Question

Cyclohexene can be converted to 1-cyclopentenecarbaldehyde by the following series of reactions. Propose a structural formula for each intermediate compound.

Short answer

Question: Outline the steps involved in converting cyclohexene to 1-cyclopentenecarbaldehyde and the intermediate structures formed during this process. Answer: The conversion of cyclohexene to 1-cyclopentenecarbaldehyde involves the following steps: 1. Oxidation of the double bond using ozonolysis (ozone and dimethyl sulfide or zinc) to form carbonyl groups (C1=O and C6=O) on former double-bonded carbons. 2. Reductive removal of the C6=O carbonyl group using Wolff-Kishner reduction (hydrazine and potassium hydroxide) to create a methylene group (C6=CH2). 3. Formation of the aldehyde functional group (H-C=O) at position C1 by DIBAL-H reduction. The intermediate structures are: (1) C1=O and C6=O, (2) C1=O and C6=CH2. The final product is 1-cyclopentenecarbaldehyde with C1=(H-C=O) and C5=CH2.

Step-by-step solution

Identify the starting material and target compound

The given starting material is cyclohexene, a six-membered ring with one double bond. The target compound is 1-cyclopentenecarbaldehyde, which is a cyclopentene ring system with an aldehyde functional group (H-C=O) at position 1.

Oxidation of the double bond

The first step is to oxidize the double bond of cyclohexene to break one of the carbon-carbon connections and introduce two carbonyl groups. This can be achieved with an oxidative cleavage agent, such as ozone (O3) in the presence of a reductive agent like dimethyl sulfide (DMS) or zinc in the subsequent step. This process is called ozonolysis. The intermediate structure here would have two carbonyl groups attached to former double-bonded carbons: C1=O and C6=O

Reductive removal of one carbonyl group

Next, we need to remove the C6=O carbonyl group while retaining the C1=O carbonyl group. This can be accomplished via a Wolff-Kishner reduction, which involves converting the carbonyl group (C6=O) to a methylene group (CH2) and hydrogen gas. The Wolff-Kishner reduction requires two reagents: hydrazine (NH2NH2) and potassium hydroxide (KOH). This results in the following intermediate structure: C1=O and C6=CH2

Formation of the aldehyde functional group

The final step is to convert the remaining carbonyl group (C1=O) to an aldehyde functional group (H-C=O). This can be achieved via the DIBAL-H reduction (Diisobutylaluminium hydride). DIBAL-H is a selective reducing agent that converts esters and nitriles to aldehydes. After the reduction, we arrive at the target compound, 1-cyclopentenecarbaldehyde: C1=O and C6=CH2 -> C1=(H-C=O) and C5=CH2 By following these steps and applying the appropriate reactions, we have successfully converted cyclohexene to 1-cyclopentenecarbaldehyde and identified the intermediate structures involved in this process.

Key concepts

Cyclohexene

Cyclohexene is a simple organic compound with a six-membered ring containing one double bond. It serves as a common starting material in various chemical reactions. The structure plays an essential role because the double bond is reactive, allowing chemists to transform it into other functional groups.
This reactivity is due to the high electron density between the two carbon atoms in the double bond. In reactions, this double bond can be targeted by oxidizing agents, like ozone in ozonolysis, to cleave the bond and introduce new functional groups. Such transformations expand its utility in synthetic organic chemistry, enabling the synthesis of more complex molecules like 1-cyclopentenecarbaldehyde.
The simplicity of cyclohexene and its accessibility make it a versatile substrate for practicing and understanding various chemical transformations.

Wolff-Kishner Reduction

The Wolff-Kishner reduction is a powerful chemical reaction used to remove carbonyl groups from compounds. In the context of the original exercise, this technique is crucial for transforming one of the two introduced carbonyl groups on the oxidized cyclohexene into a methylene group.
The reaction involves two main reagents: hydrazine (\( NH_2NH_2 \)) and potassium hydroxide (\( KOH \)). These reagents work together to convert a carbonyl group into a methylene group while releasing nitrogen gas (\( N_2 \)).

• This process allows for selective reduction, meaning you can remove specific carbonyl groups while leaving others intact, like in our transformation of cyclohexene to 1-cyclopentenecarbaldehyde.
• Moreover, the Wolff-Kishner reduction is especially useful when avoiding acidic or moisture-sensitive conditions, as it typically operates under basic and high-temperature conditions.

Understanding this reduction is key to many synthetic pathways, exemplifying the fascinating interplay of chemistry in remodeling organic structures.

Diisobutylaluminium Hydride (DIBAL-H)

Diisobutylaluminium hydride, commonly referred to as DIBAL-H, is a reducing agent used primarily for reducing esters and nitriles to aldehydes. Its selectivity and mild reaction conditions make it a favorite choice in organic synthesis for precise transformations.
In our exercise, DIBAL-H plays a pivotal role in the final step of converting one of the carbonyl groups formed post-ozonolysis into an aldehyde group, yielding 1-cyclopentenecarbaldehyde. Unlike other strong reducing agents like lithium aluminium hydride (\( LiAlH_4 \)), DIBAL-H operates selectively, stopping at the aldehyde stage without further reduction to alcohols.

• This selectivity is key when looking to introduce an aldehyde group without over-reduction.
• Moreover, reactions with DIBAL-H are typically performed at low temperatures, which helps in maintaining control over the reaction's progress.

DIBAL-H thus enables precise targeting of specific functionalities, illustrating the nuanced control chemists can exert in synthetic processes.

Cyclopentenecarbaldehyde

Cyclopentenecarbaldehyde is an organic compound characterized by a five-membered ring (cyclopentene) with an aldehyde group at one position. Synthesizing such structures from simpler compounds requires a series of carefully planned reactions.
The process of transforming cyclohexene to cyclopentenecarbaldehyde involves multiple steps, including the use of ozonolysis, Wolff-Kishner reduction, and DIBAL-H reduction. The final ring structure is locked in place, ready for further modification or application in test reactions.
Cyclopentenecarbaldehyde can often serve as a building block for more complex organic compounds, especially in the pharmaceutical and agricultural sectors, due to the reactive aldehyde group that allows for further chemistry.