Objective Chemistry for Engineering and Medical Entrance Examinations

K Rama Rao

ISBN #9789332541771

1st Edition

2,463 Questions

12,716 Students Helped

Homework Questions

Summary

Learning Objectives

Key Concepts

Example Problems

Explanations

Common Mistakes

Summary

This chapter section focuses on the controlled partial reduction of alkynes to alkenes, emphasizing the importance of selecting the right catalyst to influence product stereochemistry. Terminal alkynes typically yield a single alkene product, while non-terminal alkynes can form cis or trans isomers based on the catalyst employed, such as Lindlar's for cis and Wilkinson's for trans. Mastery of these reaction conditions is essential for chemists designing selective synthetic pathways.

Learning Objectives

Explain the mechanism of partial reduction of alkynes to alkenes.

Distinguish between terminal and non-terminal alkynes in terms of their reaction outcomes.

Identify the role of different catalysts such as Lindlar's and Wilkinson's in directing cis or trans selectivity.

Apply the understanding of reaction conditions to design selective synthetic pathways in organic chemistry.

Key Concepts

CONCEPT

DEFINITION

Partial Reduction

A chemical process where an alkyne is selectively reduced to an alkene, avoiding further hydrogenation to an alkane.

Terminal Alkyne

An alkyne with a hydrogen atom attached to the triple-bonded carbon at one end, leading typically to a single alkene product upon partial reduction.

Non-Terminal Alkyne

An alkyne that does not have a terminal hydrogen; its reduction can yield either cis or trans isomers depending on the catalyst used.

Lindlar's Catalyst

A poisoned palladium catalyst used to achieve cis-selective hydrogenation of alkynes, producing cis-alkenes.

Wilkinson's Catalyst

A catalyst known to favor trans formation during the partial reduction of certain non-terminal alkynes.

Cis and Trans Isomers

Stereoisomers where substituents are on the same side (cis) or opposite sides (trans) of the double bond in an alkene.

Example Problems

Example 1

Using which of the following reagents one can perform a simple test that can be uscd to differentiate between $\mathrm{C}_{6} \mathrm{II}_{5} \mathrm{C} \equiv \mathrm{CII}$ and $\mathrm{C}_{6} \mathrm{I}_{5} \mathrm{CII}=\mathrm{CH}_{2} ?$ (1) $\mathrm{NaOIL} / \mathrm{II}_{2} \mathrm{O}$ (2) $\mathrm{Br}_{2} / \mathrm{CCl}_{4}$ (3) $\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{OII}$ (4) $\mathrm{CrO}_{3} / \mathrm{II}_{2} \mathrm{SO}_{4}$

Example 2

By using which of the following can the conversion of $\mathrm{PhC} \equiv \mathrm{CH} \longrightarrow \mathrm{PhC}=\mathrm{CHCH}_{3}$ be achieved? (1) $\mathrm{Br}_{2} \mathrm{CCl}_{4}$, then $\mathrm{KOH}$ (2) $\mathrm{Na}$, then $\mathrm{CH}_{3} \mathrm{CH}_{3} \mathrm{I}$ (3) $\mathrm{Na}$, then $\mathrm{CH}_{3} \mathrm{I}$ (4) $\mathrm{CH}_{2} \mathrm{~N}_{2}$

Example 3

Which of the following statements is true for cthanc, cthenc and acetylene? (1) \Lambdacetylcne is the weakest acid and has the longest C II bond distance. (2) \Lambdacetylene is the strongest acid and has the strongest $\mathrm{C}$ II bond distance. (3) Ethanc is the strongest acid and has the longest C II bond distance. (4) Ethene is the strongest acid and has the shortest C II bond distance.

Example 4

On oxidation with hot aqueous $\mathrm{KMnO}_{4}$ a compound $\mathrm{X}$ gives 2 -methyl propionic acid and propionic acid. Which of the following is the strueture of $X$ ?

Example 5

On catalytic hydrogenation, a compound $\mathrm{X}\left(\mathrm{C}_{7} \mathrm{H}_{12}\right)$ absorbs 2 mol of hydrogen and yields 2 -methylhexane. On treatment with $\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{OH}, \mathrm{X}$ gives a precipitate which contains silver and which regenerates $X$ on treatment with dilute $\mathrm{HNO}_{3}$. The structure of $\mathrm{X}$ is

Step-by-Step Explanations

QUESTION

How does the partial reduction of a terminal alkyne lead to a single alkene product using a selective catalyst?

STEP-BY-STEP ANSWER:

Step 1: Identify that the alkyne is terminal, meaning there is a hydrogen directly attached to the triple bond.
Step 2: Choose an appropriate catalyst, such as Lindlar's catalyst, which is known to stop the reaction at the alkene stage.
Step 3: Introduce hydrogen gas under controlled conditions, ensuring the addition is partial and does not proceed to full hydrogenation.
Step 4: Monitor the reaction to confirm that only a cis-alkene forms, which is typical for terminal alkynes under these conditions.
Final Answer: The controlled, partial hydrogenation of terminal alkynes using a selective catalyst like Lindlar’s results in a single cis-alkene product.

Partial Reduction of Terminal Alkynes

QUESTION

How can different catalysts influence the formation of cis or trans isomers in the partial reduction of non-terminal alkynes?

STEP-BY-STEP ANSWER:

Step 1: Recognize that non-terminal alkynes lack a terminal hydrogen, allowing for multiple stereochemical outcomes.
Step 2: Use Lindlar's catalyst to promote the formation of cis-alkenes due to its catalyst poisoning that limits over-reduction.
Step 3: Alternatively, apply catalysts such as Wilkinson's catalyst which favor the formation of trans isomers through a different reaction pathway.
Step 4: Analyze the reaction conditions, as slight variations can influence the selectivity towards cis or trans products.
Final Answer: The choice of catalyst, along with precise control of reaction conditions, determines whether non-terminal alkynes yield cis or trans alkenes.

Partial Reduction of Non-Terminal Alkynes

Common Mistakes

Assuming all alkynes behave the same regardless of substitution (terminal versus non-terminal).
Overlooking the critical role of catalyst selection in influencing cis-trans isomerism.
Neglecting strict control over reaction conditions, which can lead to over-reduction beyond the alkene stage.
Confusing the end products of complete hydrogenation (alkanes) with those of partial hydrogenation (alkenes).