- An alkyne undergoes partial reduction with Lindlar catalyst (Pd/BaSO₄, quinoline) to give an alkene product. If the same alkyne is instead treated with Na/NH₃ in liquid ammonia, a different alkene stereoisomer is obtained. What is the fundamental difference between these two reduction methods that accounts for the stereochemical divergence?
Answer: Lindlar catalyst delivers hydrogen atoms simultaneously from the same face (syn addition), while Na/NH₃ involves sequential electron and proton transfers that can occur from opposite faces (anti addition)
The key difference is the stereochemistry of hydrogen delivery. Lindlar catalyst provides syn addition - both hydrogen atoms are delivered from the same face of the alkyne simultaneously, producing a Z-alkene. Na/NH₃ reduction involves sequential transfer of electrons and protons through a vinyl anion intermediate, allowing the second protonation to occur from either face, typically giving the more stable E-alkene (anti addition overall). Choice B incorrectly focuses on temperature effects rather than mechanism. Choice C wrongly discusses regioselectivity rather than stereoselectivity. Choice D reverses the mechanisms - Lindlar is actually the more concerted process.
- A chemist wants to synthesize (Z)-2-pentene with high stereoselectivity. Which of the following reaction conditions is most suitable for this transformation?
Answer: 2-pentyne with H₂ and Lindlar's catalyst
The synthesis of a Z (cis) alkene from an alkyne requires a syn-addition of H₂ that stops at the alkene stage. This is achieved using H₂ with Lindlar's catalyst. Choice A would result in complete reduction to pentane. Choice C would result in the formation of the E (trans) isomer. Choice D starts with a terminal alkyne, 1-pentyne, which upon reduction would yield 1-pentene, not 2-pentene.
- The reduction of an internal alkyne with Na in liquid NH₃ produces a trans (E) alkene. Which statement best explains the high stereoselectivity of this reaction?
Answer: The key intermediate is a vinylic radical anion, where the two alkyl groups are most stable when positioned trans to minimize steric repulsion.
The mechanism of the dissolving metal reduction involves a single electron transfer from sodium to the alkyne, forming a radical anion. This intermediate is protonated by ammonia to form a vinylic radical. The stereochemistry is determined at this stage; the vinylic radical is most stable when the two bulky alkyl groups are trans to each other, minimizing steric strain. A second electron transfer and protonation then locks in this E geometry. The mechanism does not involve carbocations (B), syn-addition (A), or surface catalysis (D).
- During the catalytic hydrogenation of an alkene, deuterium gas (D₂) is used instead of H₂ to study the mechanism. Analysis shows that both deuterium atoms end up on the same face of the original double bond in the product. However, when the reaction is performed in D₂O solvent instead of an aprotic solvent, some products show deuterium incorporation from the solvent as well. What mechanistic information do these observations provide?
Answer: The syn addition pattern demonstrates that both deuterium atoms are delivered simultaneously from the catalyst surface, while D₂O incorporation shows that the mechanism involves reversible alkene coordination that allows solvent exchange during the catalytic cycle
The syn addition of D₂ confirms the accepted mechanism of catalytic hydrogenation where both hydrogen atoms are delivered from the same catalyst surface simultaneously. The incorporation of deuterium from D₂O solvent indicates that there is reversible coordination and exchange occurring during the catalytic cycle - the alkene or partially reduced intermediate can exchange with the solvent before final product release. Choice A incorrectly suggests competing ionic mechanisms. Choice C focuses on surface exchange rather than the coordination reversibility. Choice D incorrectly invokes hydration as a competing pathway rather than understanding the exchange mechanism.
- A student intends to prepare (Z)-2-butene from 2-butyne. They set up the reaction with 2-butyne, H₂ gas, and a standard Pd-C catalyst. They allow the reaction to run for an extended period with excess hydrogen. What is the most likely major product they will isolate?
Answer: Butane
While the hydrogenation of an alkyne over Pd-C does proceed through a cis-alkene intermediate, standard, unpoisoned catalysts like Pd-C are also highly effective at reducing alkenes. Given excess hydrogen and sufficient reaction time, the (Z)-2-butene formed will be immediately hydrogenated further to the fully saturated alkane, butane. To stop the reaction at the alkene stage (A), a poisoned catalyst like Lindlar's catalyst is required.
