Alkynes: Structure, Properties, and Reactions

Chapter 9 – Alkynes

Structure and Physical Properties

Alkynes are hydrocarbons containing a carbon-carbon triple bond. Their structure and properties are distinct from alkanes and alkenes due to the linear geometry and high electron density of the triple bond.

• Structure: The carbon atoms in an alkyne triple bond are sp-hybridized, resulting in a linear geometry with a bond angle of 180°.

• General Formula: $C_nH_{2n-2}$

• Physical Properties:

  • Alkynes, like alkanes and alkenes, are nonpolar and interact via dispersion forces.

  • Boiling points and melting points increase with molecular weight and are similar to those of alkenes and alkanes.

  • Alkynes are less reactive than alkenes toward electrophilic addition due to the stability of the triple bond.

Nomenclature of Alkynes

Alkynes are named similarly to alkenes, with the suffix -yne indicating the presence of a triple bond. The parent chain is the longest chain containing the triple bond, and numbering gives the triple bond the lowest possible number.

• Common Names: Some alkynes have traditional names (e.g., acetylene for ethyne).

• Examples:

  • 1-butyne: $CH_3CH_2C CH$

  • 2-butyne: $CH_3C CCH_3$

  • Phenylacetylene: $C_6H_5C CH$

Acidity of Alkynes

The acidity of alkynes is greater than that of alkenes and alkanes due to the high s-character of the sp-hybridized carbon. Terminal alkynes (with a hydrogen attached to the triple-bonded carbon) are significantly more acidic than internal alkynes.

• pKa Values:

  • Alkanes: $pK_a \approx 50$

  • Alkenes: $pK_a \approx 44$

  • Alkynes: $pK_a \approx 25$

• Key Principle: Lower $pK_a$ = Stronger acid

• Terminal Alkynes: Can be deprotonated by strong bases (e.g., $NaNH_2$) to form acetylide ions, which are useful nucleophiles in synthesis.

Preparation of Alkynes

Alkynes can be synthesized by elimination reactions, typically from dihalides. There are two types of dihalides:

• Geminal Dihalides: Both halides on the same carbon.

• Vicinal Dihalides: Halides on adjacent carbons.

General Reaction:

• Double elimination of dihalides using strong base (e.g., $NaNH_2$) yields alkynes.

Reduction of Alkynes

Alkynes can be reduced to alkenes or alkanes using different reagents, allowing for control over the stereochemistry of the product.

• Hydrogenation: $H_2$, $Pd/C$ or $Pt$ catalyst fully reduces alkynes to alkanes.

• Lindlar's Catalyst: $H_2$, Lindlar's catalyst (Pd/CaCO3, poisoned) reduces alkynes to cis-alkenes (syn addition).

• Dissolving Metal Reduction: $Na$, $NH_3$ (liq.) reduces alkynes to trans-alkenes (anti addition).

Hydrohalogenation of Alkynes

Alkynes react with hydrogen halides (HX) to form haloalkenes and then geminal dihalides. The reaction follows Markovnikov's rule for regioselectivity.

• 1 Equivalent HX: Forms a haloalkene (Markovnikov addition).

• 2 Equivalents HX: Forms a geminal dihalide (both halides on the same carbon).

• Mechanism: Proceeds via carbocation intermediates; rearrangements are possible.

Hydration of Alkynes

Alkynes undergo hydration to yield carbonyl compounds. The reaction is catalyzed by acid and mercury(II) sulfate (Markovnikov addition) or by hydroboration-oxidation (anti-Markovnikov addition).

• Acid-Catalyzed Hydration: $H_2SO_4$, $HgSO_4$ yields ketones via enol intermediates (tautomerization).

• Hydroboration-Oxidation: $1. BH_3$, $2. H_2O_2$, $NaOH$ yields aldehydes from terminal alkynes (anti-Markovnikov addition).

• Tautomerization: Enols rapidly convert to more stable carbonyl compounds (keto form).

Halogenation of Alkynes

Alkynes react with halogens ($Br_2$, $Cl_2$) to form tetrahalides. Addition can be anti or syn, and with one equivalent, both stereoisomers are formed.

• 2 Equivalents Halogen: Complete addition to form tetrahalides.

• 1 Equivalent Halogen: Forms dihaloalkenes (mixture of E and Z isomers).

Ozonolysis of Alkynes

Ozonolysis cleaves the triple bond, fully oxidizing the carbons to carboxylic acids (or CO2 if terminal).

• Reagents: $O_3$, $H_2O$

• Products: Carboxylic acids (or CO2 for terminal alkynes)

Alkylation of Terminal Alkynes

Terminal alkynes can be deprotonated to form acetylide ions, which are strong nucleophiles and can undergo alkylation with primary alkyl halides via $S_N2$ mechanism.

• Step 1: Deprotonation with strong base (e.g., $NaNH_2$) to form acetylide ion.

• Step 2: Reaction with primary alkyl halide to form a new C–C bond.

• Double Alkylation: Possible with two equivalents of base and alkyl halide.

Summary Table: Key Reactions of Alkynes

Reaction Type	Reagents	Product	Notes
Hydrogenation	$H_2$, $Pd/C$	Alkane	Complete reduction
Partial Hydrogenation (cis)	$H_2$, Lindlar's catalyst	cis-Alkene	Syn addition
Partial Hydrogenation (trans)	$Na$, $NH_3$ (liq.)	trans-Alkene	Anti addition
Hydrohalogenation	HX (1 or 2 eq.)	Haloalkene or gem-dihalide	Markovnikov addition
Hydration	$H_2SO_4$, $HgSO_4$	Ketone (from enol)	Markovnikov
Hydroboration-Oxidation	$1. BH_3$, $2. H_2O_2$, $NaOH$	Aldehyde (from enol)	Anti-Markovnikov
Halogenation	$Br_2$, $Cl_2$	Tetrahalide	Anti & syn addition
Ozonolysis	$O_3$, $H_2O$	Carboxylic acids/CO2	Cleavage of triple bond
Alkylation	$1. NaNH_2$, $2. R-X$	Alkyne with new C–C bond	Only with primary alkyl halides

Additional info:

• Energetics and reaction coordinate diagrams for alkyne reactions show the relative activation energies and intermediates for multi-step processes.

• Mechanistic details (e.g., for hydrohalogenation and hydration) involve carbocation or enol intermediates, with regioselectivity and stereochemistry determined by the reagents and conditions.