Alkynes — Revision Notes

⚡ 30-Second Revision

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.

2-Minute Revision

Alkynes are hydrocarbons with a carbon-carbon triple bond, characterized by $sp$ hybridization and linear geometry. Their general formula is $C_nH_{2n-2}$. A key feature is the acidity of terminal alkynes ($R-C equiv CH$) due to the high $s$-character of the $sp$ carbon, allowing them to react with strong bases and metal ions to form acetylides. This property is used in distinguishing tests with Tollens' reagent (white precipitate) and ammoniacal cuprous chloride (red precipitate).

Preparation methods often involve dehydrohalogenation of dihalides. Alkynes undergo various addition reactions. Partial hydrogenation is stereoselective: Lindlar's catalyst yields *cis*-alkenes, while Na/liquid ammonia yields *trans*-alkenes.

Hydrohalogenation follows Markovnikov's rule, except for HBr with peroxides (anti-Markovnikov). Hydration (Kuccherov's reaction) forms aldehydes (from ethyne) or ketones (from other terminal alkynes) via enol-keto tautomerism.

Alkynes also undergo oxidation (cleavage with strong agents) and polymerization (e.g., ethyne to benzene). Focus on reagent-product relationships and the specific conditions that dictate reaction outcomes and stereochemistry.

5-Minute Revision

Alkynes are unsaturated hydrocarbons defined by a $C equiv C$ triple bond. The carbons are $sp$ hybridized, leading to a linear arrangement and $180^circ$ bond angles. This $sp$ hybridization also makes the C-H bond in terminal alkynes ($R-C equiv CH$) acidic, a crucial property for identification. The general formula is $C_nH_{2n-2}$.

Preparation: The most common lab method is dehydrohalogenation of vicinal or geminal dihalides using strong bases like alcoholic KOH followed by sodamide ($NaNH_2$) in liquid ammonia. Industrially, ethyne (acetylene) is produced from calcium carbide ($CaC_2$) or by thermal cracking of methane.

Reactions:

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Addition Reactions: — Alkynes undergo electrophilic addition across their two $pi$ bonds.

* Hydrogenation: Complete hydrogenation yields alkanes. Partial hydrogenation is stereoselective: Lindlar's catalyst ($Pd/CaCO_3$ poisoned) gives *cis*-alkenes, while Na in liquid ammonia (Birch reduction) gives *trans*-alkenes.

*Example: $CH_3-C equiv C-CH_3 xrightarrow{H_2/\text{Lindlar's}} \text{cis}-CH_3-CH=CH-CH_3$ *Example: $CH_3-C equiv C-CH_3 xrightarrow{Na/\text{liq. } NH_3} \text{trans}-CH_3-CH=CH-CH_3$ * Halogenation: Adds two molecules of halogen ($X_2$) to form tetrahaloalkanes.

* Hydrohalogenation: Follows Markovnikov's rule, adding two molecules of $HX$ to form geminal dihalides. For HBr with peroxides, anti-Markovnikov addition occurs for the first equivalent. *Example: $CH_3-C equiv CH xrightarrow{HBr} CH_3-CBr=CH_2 xrightarrow{HBr} CH_3-CBr_2-CH_3$ *Example: $CH_3-C equiv CH xrightarrow{HBr/\text{peroxides}} CH_3-CH=CHBr$ * Hydration (Kuccherov's Reaction): With $HgSO_4/H_2SO_4$, water adds to form an enol, which tautomerizes.

Ethyne yields ethanal ($CH_3CHO$), while other terminal alkynes yield methyl ketones ($R-CO-CH_3$).

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Acidity of Terminal Alkynes: — They react with strong bases (e.g., $NaNH_2$) and metal ions. Tests include Tollens' reagent (white silver acetylide precipitate) and ammoniacal cuprous chloride (red cuprous acetylide precipitate).
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Oxidation: — Strong oxidizing agents (hot $KMnO_4$) cleave the triple bond to form carboxylic acids (or $CO_2$ from terminal carbon).
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Polymerization: — Ethyne can cyclically polymerize to benzene in a red hot iron tube.