- A two-step synthesis is performed starting from 1-propyne. First, 1-propyne is treated with NaNH₂ followed by iodomethane (CH₃I). Second, the product of the first step is treated with sodium metal in liquid ammonia. What is the final major product?
Answer: (E)-2-butene
This question tests your understanding of alkyne alkylation followed by dissolving metal reduction, two fundamental transformations in organic synthesis.
When you treat 1-propyne with NaNH₂, the strong amide base deprotonates the terminal alkyne, forming a sodium acetylide anion. This nucleophilic anion then attacks CH₃I in an SN2 reaction, extending the carbon chain by one unit to give 2-butyne (HC≡C-CH₂CH₃ becomes CH₃-C≡C-CH₂CH₃).
In the second step, sodium metal in liquid ammonia performs a dissolving metal reduction of the internal alkyne. This reaction proceeds through radical intermediates and specifically produces the trans (E) alkene as the major product. The mechanism involves anti addition of hydrogen atoms across the triple bond, leading to (E)-2-butene where the larger groups (CH₃) are on opposite sides of the double bond.
Looking at the wrong answers: B) (Z)-2-butene would result from syn addition, but dissolving metal reduction gives anti addition, favoring the E isomer. C) Butane would require complete reduction of the alkyne to an alkane, but Na/NH₃ stops at the alkene stage under these conditions. D) 1-butene would require rearrangement or a different reduction pattern that doesn't occur with this reagent system.
The correct answer is A) (E)-2-butene.
Study tip: Remember that dissolving metal reduction (Na or Li in NH₃) is stereoselective for trans alkenes, while catalytic hydrogenation typically gives cis alkenes. This stereochemical outcome is a favorite testing point in organic chemistry.
- Which statement most accurately contrasts the reduction of 3-heptyne using H₂/Lindlar's catalyst versus using Na/NH₃?
Answer: H₂/Lindlar's catalyst produces (Z)-3-heptene via syn-addition, while Na/NH₃ produces (E)-3-heptene via an anti-addition pathway.
This question directly compares the two primary methods for the partial reduction of internal alkynes. H₂ with Lindlar's catalyst is a stereospecific syn-addition that yields a Z (cis) alkene. In contrast, the dissolving metal reduction with Na in liquid NH₃ proceeds through a mechanism that favors an anti-addition pathway, yielding an E (trans) alkene. All other options contain factual errors: A and D misidentify the product of one or both reactions, and C incorrectly states that Lindlar's catalyst leads to complete reduction.
- Consider the catalytic hydrogenation of methylenecyclohexane using different catalysts: Pt/C, Pd/C, and Rh/C. All reactions are carried out under identical conditions (same temperature, pressure, and reaction time). Which factor most significantly influences the relative rates of hydrogenation among these three catalysts?
Answer: The crystal lattice structure of each metal creates different active site geometries that either facilitate or hinder the simultaneous adsorption of both H₂ and the alkene substrate
The activity differences among Pt, Pd, and Rh catalysts primarily arise from their different surface structures and active site geometries. Each metal has a characteristic crystal structure that creates specific arrangements of surface atoms, affecting how well both H₂ and alkene substrates can simultaneously adsorb and interact on the catalyst surface. This geometric factor is crucial for the catalytic mechanism. Choice A oversimplifies by focusing only on electronic effects. Choice C incorrectly emphasizes atomic radius as the primary factor. Choice D misrepresents the mechanism - these metals don't undergo significant oxidation state changes during hydrogenation catalysis.
- The heat of hydrogenation (ΔH°hydrog) is the enthalpy change when one mole of an unsaturated compound is hydrogenated. A more negative ΔH°hydrog corresponds to a less stable starting alkene. Which of the following isomeric pentenes would be expected to have the most negative (most exothermic) heat of hydrogenation?
Answer: 1-pentene
Alkene stability increases with the degree of substitution on the double bond. 2-methyl-2-butene is tetrasubstituted (most stable). (E)- and (Z)-2-pentene are disubstituted. 1-pentene is monosubstituted (least stable). The heat of hydrogenation is a measure of the alkene's stability; a less stable alkene is higher in potential energy and will release more heat upon hydrogenation to the same alkane. Therefore, the least stable alkene, 1-pentene, will have the most exothermic (most negative) heat of hydrogenation.
