Organic Chemistry - Alkynes

Alkynes

9.1 Alkynes / Overview

• Alkynes are molecules containing a carbon-carbon triple bond (C≡C).

9.2 Nomenclature of Alkynes / Steps

• Alkynes are named similarly to alkanes (Chapter 4) with a few modifications:

  1. Identify the Parent Chain: The parent chain must include the C≡C triple bond.

  2. Identify and Name Substituents: Identify and name any substituents attached to the parent chain.

  3. Assign Locants:

     • Assign a locant to each substituent, giving the C≡C triple bond the lowest possible number.

     • The locant for the triple bond is the lower number of the two carbons it connects (e.g., if the triple bond is between carbons 2 and 3, the locant is 2).

  4. List Substituents: List the numbered substituents before the parent name in alphabetical order. Ignore prefixes (except "iso") when alphabetizing.

  5. Triple Bond Locant Placement: The C≡C triple bond locant can be placed either just before the parent name or just before the "-yne" suffix.

9.2 Nomenclature of Alkynes / Common Names

• Common names derived from acetylene are frequently used.

• Alkynes are classified as either terminal (triple bond at the end of the chain) or internal (triple bond within the chain).

9.3 Acidity of Terminal Alkynes / Overview

• Terminal alkynes are more acidic (lower pKa) than other hydrocarbons.

• Acetylene is significantly more acidic than ethylene (by 19 pKa units), making it 10^{19} times stronger as an acid. This is due to the higher s-character of the sp-hybridized carbon in alkynes, which stabilizes the resulting negative charge on the conjugate base.

9.3 Acidity of Terminal Alkynes / Comparison

• A base must have a conjugate acid with a pKa greater than 25 to deprotonate a terminal alkyne.

9.3 Acidity of Terminal Alkynes / Choosing a Suitable Base

• Any terminal alkyne can be deprotonated using a suitable base.

• Sodium amide (NaNH2) is commonly used as a base.

9.3 Acidity of Terminal Alkynes / Examples

• Bases that will deprotonate a terminal alkyne include:

  • H-C≡C:^-; Conjugate acid: H-C≡C-H, pKa = 25

  • H2N:^-; Conjugate acid: NH3, pKa = 38

  • H:^-; Conjugate acid: H_2, pKa = 35

• Bases that will not deprotonate a terminal alkyne include:

  • HO:^-; Conjugate acid: H_2O, pKa = 15.7

  • -OH; Conjugate acid: H_2O, pKa = 16

9.4 Preparation of Alkynes / Overview

• Alkynes can be prepared through elimination reactions, similar to alkenes.

• The preparation requires a dihalide as a starting material.

9.4 Preparation of Alkynes / Dihalides

• These eliminations usually occur via an E2 mechanism.

• Either geminal (both halides on the same carbon) or vicinal (halides on adjacent carbons) dihalides can be used

9.4 Preparation of Alkynes / Excess Sodium Amide

• Excess sodium amide (NaNH2) is used to shift the equilibrium towards the elimination products.

• An aqueous workup is necessary to produce the neutral alkyne.

9.4 Preparation of Alkynes / Terminal Alkynes

• A terminal alkyne is prepared by treating a dihalide with excess sodium amide (xs NaNH2), followed by water.

9.11 Reactions of Alkynes – Summary / List

• Review of Reactions

  1. Elimination

  2. Hydrohalogenation (two equivalents)

  3. Hydrohalogenation (one equivalent)

  4. Acid-catalyzed hydration

  5. Hydroboration-oxidation

  6. Halogenation (one equivalent)

  7. Halogenation (two equivalents)

  8. Ozonolysis

  9. Alkylation

  10. Dissolving metal reduction

  11. Hydrogenation

  12. Hydrogenation with a poisoned catalyst

9.5 Reduction of Alkynes / Catalytic Hydrogenation

• Catalytic Hydrogenation: Converts an alkyne to an alkane by adding two equivalents of H_2.

• The first addition produces a cis alkene (via syn addition), which then undergoes further addition to yield the alkane.

9.5 Reduction of Alkynes / Lindlar’s Catalyst

• A deactivated or poisoned catalyst can be used to halt the reaction at the cis alkene stage.

• Lindlar’s catalyst and P-2 (Ni2B complex) are common examples of poisoned catalysts.

9.5 Reduction of Alkynes / Graphical Interpretation

• The poisoned catalyst facilitates the first addition of H_2 but not the second, allowing for the selective formation of a cis alkene.