Key takeaway: Master the specific reagents and their products, especially for stereochemistry and distinguishing tests.

Prelims Revision Notes

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Definition & Structure: — Alkynes are unsaturated hydrocarbons with $C equiv C$ triple bond. General formula $C_nH_{2n-2}$. Triple-bonded carbons are $sp$ hybridized, linear geometry, $180^circ$ bond angle. Bond length $C equiv C approx 1.20,\text{Å}$.
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Nomenclature: — Suffix '-yne'. Number chain to give triple bond lowest number. If both double and triple bonds, it's an 'enyne'.
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**Acidity of Terminal Alkynes ($R-C equiv CH$):**

* Due to 50% $s$-character of $sp$ carbon, making C-H bond slightly acidic. * Acidity order: Alkynes > Alkenes > Alkanes. * Tests: * Tollens' Reagent ($[Ag(NH_3)_2]^+OH^-$): Forms white precipitate of silver acetylide ($R-C equiv CAg$). * Ammoniacal Cuprous Chloride ($[Cu(NH_3)_2]^+Cl^-$): Forms red precipitate of cuprous acetylide ($R-C equiv CCu$). * *Note: Internal alkynes do not give these tests.*

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Preparation Methods:

* Dehydrohalogenation: From vicinal or geminal dihalides with strong bases (e.g., alc. KOH then $NaNH_2$/liq. $NH_3$). *Example: $CH_3-CHBr-CH_2Br xrightarrow{\text{alc. KOH}} CH_3-CH=CHBr xrightarrow{NaNH_2} CH_3-C equiv CH$ * From Calcium Carbide: $CaC_2 + 2H_2O \rightarrow HC equiv CH + Ca(OH)_2$

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Reactions (Addition):

* Hydrogenation: * Complete: $H_2$/Ni, Pt, or Pd $ightarrow$ Alkane. * Partial: * Lindlar's Catalyst ($H_2/Pd/CaCO_3$ poisoned with quinoline/lead acetate) $ightarrow$ *cis*-Alkene (syn-addition).

* **Na/liquid $NH_3$** (Birch Reduction) $ightarrow$ *trans*-Alkene (anti-addition). * Halogenation: $X_2$ (e.g., $Br_2$) $ightarrow$ Dihaloalkene $ightarrow$ Tetrahaloalkane. Decolorizes bromine water (test for unsaturation).

* **Hydrohalogenation ($HX$):** Follows Markovnikov's rule. Two equivalents add to form geminal dihalides. *Example: $CH_3-C equiv CH + 2HBr \rightarrow CH_3-CBr_2-CH_3$ * **Hydrohalogenation ($HBr$ with Peroxides):** Anti-Markovnikov's rule for the first addition.

*Example: $CH_3-C equiv CH + HBr xrightarrow{\text{peroxides}} CH_3-CH=CHBr$ * Hydration (Kuccherov's Reaction): $H_2O/HgSO_4/H_2SO_4$. Forms enol, which tautomerizes. * Ethyne ($HC equiv CH$) $ightarrow$ Ethanal ($CH_3CHO$).

* Other Terminal Alkynes ($R-C equiv CH$) $ightarrow$ Methyl Ketones ($R-CO-CH_3$). * Internal Alkynes ($R-C equiv C-R'$) $ightarrow$ Ketones.

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Oxidation:

* Baeyer's Reagent (cold, dil., alk. $KMnO_4$): Decolorizes (test for unsaturation), forms dicarbonyl compounds. * **Hot, acidic $KMnO_4$:** Cleaves triple bond, forms carboxylic acids (terminal C $ightarrow CO_2$).

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Polymerization: — Ethyne $xrightarrow{\text{red hot iron tube}} \text{Benzene}$ (cyclic trimerization).

Vyyuha Quick Recall

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.