Alkynes: Structure, Properties, Preparation, and Reactions

CHAPTER 13: ALKYNES – Carbon-Carbon Triple Bond

Naming and Isomerism

Alkynes are hydrocarbons containing at least one carbon-carbon triple bond. Their names end with -yne. The position of the triple bond is indicated by a number.

• Terminal alkyne: The triple bond is at the end of the carbon chain (mono-substituted).

• Internal alkyne: The triple bond is within the carbon chain (di-substituted).

• No cis-trans isomerism: Triple bonds do not allow for cis-trans (E/Z) isomerism due to their linear geometry.

• Examples:

  • Eth-yne: $\mathrm{HC \equiv CH}$

  • 2-But-yne: $\mathrm{CH_3C \equiv CCH_3}$

Structure of Alkynes

Alkynes feature a linear geometry due to sp hybridization of the triple-bonded carbons. The triple bond consists of one sigma ($\sigma$) bond and two perpendicular pi ($\pi$) bonds.

• Bonding: $\mathrm{R-C \equiv C-R}$

• Bond lengths:

  • C–C triple bond: 1.203 Å

  • C–H bond: 1.061 Å

• Bond angle: 180°, resulting in a linear molecule.

• Orbital description: Each carbon in the triple bond uses sp hybrid orbitals for $\sigma$ bonding and unhybridized p orbitals for $\pi$ bonding.

Relative Stability of Alkynes

The stability of alkynes can be compared by measuring their heat of hydrogenation ($\Delta H$). This value reflects the energy released when an alkyne is converted to an alkane.

• General reaction: $\mathrm{Alkyne + 2H_2 \xrightarrow{catalyst} Alkane}$

• Example:

  • Terminal alkyne: $\mathrm{CH_3CH_2C \equiv CH + 2H_2 \xrightarrow{catalyst} CH_3CH_2CH_2CH_3}$, $\Delta H = -69.9$ kcal mol-1

  • Internal alkyne: $\mathrm{CH_3C \equiv CCH_3 + 2H_2 \xrightarrow{catalyst} CH_3CH_2CH_2CH_3}$, $\Delta H = -65.1$ kcal mol-1

• Key points:

  • Alkynes have more potential energy than alkenes due to electron repulsion.

  • Internal triple bonds are more stable than terminal triple bonds due to hyperconjugation.

Acidity of Alkynes

Alkynes are more acidic than alkenes and alkanes due to the high s-character (50%) of the sp-hybridized carbon, which pulls electron density toward the nucleus and stabilizes the conjugate base.

• Hybridization and pKa values:

  Compound	Hybridization	pKa
  Alkyne (HC≡CH)	sp	~25
  Alkene (HC=CH2)	sp2	~44
  Alkane (CH3CH3)	sp3	~50

• Deprotonation: Terminal alkynes can be deprotonated by strong bases (e.g., NaNH2, organometallic reagents) to form alkynyl anions.

• Example reaction: $\mathrm{RC \equiv CH + CH_3MgBr \rightarrow RC \equiv CMgBr + CH_4}$

Preparation of Alkynes

1. Using Alkynyl Organometallic Reagents

Alkynyl anions (generated from terminal alkynes) act as nucleophiles in SN2 reactions to form new carbon-carbon bonds.

• Example:

  • $\mathrm{RC \equiv C^- + R'Br \rightarrow RC \equiv CR'}$

  • Alkynyl lithium or magnesium reagents can add to epoxides or carbonyl compounds to form alcohols after hydrolysis.

2. Elimination of Dihaloalkanes via E2 Mechanism

Alkynes can be synthesized by double dehydrohalogenation of vicinal or geminal dihaloalkanes using strong bases.

• General steps:

  1. Halogenation of alkene to form dihaloalkane.

  2. Treatment with excess base (e.g., NaNH2) to eliminate two equivalents of HX, forming the alkyne.

• Example:

  • $\mathrm{Br_2}$, $\mathrm{CCl_4}$: Alkene → Dihaloalkane

  • $\mathrm{2NaNH_2}$: Dihaloalkane → Alkyne

Reactions of Alkynes

1. Reductions

Alkynes can be reduced to alkenes or alkanes using different reagents and conditions.

• A. Complete hydrogenation: $\mathrm{RC \equiv CR' + 2H_2 \xrightarrow{Pt} RC H_2 CR'}$ (alkane)

• B. Partial hydrogenation (Lindlar's catalyst): Syn addition produces cis-alkene.

  • Lindlar's catalyst: Pd-CaCO3 poisoned with Pb(OAc)2 or quinoline.

  • Product: cis-alkene (e.g., cis-3-heptene)

• C. Sodium in liquid ammonia (Na/NH3): Anti addition produces trans-alkene.

  • Product: trans-alkene (e.g., trans-3-heptene)

Mechanism of Na Reduction of Alkynes

1. Step 1: One-electron transfer from Na to alkyne forms a radical anion.

2. Step 2: Protonation of the radical anion by NH3 forms an alkenyl radical.

3. Step 3: Second one-electron transfer forms an alkenyl anion.

4. Step 4: Second protonation yields the trans-alkene.

2. Electrophilic Additions

Alkynes undergo electrophilic addition reactions similar to alkenes, but with distinct regioselectivity and stereochemistry.

B. Halogenation

• Anti addition: Addition of Br2 or Cl2 across the triple bond yields trans-dihaloalkenes and then tetrahaloalkanes.

• Mechanism: Proceeds via a cyclic bromonium ion intermediate.

A. Hydrohalogenation using HX

• Markovnikov addition: HX adds to the more substituted carbon, forming a vinyl halide, then a geminal dihalide upon excess HX.

• Example: $\mathrm{CH_3C \equiv CH + 2HBr \rightarrow CH_3CBr_2CH_3}$

C. Hydration (Markovnikov)

• Reagents: H2O, H+, HgSO4

• Product: Enol intermediate tautomerizes to a ketone.

  • $\mathrm{RC \equiv CH \xrightarrow{H_2O, HgSO_4} RC(OH)=CH_2 \rightarrow RCCH_3}$ (ketone)

D. Hydroboration-Oxidation (Anti-Markovnikov)

• Reagents: R2BH, H2O2, OH-

• Product: Enol intermediate tautomerizes to an aldehyde.

  • $\mathrm{RC \equiv CH \xrightarrow{R_2BH, H_2O_2} RCH_2CHO}$ (aldehyde)

Additional info: These notes cover the essential structure, properties, preparation, and reactions of alkynes, including mechanistic details and key examples relevant for college-level organic chemistry.