- In the reduction of alkynes to alkanes, two sequential hydrogenation steps occur. The first step (alkyne → alkene) typically proceeds more slowly than the second step (alkene → alkane) under standard catalytic conditions. However, with Lindlar catalyst, the reaction can be stopped at the alkene stage. What structural and electronic factors explain this selectivity pattern?
Answer: Alkynes coordinate more strongly to metal surfaces than alkenes due to their higher π electron density, slowing their reduction rate, while Lindlar catalyst contains additives that selectively poison the catalyst toward alkene reduction
Alkynes actually coordinate more strongly to metal catalyst surfaces than alkenes due to their higher π electron density (two π bonds vs. one). This stronger binding paradoxically makes them react more slowly because they are less readily displaced from the surface after partial reduction. Lindlar catalyst (Pd/BaSO₄ with quinoline) is specifically 'poisoned' with additives that reduce its activity, particularly toward alkene reduction, allowing selective stopping at the alkene stage. Choice A incorrectly states alkynes have lower π electron density. Choice B wrongly emphasizes geometric factors. Choice D misrepresents the coordination requirements and mechanism.
- A student attempts to reduce 2-methyl-2-butene using catalytic hydrogenation but observes no reaction even after extended reaction time under standard conditions (H₂, Pd/C, room temperature, 1 atm). The student then tries the same reaction with elevated temperature and pressure, and again observes no significant conversion. What is the most likely explanation for this observation?
Answer: The alkene is too sterically hindered around the double bond for effective coordination to the catalyst surface, preventing the normal catalytic mechanism from proceeding
2-Methyl-2-butene is a highly substituted alkene (tetrasubstituted) with significant steric crowding around the double bond. This steric hindrance prevents effective coordination of the alkene to the catalyst surface, which is required for the catalytic hydrogenation mechanism. Even elevated conditions cannot overcome this fundamental geometric limitation. Choice B is incorrect because electronic effects from alkyl groups are generally favorable for hydrogenation. Choice C assumes impurities without evidence. Choice D is wrong because alkene to alkane conversion is thermodynamically favorable; the issue is kinetic (mechanistic) rather than thermodynamic.
- In the catalytic hydrogenation of an alkene using H₂ and a solid metal catalyst like palladium on carbon (Pd-C), what is the primary role of the metal surface?
Answer: To provide a surface that adsorbs both H₂ and the alkene, facilitating cleavage of the H-H bond and delivery of H atoms to one face of the C=C bond.
Catalytic hydrogenation is a heterogeneous process. The metal surface (e.g., Pd, Pt) serves to adsorb both the hydrogen gas and the alkene. This adsorption weakens the H-H bond and orients the alkene, allowing for the stepwise, syn-addition of hydrogen atoms across one face of the double bond. The catalyst is not consumed (ruling out D) and it does not create a homogeneous solution (ruling out B). While there is an electronic interaction, describing it as a simple Lewis acid polarization (A) is less accurate than describing the full role of adsorption and bond activation.
- The experimental heat of hydrogenation for cyclohexene is -120 kJ/mol, and for 1,3-cyclohexadiene it is -232 kJ/mol. If 1,3-cyclohexadiene contained two isolated double bonds, its heat of hydrogenation would be expected to be approximately 2 × (-120) = -240 kJ/mol. What does this difference between the experimental (-232 kJ/mol) and expected (-240 kJ/mol) values suggest about 1,3-cyclohexadiene?
Answer: 1,3-cyclohexadiene possesses approximately 8 kJ/mol of stabilization energy due to conjugation.
When you encounter heat of hydrogenation problems comparing experimental vs. theoretical values, you're looking at how molecular structure affects stability. The key insight is that when experimental energy release is less than expected, the starting molecule must be more stable than predicted.
Here's the logic: 1,3-cyclohexadiene experimentally releases 232 kJ/mol upon hydrogenation, but if it had two isolated double bonds, you'd expect 2 × 120 = 240 kJ/mol. The difference is 240 - 232 = 8 kJ/mol. This means the diene is 8 kJ/mol more stable than a hypothetical molecule with isolated double bonds. This extra stability comes from conjugation - the overlap of p-orbitals in the alternating double bond system that delocalizes electrons and lowers the molecule's energy.
