Chapter 9 - Alkynes Flashcards
What is Parkinson’s disease?
A motor system disorder that affects an estimated 3% of the U.S. population over the age of 60.
What are the main symptoms of Parkinson’s disease?
- Trembling and stiffness of the limbs
- Slowness of movement
- Impaired balance
What causes the symptoms of Parkinson’s disease?
Degeneration of neurons in the substantia nigra that cease to produce dopamine.
At what percentage of dopamine-producing neurons’ death do symptoms of Parkinson’s disease begin to appear?
When 50–80% of dopamine-producing neurons have died.
Is there a known cure for Parkinson’s disease?
No, there is no known cure for the disease.
What is selegiline?
A drug used to treat symptoms of Parkinson’s disease, which has a molecular structure containing a C≡C bond.
What is the significance of the C≡C bond in selegiline?
It plays an important role in the action of the drug.
What is the hybridization state of carbon atoms in alkynes?
sp hybridization.
What types of bonds comprise a triple bond?
- One σ bond
- Two π bonds
What is the geometry of acetylene?
Linear geometry with bond angles of 180°.
What trend is observed in the bond lengths of ethane, ethylene, and acetylene?
- Ethane: longest C–C and C–H bond lengths
- Ethylene: intermediate bond lengths
- Acetylene: shortest C–C and C–H bond lengths
What is the s-character of sp3, sp2, and sp hybridized orbitals?
- sp3: 25%
- sp2: 33%
- sp: 50%
What is the simplest alkyne?
Acetylene (H–C≡C–H).
What is the combustion temperature of acetylene?
2800°C.
What is histrionicotoxin?
A toxin secreted by the South American frog Dendrobates histrionicus.
What is ethynylestradiol used for?
Found in many birth control formulations, it elevates hormone levels in women and prevents ovulation.
What role does the triple bond play in ethynylestradiol?
It imparts structural rigidity, making the compound a more potent contraceptive.
What is the first selective MAO B inactivator approved by the FDA for Parkinson’s disease?
Selegiline.
What is polyacetylene?
The first known example of an organic polymer capable of conducting electricity.
What is the significance of doping in conducting polymers?
It allows polymers like polyacetylene to conduct electricity almost as well as a copper wire.
What suffix is used to indicate the presence of a C≡C bond in alkyne nomenclature?
The suffix ‘yne’.
What are the four steps to name alkynes systematically?
- Identify and name the parent
- Identify and name the substituents
- Number the parent chain and assign a locant to each substituent
- Assemble the substituents alphabetically
Fill in the blank: The position of the triple bond in an alkyne is indicated using a _______.
[single locant]
What is the lowest locant assigned to a triple bond in a compound with alkyl substituents?
The triple bond should receive the lowest number possible.
How is the position of the triple bond indicated in IUPAC nomenclature?
Using a single locant placed immediately before the parent or before the suffix ‘yne’.
What are the two acceptable IUPAC names for the compound with a triple bond at C2?
- 6,5,5,6-Trimethyl-2-heptyne
- 5,5,6-Trimethylhept-2-yne
What is the common name for ethyne?
Acetylene.
What are terminal alkynes?
Monosubstituted acetylenes with only one alkyl group.
What are internal alkynes?
Disubstituted acetylenes with two alkyl groups.
What is the process for numbering the parent chain in alkyne nomenclature?
Number the parent chain and assign a locant to each substituent.
When naming substituents, how should they be organized?
Assemble the substituents alphabetically.
In cycloalkynes, does the triple bond require a locant?
No, it is assumed to be between C1 and C2.
What is the pKa value of ethane?
pKa = 50.
What is the pKa value of ethylene?
pKa = 44.
What is the pKa value of acetylene?
pKa = 25.
How does acetylene’s acidity compare to ethylene and ethane?
Acetylene is significantly more acidic than both ethylene and ethane.
What is the conjugate base of acetylene called?
Acetylide ion.
How is the stability of the acetylide ion explained?
Using hybridization theory; the negative charge occupies an sp-hybridized orbital.
What happens to the equilibrium when a strong base is used to deprotonate acetylene?
It favors formation of the weaker acid and weaker base.
Which base is not suitable for deprotonating acetylene?
Sodium hydroxide (NaOH).
