Chapter 10: Alkynes – Properties, Nomenclature, Reactions, and Synthesis

Alkynes: Structure and Properties

Definition and General Features

Alkynes are a class of hydrocarbons characterized by the presence of a carbon-carbon triple bond. This triple bond imparts unique chemical and physical properties to alkynes, making them important in organic synthesis and industrial applications.

• Alkynes are unsaturated molecules, meaning they contain fewer hydrogen atoms than alkanes or alkenes.

• Acyclic alkynes follow the general formula $C_nH_{2n-2}$.

• Cyclic alkynes follow the formula $C_nH_{2n-4}$.

• Examples include acetylene (ethyne), pargyline (antihypertensive), and ishtytylphenol (convulsant).

Bonding and Hybridization

The carbon atoms in the triple bond of alkynes are sp-hybridized, resulting in a linear geometry and high electron density around the bond.

• The triple bond consists of one sigma ($\sigma$) and two pi ($\pi$) bonds.

• This bond is the shortest and strongest among single, double, and triple bonds.

• Bond dissociation energies (BDE):

  • Ethane (single bond): $\sim 90$ kcal/mol

  • Ethene (double bond): $\sim 146$ kcal/mol

  • Ethyne (triple bond): $\sim 200$ kcal/mol

• High electron density makes alkynes good nucleophiles.

Classification: Terminal vs. Internal Alkynes

Alkynes are classified based on the position of the triple bond:

Type	General Form	Example
Terminal alkyne	R–C≡C–H
Internal/symmetrical alkyne	R–C≡C–R
Internal/unsymmetrical alkyne	R–C≡C–R'

• Internal alkynes are more stable than terminal alkynes due to alkyl group stabilization.

Physical Properties

Alkynes exhibit distinct physical properties compared to other hydrocarbons.

• Boiling points increase with molecular weight.

• Alkynes are insoluble in water due to their nonpolar nature.

• Terminal alkynes can be deprotonated easily, forming acetylide ions.

C5 compound	Boiling point (°C)
Pentane	36
trans-pent-2-ene	37
pent-3-yne	56

Nomenclature of Alkynes

Basic Rules

Naming alkynes follows IUPAC conventions similar to alkanes and alkenes, with specific modifications for the triple bond.

• Replace -ane (alkane) or -ene (alkene) with -yne for alkynes.

• The longest chain containing the triple bond is chosen as the parent.

• Number the chain to give the triple bond the lowest possible number.

• Substituents are listed in alphabetical order and assigned the lowest possible locants.

Example: 3-bromo-2-chloro-4-octyne

Multiple Triple Bonds and Functional Group Priority

• If two or more triple bonds are present, use prefixes such as di-, tri-, etc.

• If both alkene and alkyne are present, the alkene gets priority for the lowest number.

• Functional groups with higher priority (e.g., acids, esters, alcohols) receive the lowest locant.

Functional group	Priority
Acids	1 (highest)
Esters	2
Acid chlorides	3
Anhydrides	4
Amides	5
Nitriles	6
Aldehydes	7
Ketones	8
Alcohols	9
Amines	10
Alkenes, alkynes	11
Alkanes, ethers, alkyl halides	lowest

Reactivity of Alkynes

General Reactivity

Alkynes are electron-rich and act as nucleophiles in chemical reactions. Their reactivity is largely governed by the triple bond.

• Alkynes undergo electrophilic addition reactions.

• For alkenes, the rate-determining step is carbocation formation; for alkynes, it is the formation of a vinylic cation.

• Vinylic cations are less stable than carbocations due to the higher electronegativity of sp-hybridized carbon.

Relative stabilities of carbocations:

Carbocation Type	Stability
Tertiary	Most stable
Secondary
Primary
Methyl
Vinylic	Least stable

Formation of Carbon-Carbon Bonds

Mechanisms

Carbon-carbon bond formation is central to organic synthesis and can occur via nucleophilic attack or radical combination.

• Alkylation reaction: Terminal alkynes react with a strong base (e.g., sodium amide, NaNH2) to form an acetylide ion.

• The acetylide ion can then undergo substitution with alkyl halides to form new C–C bonds.

Equation:

• $\text{R–C≡C–H} + \text{NaNH}_2 \rightarrow \text{R–C≡C}^- \text{Na}^+ + \text{NH}_3$

• $\text{R–C≡C}^- + \text{R'–X} \rightarrow \text{R–C≡C–R'} + \text{X}^-$

Addition of Hydrogen

Catalytic Hydrogenation

Alkynes can be reduced to alkanes via catalytic hydrogenation, similar to alkenes.

