What is the general formula for alkynes and how does it relate to their unsaturation?

The general molecular formula for acyclic alkynes with one triple bond is $C_nH_{2n-2}$, where 'n' is the number of carbon atoms ($n ge 2$). This formula indicates a high degree of unsaturation. Compared to alkanes ($C_nH_{2n+2}$) and alkenes ($C_nH_{2n}$), alkynes have four and two fewer hydrogen atoms, respectively, for the same number of carbon atoms. Each triple bond accounts for two degrees of unsaturation, meaning it can add two molecules of hydrogen or other reagents across it, making them highly reactive towards addition reactions.

Why are terminal alkynes acidic, and how is this property used in identification?

Terminal alkynes ($R-C equiv CH$) are acidic because the hydrogen atom attached to the $sp$ hybridized carbon is relatively easily removed. The $sp$ hybrid orbital has 50% $s$-character, which means the electrons in the $C-H$ bond are held closer to the more electronegative $sp$ carbon nucleus. This makes the hydrogen slightly positive and the $C-H$ bond weaker, allowing strong bases to abstract the proton. This acidity is utilized in identification tests: terminal alkynes react with Tollens' reagent (ammoniacal silver nitrate) to form a white precipitate of silver acetylide, and with ammoniacal cuprous chloride to form a red precipitate of cuprous acetylide. Internal alkynes, alkanes, and alkenes do not give these tests.

Explain the difference in products when an alkyne is partially hydrogenated using Lindlar's catalyst versus sodium in liquid ammonia.

Partial hydrogenation of an alkyne to an alkene can yield different stereoisomers depending on the catalyst. Lindlar's catalyst, which is palladium on calcium carbonate poisoned with lead acetate and quinoline, facilitates *syn*-addition of hydrogen, meaning both hydrogen atoms add to the same face of the triple bond. This results in the formation of a *cis*-alkene. In contrast, reduction with sodium metal in liquid ammonia (Birch reduction) involves a radical mechanism that favors *anti*-addition, where the hydrogen atoms add to opposite faces of the triple bond, leading to the formation of a *trans*-alkene. This stereoselectivity is a key concept for NEET.

What is Kuccherov's reaction and what type of products does it yield for different alkynes?

Kuccherov's reaction is the hydration of alkynes, where water is added across the triple bond in the presence of mercuric sulfate ($HgSO_4$) and dilute sulfuric acid ($H_2SO_4$). The reaction proceeds via an unstable enol intermediate, which rapidly tautomerizes to a more stable keto form. For ethyne ($HC equiv CH$), the product is ethanal (acetaldehyde). For other terminal alkynes ($R-C equiv CH$), the addition follows Markovnikov's rule, forming a methyl ketone ($R-CO-CH_3$). For internal alkynes ($R-C equiv C-R'$), if symmetrical, a single ketone is formed; if unsymmetrical, a mixture of two different ketones can be formed.

How can alkynes be distinguished from alkenes and alkanes using chemical tests?

Alkynes, like alkenes, are unsaturated and can be distinguished from alkanes by their ability to decolorize bromine water (reddish-brown to colorless) and Baeyer's reagent (purple to colorless), indicating the presence of pi bonds. However, to distinguish alkynes from alkenes, or specifically terminal alkynes from internal alkynes/alkenes/alkanes, we use the acidity of terminal alkynes. Terminal alkynes react with Tollens' reagent (ammoniacal silver nitrate) to form a white precipitate of silver acetylide, and with ammoniacal cuprous chloride to form a red precipitate of cuprous acetylide. Alkenes, alkanes, and internal alkynes do not give these precipitate tests.

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Alkynes

Chemistry

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Version 1Updated 22 Mar 2026

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Alkynes are unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon triple bond ( $C equiv C$ ) within their molecular structure. This triple bond consists of one sigma ( $sigma$ ) bond and two pi ( $pi$ ) bonds. The carbon atoms involved in the triple bond are $sp$ hybridized, resulting in a linear geometry around these carbon centers with a bond angle of $180^circ$ . Their …

Quick Summary

Alkynes are unsaturated hydrocarbons containing at least one carbon-carbon triple bond ( $C equiv C$ ). The carbon atoms involved in the triple bond are $sp$ hybridized, resulting in a linear geometry with $180^circ$ bond angles.

The general formula for acyclic alkynes with one triple bond is $C_nH_{2n-2}$ . They are highly reactive due to the presence of two pi bonds. Key preparation methods include dehydrohalogenation of vicinal or geminal dihalides using strong bases like $NaNH_2$ .

Characteristic reactions include addition reactions such as hydrogenation (partial hydrogenation with Lindlar's catalyst yields *cis*-alkenes, while Na/liq. $NH_3$ yields *trans*-alkenes), halogenation, hydrohalogenation (following Markovnikov's rule), and hydration (Kuccherov's reaction, forming aldehydes or ketones via enol-keto tautomerism).

Terminal alkynes possess an acidic hydrogen, allowing them to react with strong bases and form metal acetylides, which is a key distinguishing feature used in tests with Tollens' reagent and ammoniacal cuprous chloride.

Alkynes also undergo oxidation and polymerization reactions.

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Key Concepts

Acidity of Terminal Alkynes and its Tests

The unique acidity of terminal alkynes is a direct consequence of the $sp$ hybridization of the carbon atom…

Dehydrohalogenation for Alkyne Synthesis

Dehydrohalogenation is a primary method for synthesizing alkynes from dihaloalkanes. This process involves…

Kuccherov's Reaction (Hydration of Alkynes)

Kuccherov's reaction is the specific name for the acid-catalyzed hydration of alkynes, which involves the…

General Formula: —

C_nH_{2n-2}

(for acyclic, one triple bond)

Hybridization: —

sp

for triple-bonded carbons

Geometry: — Linear (

180^circ

bond angle)

Acidity: — Terminal alkynes (

R-C equiv CH

) are acidic due to

sp

carbon's high

s

-character.

Preparation:

- Dehydrohalogenation of vicinal/geminal dihalides: $R-CHX-CH_2X xrightarrow{NaNH_2} R-C equiv CH$ - From $CaC_2$ : $CaC_2 + 2H_2O \rightarrow HC equiv CH + Ca(OH)_2$

Reactions:

- Hydrogenation: - Complete: $R-C equiv C-R' xrightarrow{H_2/Ni, Pt, Pd} R-CH_2-CH_2-R'$ - Partial (*cis*): $R-C equiv C-R' xrightarrow{H_2/\text{Lindlar's}} \text{cis}-R-CH=CH-R'$ - Partial (*trans*): $R-C equiv C-R' xrightarrow{Na/ ext{liq.

For Alkyne Reductions, remember: 'LINdlar's is CINematic, Na is TRANS-gender.'

LINdlar's catalyst gives CIS-alkenes.

Na — (sodium in liquid ammonia) gives TRANS-alkenes.

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