What is the result of treating acetylene with a suitable base?
Formation of lithium acetylide.
What types of dihalides can be used to prepare alkynes?
- Geminal dihalides
- Vicinal dihalides
What is required for the preparation of terminal alkynes from alkyl dihalides?
Two successive elimination (E2) reactions.
What is a suitable base for achieving two successive elimination reactions?
Sodium amide (NaNH2), dissolved in liquid ammonia (NH3).
How many equivalents of amide ion are required for the formation of an alkynide ion?
Three equivalents.
What is the final step after forming the alkynide ion?
Introduce a proton source to regenerate the terminal alkyne.
Fill in the blank: Acetylene is _______ times more acidic than ethylene.
10,000,000,000,000,000,000 (19 orders of magnitude).
True or False: The negative charge in the acetylide ion is associated with an sp2-hybridized orbital.
False.
What are the three bases that can be used to deprotonate a terminal alkyne?
- Sodium amide (NaNH2)
- Lithium diisopropylamide (LDA)
- Butyllithium (BuLi)
What type of acid is the conjugate base of a terminal alkyne?
Weaker acid.
What reactions can be used to prepare a terminal alkyne?
Treating a dihalide with excess sodium amide followed by water.
Dihalides can include compounds like 3,3-dichloropentane.
What is the role of sodium amide in the preparation of terminal alkynes?
It acts as a strong base to deprotonate the terminal alkyne and form the alkynide ion.
What is the pKa value for a weaker acid compared to a stronger acid?
Stronger acid (pKa = 15.7); Weaker acid (pKa ∼ 25).
What does the equilibrium favor in terms of acid-base reactions?
Formation of weaker acid and weaker base.
What is the major product when 3,3-dichloropentane is treated with excess sodium amide in liquid ammonia?
2-pentyne.
What is the process of catalytic hydrogenation in relation to alkynes?
Alkynes undergo addition reactions with hydrogen, consuming two equivalents of molecular hydrogen.
What is a poisoned catalyst, and how does it affect the reduction of alkynes?
A poisoned catalyst allows the conversion of an alkyne into a cis alkene without further reduction to an alkane.
Name a common example of a poisoned catalyst.
Lindlar’s catalyst.
What is the stereochemical outcome of alkyne hydrogenation?
Both hydrogen atoms are added to the same face of the alkyne (syn addition) to give the cis alkene.
What is dissolving metal reduction?
A reaction that reduces alkynes to trans alkenes using sodium metal in liquid ammonia.
What are the steps in the dissolving metal reduction mechanism?
- Electron transfer
- Proton transfer
- Electron transfer
- Proton transfer.
What is the expected product when an alkyne is treated with sodium metal in liquid ammonia?
A trans alkene.
What is the difference in the reaction mechanism of HX addition to alkynes compared to alkenes?
Alkynes may proceed through a vinylic carbocation or a termolecular mechanism involving simultaneous interaction with two molecules of HX.
What is the rate expression for the addition of HX to alkynes?
Rate = k [alkyne] [HX]^2.
What type of addition occurs when alkynes react with HBr in the presence of peroxides?
Anti-Markovnikov addition.
What is the role of ammonia in the dissolving metal reduction mechanism?
Ammonia donates a proton to the anion.
Fill in the blank: The conversion of an internal alkyne into a _______ occurs rapidly under certain conditions.
terminal alkyne.
True or False: A terminal alkyne can be reduced to an alkene using dissolving metal reduction.
False.
What type of addition occurs when HBr reacts with an alkene in the presence of peroxides?
Anti-Markovnikov addition
The Br is installed at the less substituted position.
Which reagent is necessary for radical addition of HBr to occur?
Peroxides
Radical addition only occurs with HBr, not with HCl or HI.
What is the product of a terminal alkyne treated with HBr in the presence of peroxides?
A mixture of E and Z isomers
The Br is installed at the terminal position.
What are the two main products formed from the interconversion of dihalides and terminal alkynes?
Dihalides and terminal alkynes
The process can be facilitated by reagents such as excess HX and sodium amide (NaNH2).
What is the primary mechanism for acid-catalyzed hydration of alkynes?
Markovnikov addition
The hydroxyl group is installed at the more substituted position.