9.5 Reduction of Alkynes / Dissolving Metal Reduction

• Dissolving Metal Reduction: Reduces an alkyne to a trans alkene using sodium metal (Na) and ammonia (NH3).

• This reaction is stereoselective for anti addition of H and H.

9.5 Reduction of Alkynes - Summary

• Be familiar with the reagents required to reduce an alkyne to an alkane, a cis alkene, or a trans alkene.

9.6 Hydrohalogenation of Alkynes / Overview

• Hydrohalogenation results in Markovnikov addition of H and X to an alkyne, similar to alkenes.

• Excess HX leads to the formation of a geminal dihalide.

9.6 Hydrohalogenation of Alkynes / Proposed Mechanism

• The precise mechanism is still under investigation, and several competing mechanisms may occur.

9.6 Hydrohalogenation of Alkynes / Radical Reaction

• HBr in the presence of peroxides promotes anti-Markovnikov addition, analogous to alkenes.

• This reaction is specific to HBr; it does not occur with HCl or HI.

9.6 Dihalide/Alkyne Interconversion

• Hydrohalogenation of alkynes and elimination of dihalides are complementary reactions.

9.7 Hydration of Alkynes / Overview

• Alkynes undergo acid-catalyzed Markovnikov hydration.

• The process typically requires a catalyst such as HgSO4 to compensate for the slow reaction rate resulting from the formation of a vinylic carbocation.

9.7 Hydration of Alkynes / Final Steps

• The resulting enol tautomerizes to a ketone.

• This process is known as keto-enol tautomerization.

  • The enol and ketone are tautomers of each other.

  • The equilibrium generally favors the ketone.

9.7 Hydroboration-Oxidation of Alkynes / Overview

*Hydroboration-oxidation of alkynes proceeds similarly to alkenes.
*Results in anti-Markovnikov addition.
*It produces an enol that tautomerizes to an aldehyde.
*Tautomerization is base-catalyzed (OH-).

9.7 Hydroboration-Oxidation of Alkynes / In Base

• Mechanism of base-catalyzed tautomerization:

  • The enol is deprotonated to form an enolate, which is then protonated at the carbon to yield the aldehyde.

9.7 Hydroboration-Oxidation of Alkynes / Use of Borane

• If BH_3 is used, the alkyne can undergo two successive additions.

• To prevent the second addition, a dialkyl borane is used instead of BH_3.

9.7 Controlling Hydration Regiochemistry

• For a terminal alkyne:

  • Markovnikov hydration yields a ketone.

  • Anti-Markovnikov hydration yields an aldehyde.

9.8 Halogenation of Alkynes / Overview

• Halogenation of alkynes yields a tetrahalide.

• Two equivalents of halogen are added with excess X_2.

9.8 Halogenation of Alkynes / Anti and Syn

• When one equivalent of halogen is added to an alkyne, both anti and syn addition are observed.

• The mechanism for alkyne halogenation is not fully understood. If it were similar to the halogenation of an alkene, only the anti product would be obtained.

9.9 Ozonolysis of Alkynes / Overview

• Ozonolysis of an internal alkyne produces two carboxylic acids.

• Ozonolysis of a terminal alkyne yields a carboxylic acid and carbon dioxide (CO_2).

9.9 Ozonolysis of Alkynes / Practice Answer

• Ozonolysis of symmetrical alkynes is particularly useful for preparing carboxylic acids, as it yields two equivalents of a single product.

9.10 Alkylation of Terminal Alkynes / Overview

• Terminal alkynes are completely converted to an alkynide ion using NaNH_2.

• Alkynide ions are strong nucleophiles.

• They undergo S_N2 reactions with alkyl halides.

9.10 Alkylation of Terminal Alkynes / SN2

• Alkylation of an alkynide ion is an S_N2 substitution, which proceeds most effectively with methyl and primary (1°) halides.

  • E_2 elimination dominates with secondary (2°) and tertiary (3°) halides.

• Acetylene can undergo two successive alkylations.

9.10 Alkylation of Terminal Alkynes / Stepwise Process

• Double alkylation of acetylene must be performed stepwise.

• Complex target molecules can be synthesized by constructing a carbon skeleton and interconverting functional groups.

9.11 Synthesis Strategies / Practice Answers

• Halogenation of an alkene followed by elimination yields an alkyne.

• These reactions provide a method for interconverting single, double, and triple bonds.