• Typical catalysts: Pd/C, Pt, or Ni.

• Stoichiometry determines whether the reaction stops at the alkene or proceeds to the alkane.

• To selectively form a cis-alkene, use the Lindlar catalyst (Pd, CaCO3, Pb(OAc)2).

• For trans-alkene formation, use Na or Li in liquid ammonia (dissolving metal reduction).

Equations:

• $\text{R–C≡C–R'} + 2\text{H}_2 \xrightarrow{\text{Pd/C}} \text{R–CH}_2\text{CH}_2\text{R'}$

• $\text{R–C≡C–R'} + \text{H}_2 \xrightarrow{\text{Lindlar}} \text{cis-alkene}$

• $\text{R–C≡C–R'} + 2\text{Na} + 2\text{NH}_3 \rightarrow \text{trans-alkene}$

Electrophilic Addition Reactions

Overview Table

Reagent	X	Y	Representative alkyne	Initial product/intermediate	Process
HBr	H	Br	pent-2-yne	alkenyl halide	Hydrohalogenation
HgSO4, H2O	H+	OH	pent-2-yne	enol	Oxymercuration
BH3	H	OH	pent-2-yne	aldehyde/ketone	Hydroboration
Br2	Br	Br	pent-2-yne	dihaloalkane	Halogenation

Addition of Hydrogen Halide

• Addition of HX (e.g., HBr, HCl) to an alkyne yields an alkenyl halide; excess reagent leads to a dihaloalkane.

• Regioselectivity depends on the stability of the carbocation intermediate.

• Internal alkynes (symmetrical vs. unsymmetrical) yield different products.

Equation:

• $\text{R–C≡C–R'} + \text{HBr} \rightarrow \text{R–CH=CHBr–R'}$

• Excess $\text{HBr}$: $\text{R–CHBr–CHBr–R'}$

Addition of Water

• Addition of water to an alkyne forms an enol (alkenyl alcohol), which rapidly tautomerizes to a ketone or aldehyde.

• Tautomers are constitutional isomers in equilibrium.

• Hydration of alkynes requires a catalyst (e.g., HgSO4, H2SO4), resulting in an oxymercuration-type reaction.

Equation:

• $\text{R–C≡C–R'} + \text{H}_2\text{O} \xrightarrow{\text{HgSO}_4, \text{H}_2\text{SO}_4} \text{enol} \rightarrow \text{ketone/aldehyde}$

Addition of Borane

• Hydroboration-oxidation of alkynes yields aldehydes (from terminal alkynes) or ketones (from internal alkynes).

• Tautomerization occurs under basic conditions.

Equation:

• $\text{R–C≡C–H} \xrightarrow{1. \text{HB(sia)}_2, 2. \text{H}_2\text{O}_2, \text{NaOH}} \text{aldehyde}$

• $\text{R–C≡C–R'} \xrightarrow{1. \text{BH}_3, 2. \text{H}_2\text{O}_2, \text{NaOH}} \text{ketone}$

Addition of Halogens

• Halogenation of alkynes (e.g., Br2, Cl2) proceeds via the same mechanism as alkenes.

• Stoichiometry determines whether mono- or tetrahalogenated products are formed.

Equation:

• $\text{R–C≡C–R'} + \text{Br}_2 \rightarrow \text{R–CBr=CHBr–R'}$

• Excess $\text{Br}_2$: $\text{R–CBr_2–CBr_2–R'}$

Synthetic Design and Retrosynthetic Analysis

Retrosynthetic Analysis

Retrosynthetic analysis is a strategy for planning organic syntheses by working backward from the target molecule to simpler starting materials.

• Identify key bonds to break and possible intermediates.

• Apply known reactions (e.g., alkylation, addition, reduction) to construct the target molecule.

• Use reaction maps to visualize possible synthetic routes.

Example: Synthesis of pentan-2-ol from propene via alkynes and addition reactions.

Practice Problems and Examples

Reaction Products

• Predict the products for reactions such as:

  • Alkyne + HCl

  • Alkyne + NaNH2 + alkyl halide

Synthesis from Ethyne

• Design synthetic routes to prepare ketones, alkenes, and other compounds from ethyne using alkylation and addition reactions.

Extra Practice

• Name and draw structures for compounds such as hepta-1,3,5-triyne, (E)-4-hepten-1-yne, and (S)-5-fluoro-5-methyl-2-heptyne.

• Predict products for reactions starting from given alkynes.

Additional info: Reaction mechanisms, stereochemistry, and regioselectivity are important considerations in alkyne chemistry. Practice problems reinforce understanding of nomenclature, reactivity, and synthetic strategies.