Which ion enhances the rate of alkyne hydration in acid-catalyzed reactions?
Mercuric ion (Hg2+)
It activates the alkyne toward nucleophilic attack.
What is the term for the compound formed during the hydration of an alkyne before it converts into a ketone?
Enol
Enols cannot be isolated under acidic conditions as they rapidly convert to ketones.
What is keto-enol tautomerization?
The interconversion between an enol and a ketone
It is an equilibrium process catalyzed by acid or base.
What type of isomers are enols and ketones in keto-enol tautomerization?
Constitutional isomers
They rapidly interconvert via the migration of a proton.
What is the effect of trace amounts of acid on keto-enol tautomerization?
It catalyzes the tautomerization process
This makes it difficult to prevent reaching equilibrium.
What is the final product of hydroboration-oxidation of a terminal alkyne?
Aldehyde
This occurs via anti-Markovnikov addition.
What reagents are used in hydroboration-oxidation of alkynes?
BH3 ∙ THF followed by H2O2, NaOH
This process results in an aldehyde product due to tautomerization.
What is the role of disiamylborane or 9-BBN in hydroboration-oxidation?
Preventing the second addition of borane
Steric hindrance from the two alkyl groups allows selective addition.
What type of product results from the acid-catalyzed hydration of unsymmetrical internal alkynes?
A mixture of ketones
This process lacks regiochemical control.
What is the major product when a terminal alkyne is hydrated with aqueous acid and mercuric sulfate?
Methyl ketone
This follows Markovnikov regiochemistry.
What is the mechanism of acid-catalyzed keto-enol tautomerization?
Protonation of the π bond followed by deprotonation
This results in the conversion of an enol to a ketone.
What are the two steps in the base-catalyzed tautomerization of an enol?
Deprotonation followed by protonation
This generates a resonance-stabilized enolate ion.
What is the expected outcome when a terminal alkyne is treated with H2SO4 and water?
Formation of a methyl ketone
This is achieved through Markovnikov addition.
What does id-catalyzed hydration of a terminal alkyne produce?
A methyl ketone.
What does hydroboration-oxidation of a terminal alkyne produce?
An aldehyde.
What governs the regiochemical outcome of alkyne hydration?
The choice of reagents.
What is the major product of hydration when the oxygen atom is installed at the more substituted position?
A methyl ketone.
What reagents can be used to achieve Markovnikov addition in alkyne hydration?
H2SO4, H2O, HgSO4.
True or False: Alkynes can only undergo halogenation with one equivalent of halogen.
False.
What is the result of halogenation of alkynes with excess halogen?
Formation of a tetrahalide.
What type of addition occurs when one equivalent of halogen is added to an alkyne?
Anti addition.
What happens when alkynes are treated with ozone followed by water?
Oxidative cleavage to produce carboxylic acids.
What is produced from the terminal carbon atom of a terminal alkyne during oxidative cleavage?
Carbon dioxide.
What is the synthetic utility of deprotonating a terminal alkyne with a strong base?
It generates an alkynide ion that can act as a nucleophile.
Fill in the blank: An alkynide ion can function as a _______ when treated with an alkyl halide.
Nucleophile.
What type of alkyl halides must be used for efficient alkylation of alkynides?
Methyl or primary alkyl halides.
What happens when secondary or tertiary alkyl halides are used in alkylation?
Elimination products are obtained.
What does the overall synthesis of a compound from acetylene require?
Two separate alkylation steps.
What is the first step in the synthesis of a compound from acetylene?
Deprotonation of the alkyne with NaNH2.
What is the purpose of retrosynthetic analysis in organic synthesis?
To determine the starting materials needed for the target molecule.
What method can be used to convert an alkene into an alkyne?
Bromination followed by elimination.
What is the result of partial reduction of an alkyne?
Formation of a cis alkene.
What is a protease inhibitor?
A compound that inhibits the function of proteases.
What is the major product of ozonolysis of an alkyne with the molecular formula C4H6?
A carboxylic acid and carbon dioxide.
What are disiamylborane and H2O2, NaOH used for in alkyne reactions?
Hydroboration-oxidation.
What is the expected product when an alkyne is treated with H2SO4, H2O, and HgSO4?
A methyl ketone.
What is the role of ozone in the ozonolysis of alkynes?