Answer A correctly identifies this 8 kJ/mol as conjugation stabilization energy. Answer B incorrectly suggests hydrogenation is "less favorable," but thermodynamic favorability depends on the overall energy change, not the comparison to theoretical values. Answer C makes a fundamental error - conjugated dienes are actually more stable than isolated systems, which is why they release less energy (they start from a lower energy state). Answer D focuses on ring strain relief, which isn't the concept being tested here and doesn't explain the energy difference.
Remember: when experimental heat of hydrogenation is less negative than expected, the starting compound has extra stabilization. Calculate the difference to find the stabilization energy.
- An alkene substrate contains both a C=C double bond and an aromatic benzene ring. Under mild catalytic hydrogenation conditions (H₂, Pd/C, room temperature, 1 atm), selective reduction occurs. Under more forcing conditions (higher temperature and pressure), additional reduction is observed. What selectivity pattern is expected and why?
Answer: The isolated alkene double bond reduces selectively under mild conditions because it is more reactive than the aromatic system, while the benzene ring requires forcing conditions due to its aromatic stabilization energy
When you encounter selectivity problems in catalytic hydrogenation, think about relative reactivity and thermodynamic stability. Different π systems have vastly different reduction barriers due to their electronic structures.
Under mild hydrogenation conditions, isolated alkenes reduce readily because they can easily coordinate to the palladium surface and have no special stabilization to overcome. The π electrons in a C=C double bond are localized and relatively high in energy, making them reactive toward hydrogen addition. In contrast, benzene rings possess aromatic stabilization (resonance energy of ~36 kcal/mol), which must be overcome before reduction can occur. This enormous energy barrier means aromatic rings remain untouched under gentle conditions but will eventually reduce under forcing conditions when sufficient thermal energy is provided to overcome the activation barrier.
Answer A incorrectly suggests aromatics reduce first due to higher π electron density—this ignores the crucial role of aromatic stabilization. Answer B wrongly claims aromatic reduction involves ring opening; catalytic hydrogenation of benzene produces cyclohexane directly without ring cleavage. Answer C incorrectly describes aromatic reduction as reversible under these conditions; once reduced, the cyclohexane product doesn't spontaneously dehydrogenate back to benzene under typical hydrogenation conditions.
Answer D correctly identifies that alkenes reduce selectively under mild conditions due to higher reactivity, while aromatics require forcing conditions to overcome their stabilization energy.
Study tip: Remember the hierarchy of π system reactivity in hydrogenation: isolated alkenes > conjugated alkenes > aromatic systems. Aromatic stabilization always creates a high barrier to reduction.
- A researcher needs to synthesize (E)-4,4-dimethyl-2-pentene starting from 4,4-dimethyl-2-pentyne. They mistakenly use H₂ gas with Lindlar's catalyst. Which statement accurately describes the primary outcome of this procedural error?
Answer: The (Z)-alkene will be formed as the major product, which is the geometric isomer of the desired product.
When you encounter alkyne reduction reactions, the key is recognizing how different catalysts control both the extent of reduction and stereochemical outcome.
Lindlar's catalyst (Pd/CaCO₃ with quinoline) is specifically designed for partial reduction of alkynes to alkenes through syn addition of hydrogen. The catalyst's surface forces both hydrogen atoms to add from the same face of the triple bond, creating a cis (Z) configuration. Since 4,4-dimethyl-2-pentyne is an internal alkyne with substituents on both sides, reduction will produce the Z-alkene where the larger groups are on the same side of the double bond.
Choice A incorrectly suggests the catalyst has minimal stereochemical influence. In reality, Lindlar's catalyst has very predictable stereoselectivity, consistently producing Z-alkenes through its syn addition mechanism.
Choice B is wrong because Lindlar's catalyst works effectively on both terminal and internal alkynes. The key difference is that terminal alkynes give Z-alkenes with the hydrogen and alkyl group cis to each other.
