular weight, and lower members are gases. A key property is the acidity of terminal alkynes, where the hydrogen attached to the $sp$ carbon is weakly acidic, allowing reactions with strong bases and heavy metal ions (e.

g., Tollens' reagent, ammoniacal cuprous chloride) to form acetylides. Chemically, the electron-rich triple bond undergoes electrophilic addition reactions. These include hydrogenation (complete to alkanes, partial to cis-alkenes with Lindlar's catalyst, or trans-alkenes with Na/liq.

NH3), halogenation (to tetrahaloalkanes), hydrohalogenation (to geminal dihalides following Markovnikov's rule), and hydration (to enols that tautomerize to ketones/aldehydes, also following Markovnikov's rule).

Alkynes also undergo oxidation, with strong agents like hot $KMnO_4$ leading to oxidative cleavage and formation of carboxylic acids or $CO_2$ . Polymerization can also occur, such as the trimerization of ethyne to benzene.

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Key Concepts

Acidity of Terminal Alkynes and Acetylide Formation

The acidity of terminal alkynes is a unique property stemming from the $sp$ hybridization of the carbon atom…

Selective Hydrogenation of Alkynes

Alkynes can undergo partial hydrogenation to alkenes with specific catalysts, allowing for control over the…

Hydration of Alkynes and Keto-Enol Tautomerism

The addition of water to alkynes, typically catalyzed by mercuric sulfate ( $HgSO_4$ ) and dilute sulfuric acid…

General formula:

C_nH_{2n-2}

Hybridization:

sp

(linear geometry,

180^circ

bond angle)

Acidity of terminal alkynes:

R-C equiv C-H

(acidic H due to

sp

carbon)

Key Reagents & Products:

- $H_2/$ Lindlar's catalyst: Alkyne $ightarrow$ cis-alkene - $Na/liq. NH_3$ : Alkyne $ightarrow$ trans-alkene - $H_2/Pt/Pd/Ni$ : Alkyne $ightarrow$ Alkane - $X_2/CCl_4$ : Alkyne $ightarrow$ Tetrahaloalkane - $HX$ (excess): Alkyne $ightarrow$ Geminal dihalide (Markovnikov) - $H_2O/HgSO_4, H_2SO_4$ : Alkyne $ightarrow$ Ketone/Aldehyde (via enol, Markovnikov) - Cold, dilute $KMnO_4$ : Alkyne $ightarrow$ Vicinal diketone - Hot, conc.

$KMnO_4$ : Alkyne $ightarrow$ Carboxylic acids/ $CO_2$ - $NaNH_2$ : Terminal alkyne $ightarrow$ Acetylide - Tollens' reagent ( $[Ag(NH_3)_2]OH$ ): Terminal alkyne $ightarrow$ White Ag-acetylide ppt. - Ammoniacal $Cu_2Cl_2$ : Terminal alkyne $ightarrow$ Red Cu-acetylide ppt.

To remember alkyne hydrogenation products: Lindlar's gives Cis, Na/liq. Ammonia gives Trans. (LC NAT - 'L' for Lindlar, 'C' for Cis; 'N' for Na/liq. Ammonia, 'T' for Trans).

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