It cleaves the triple bond to form carboxylic acids.
How can the cis alkene be prepared?
By partial reduction of the corresponding alkyne.
What is required for a C–C bond disconnection to occur?
It must take place adjacent to a C≡C triple bond.
What type of carbon atom can bear a negative charge and form a stable carbanion?
An sp hybridized carbon atom.
Why is deprotonation of an alkene not allowed?
Because the proton attached to an sp2 hybridized carbon is not acidic.
What is the first step in the synthesis process described?
Alkylation of the alkyne via treatment with NaNH2 followed by ethyl iodide.
What is used to achieve the reduction of the alkyne to a cis alkene?
Hydrogenation reaction in the presence of a poisoned catalyst, such as Lindlar’s catalyst.
What does bromination of the alkene produce?
A racemic mixture of the target molecule.
What does metabolism refer to?
All of the chemical reactions that occur in living organisms.
What is deuterium?
An isotope of hydrogen with one proton and one neutron in the nucleus.
Which reactions can prepare alkynes from dihalides?
Two successive E2 reactions.
What product is formed from catalytic hydrogenation of an alkyne?
An alkane.
What does Lindlar’s catalyst yield when hydrogenating an alkyne?
A cis alkene.
What is the result of oxidative cleavage of internal alkynes with ozone?
Carboxylic acids.
What is the product when a terminal alkyne undergoes oxidative cleavage?
The terminal side is converted into carbon dioxide.
What type of addition occurs when treated with HBr and peroxides?
Anti-Markovnikov addition.
What is keto-enol tautomerization?
The interconversion between an enol and a ketone.
What is the role of mercuric sulfate (HgSO4) in hydration of alkynes?
It catalyzes the acid-catalyzed hydration to produce an enol that converts into a ketone.
What is the effect of using trace amounts of acid or base during tautomerization?
It catalyzes the interconversion between enols and ketones.
What is the systematic naming rule for alkynes?
The suffix ‘ane’ is replaced with ‘yne.’
What is a terminal alkyne?
A monosubstituted alkyne.
What is an internal alkyne?
A disubstituted alkyne.
What is the general structure of an alkyne?
A triple bond comprised of one σ bond and two π bonds.
What type of geometry do alkynes exhibit?
Linear geometry.
What is required for the formation of an alkynide ion?
A sufficiently strong base, such as NaNH2.
What is the outcome of hydroboration-oxidation of a terminal alkyne?
An anti-Markovnikov addition produces an enol that is converted into an aldehyde.
What happens in the mechanism of hydrohalogenation of alkynes?
It involves a vinylic carbocation or a termolecular mechanism.
What is the outcome of adding HX to alkynes?
A Markovnikov addition occurs.
What does ozonolysis of alkynes produce?
Ozonolysis produces carboxylic acids or carbon dioxide depending on the alkyne.
What is the significance of using a poisoned catalyst in hydrogenation?
It selectively yields a cis alkene instead of an alkane.
What must be done to convert an alkene into an alkyne?
Bromination followed by elimination with excess NaNH2.
What is the first step in assembling the systematic name of an alkyne?
Identify the parent: choose the longest chain that includes the triple bond.
What base is unsuitable for deprotonating a terminal alkyne?
Hydroxide (NaOH) is not suitable.
What is the characteristic of an alkylation reaction involving alkynes?
Alkynes undergo alkylation when treated with an alkyl halide.
What is the process to achieve the hydration of an alkyne?
Use H2SO4 and HgSO4.
What is the product of the reaction of 1-pentyne with H2 in the presence of Lindlar’s catalyst?
A cis alkene.
What do you call a pair of compounds that can interconvert via keto-enol tautomerization?
Tautomers.
What does the term ‘alkylation’ refer to in this context?
Introducing alkyl groups to a terminal alkyne.
What is the outcome of treating an alkyne with ozone followed by water?
Oxidative cleavage occurs.
What type of base can deprotonate a terminal alkyne effectively?
Strong bases like NaNH2.
What is the effect of using a dissolving metal reduction on an internal alkyne?
It converts it into a trans alkene.
What is the product when (R)-4-bromohept-2-yne is treated with H2 in the presence of Pt?
Optically inactive product
The reaction leads to a loss of chirality.
