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:
Identify the Parent Chain: The parent chain must include the C≡C triple bond.
Identify and Name Substituents: Identify and name any substituents attached to the parent chain.
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).
List Substituents: List the numbered substituents before the parent name in alphabetical order. Ignore prefixes (except "iso") when alphabetizing.
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
Elimination
Hydrohalogenation (two equivalents)
Hydrohalogenation (one equivalent)
Acid-catalyzed hydration
Hydroboration-oxidation
Halogenation (one equivalent)
Halogenation (two equivalents)
Ozonolysis
Alkylation
Dissolving metal reduction
Hydrogenation
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.