Choice C misunderstands Lindlar's selectivity. The catalyst is deliberately poisoned with quinoline to prevent over-reduction to alkanes. It stops at the alkene stage because the catalyst surface becomes less active toward alkene reduction.
Choice D correctly identifies that the major product will be (Z)-4,4-dimethyl-2-pentene, which is indeed the geometric isomer opposite to the desired E-alkene.
Remember: Lindlar's catalyst always gives Z-alkenes via syn addition, while dissolving metal reductions (Na/NH₃) give E-alkenes via anti addition. Choose your reduction method based on the desired stereochemistry.
- A student observes that when 1-octene is subjected to catalytic hydrogenation using Pd/C, the reaction rate decreases significantly as the reaction progresses, even though substrate is still present and hydrogen pressure remains constant. The student suspects catalyst deactivation but finds that adding fresh catalyst does not restore the original rate. What is the most likely explanation for this kinetic behavior?
Answer: As the alkene concentration decreases, the reaction shifts from zero-order kinetics in substrate to first-order kinetics, causing the apparent rate decrease due to lower substrate availability for surface adsorption
When analyzing heterogeneous catalysis kinetics, you need to understand how reaction order can change as substrate concentration varies. In catalytic hydrogenation, the alkene substrate must adsorb onto the catalyst surface before reaction can occur.
At high substrate concentrations (early in the reaction), the catalyst surface becomes saturated with alkene molecules. Under these conditions, the reaction rate becomes independent of substrate concentration—this is zero-order kinetics. The rate-determining step is the surface reaction itself, not substrate availability.
As the reaction progresses and alkene concentration decreases, the catalyst surface is no longer saturated. Now substrate availability becomes limiting, and the reaction shifts to first-order kinetics where rate directly depends on substrate concentration. This transition explains why the rate decreases even with constant hydrogen pressure and functional catalyst.
Option A incorrectly suggests competitive inhibition by the octane product, but saturated hydrocarbons typically have weak affinity for metal catalyst surfaces compared to alkenes. Option B proposes hydrogen mass transfer limitations, but the problem states hydrogen pressure remains constant, and this wouldn't explain why fresh catalyst fails to restore the original rate. Option C suggests accumulating impurities poison the catalyst, but this would be irreversible poisoning that adding fresh catalyst should overcome.
The correct answer is D because it accurately describes the kinetic transition from zero-order to first-order behavior as substrate concentration decreases.
Remember: In heterogeneous catalysis, always consider whether the catalyst surface is saturated or unsaturated with substrate—this determines the apparent reaction order and explains many kinetic observations.
- A researcher compares the hydrogenation rates of cyclohexene and 1-hexene under identical catalytic conditions. The cyclohexene reacts significantly faster than 1-hexene. When the same comparison is made using a homogeneous catalyst system instead of heterogeneous catalysis, the rate difference becomes much smaller. What factor primarily accounts for this observation?
Answer: Cyclohexene can adsorb more favorably on solid catalyst surfaces due to its compact shape and ability to maximize surface contact, while this geometric advantage is less important in homogeneous systems where catalyst-substrate interactions occur in solution
The key difference lies in surface adsorption effects that are crucial for heterogeneous catalysis but less important in homogeneous systems. Cyclohexene's compact, cyclic structure allows it to make better surface contact with the solid catalyst, leading to stronger adsorption and faster reaction. In homogeneous catalysis, both substrates are freely dissolved and interact with the catalyst in solution, minimizing the geometric advantages. Choice A incorrectly focuses on orbital overlap differences. Choice B wrongly suggests significant ring strain relief in cyclohexene hydrogenation. Choice D incorrectly implies different hybridization states between the alkenes.
- The reduction of 4-octyne with sodium metal in liquid ammonia (Na/NH₃) primarily yields which product?
Answer: (E)-4-octene
The dissolving metal reduction (Na in liquid NH₃) of an internal alkyne is stereoselective for the formation of a trans (E) alkene. The mechanism proceeds through a vinylic radical anion intermediate, which adopts the more stable trans conformation to minimize steric repulsion between the alkyl groups before being fully reduced. Therefore, the major product is (E)-4-octene. Choice B would be formed using H₂/Lindlar's catalyst. Choice A is the fully reduced alkane. Choice D is incorrect because the reaction is highly stereoselective.