What is the result when (R)-4-bromohex-2-yne is treated with H2 in the presence of Pt?
Optically active product
The structure allows for retention of chirality.
Draw the structure of an alkyne that can be converted into 3-ethyl-pentane upon hydrogenation.
Alkyne structure that produces 3-ethyl-pentane
Systematic name should be provided.
What is the molecular formula of compound A?
C5H8
Compound A is an alkyne.
What are the products obtained when compound A is treated with aqueous sulfuric acid and mercuric sulfate?
Two different products with the molecular formula C5H10O
Equal amounts of both products are formed.
How many chiral centers are present in compound A, which reacts to give 2,4,6-trimethyloctane?
Chiral centers count
Specific count should be determined based on structure.
What are the locants for the methyl groups in compound A?
Locants are not 2, 4, and 6
Explanation needed for the correct locants.
What is produced upon hydroboration-oxidation of compound A?
An aldehyde
The aldehyde results from the specific transformation.
Draw the structure of each possible dichloride that can be used to prepare a specific alkyne via elimination.
Structures of possible dichlorides
Specific alkyne should be stated.
What is the molecular formula of the terminal alkynes that can be drawn with C6H10?
Four terminal alkynes
Names and structures needed for each alkyne.
Why can’t 2,2-dimethyl-3-octyne be prepared via alkylation of acetylene?
Preparation cannot be achieved
Explanation of the specific limitations in the synthesis.
What fatal flaw exists in the following synthesis: excess NaNH2?
Identify the error in the synthesis
Explanation of why the synthesis fails.
What is the structure of the ketone produced from compound X after hydration?
Structure of the ketone
Derived from the specified reaction sequence.
What is the structure of the unknown alkyne that yields acetic acid and carbon dioxide upon ozonolysis?
Structure of the unknown alkyne
Relevant details about the transformations should be included.
Which reagent will achieve the transformation of 1,2-Dichloropentane with excess sodium amide?
Compound X produced
Followed by water workup.
Which transformations will yield the desired target molecule?
Specific reagents needed
A comprehensive list of reagents should be included.
What is the major product of the reaction sequence involving NaNH2?
Major product of the reaction sequence
Requires additional context about the specific transformations.
Explain why the enol of compound 2 is favored over compound 1.
Enol concentration is higher
Steric effects influencing stability should be detailed.
Draw a complete mechanism showing the conversion of compound 1 to compound 2.
Mechanism of conversion
Should include steps showing the intramolecular attack.
What is an N-reverse prenyl group?
A group often found in natural products
Known for antitumor activity.
What does the transformation of terminal alkyne with NaNH2 followed by 1-iodopropane yield?
An internal alkyne
This alkyne is treated further with ozone.
What is the structure of 3-heptyne?
An elongated analogue of pentane with a carbon-carbon triple bond inserted between C2 and C3.
In the most stable conformations of 3-heptyne, which carbons are nearly eclipsed?
C1 and C6.
What is the relationship between C4 and C7 in one of the stable conformations of 3-heptyne?
C4 and C7 are anti to each other.
What interaction do C4 and C7 experience in the other stable conformation of 3-heptyne?
Gauche interaction.
What type of structure is used to represent the lowest energy conformations of pentane?
Wedge-and-dash structure.
What is required to illustrate the eclipsed nature of the low-energy conformations of 3-heptyne?
A Newman projection.
What type of projections illustrate the difference between the two lowest energy conformations of 3-heptyne?
Newman projections.
What happens when phenyl-substituted acetylenes are treated with HCl?
A mixture of E and Z isomers is produced.
What can form if vinyl carbocations are stabilized by resonance?
Vinyl carbocations.
What is observed in the regioselectivity when phenyl-substituted acetylenes are protonated by HCl?
The formation of a predominantly formed vinyl carbocation.
How can transition state stabilities explain the stereoselectivity in the reactions of phenyl-substituted acetylenes?
By comparing the stability of the transition states during vinyl carbocation capture by chloride ion.
What is the E:Z ratio for 1a (R = Me)?
70:30.
What is the E:Z ratio for 1b (R = Et)?
80:20.
What is the E:Z ratio for 1c (R = i-Pr)?
95:5.
What is the E:Z ratio for 1d (R = t-Bu)?
100:0.
