Question

Using your reaction roadmap as a guide, show how to convert cyclohexane into racemic 3-bromocyclohexene. Show all reagents and all molecules synthesized along the way.

Short answer

Question: Describe the process of converting cyclohexane into racemic 3-bromocyclohexene in two steps. Answer: To convert cyclohexane into racemic 3-bromocyclohexene, the first step is to form a double bond on the cyclohexane ring by treating it with a strong base, such as potassium tert-butoxide (t-BuOK). This will convert cyclohexane into cyclohexene. In the second step, add a bromine atom at the 3-position of the cyclohexene ring using N-bromosuccinimide (NBS). This bromination reaction will result in a racemic mixture of 3-bromocyclohexene.

Step-by-step solution

Formation of the double bond

To form a double bond on the cyclohexane ring, an elimination reaction must be carried out. This can be achieved by treating cyclohexane with a strong base, such as potassium tert-butoxide (t-BuOK), which will abstract a proton from the beta-carbon, leading to the formation of the cyclohexene double bond. The reaction will occur as follows: Cyclohexane + t-BuOK → Cyclohexene + t-BuOH This will convert cyclohexane to cyclohexene.

Bromination of the cyclohexene ring

In order to add a bromine atom at the 3-position of the cyclohexene ring, a bromination reaction using N-bromosuccinimide (NBS) can be performed. The addition of NBS to the reaction mixture under the appropriate conditions will lead to bromination of the cyclohexene double bond, creating two isomers; as a result, we will obtain a racemic mixture of 3-bromocyclohexene. The reaction will proceed as follows: Cyclohexene + NBS → Racemic 3-bromocyclohexene By combining these two reactions, we can convert cyclohexane into racemic 3-bromocyclohexene using a reaction roadmap: 1. Cyclohexane →(t-BuOK)→ Cyclohexene 2. Cyclohexene →(NBS)→ Racemic 3-bromocyclohexene

Key concepts

Reaction Roadmap

A reaction roadmap is an essential tool for chemists to outline the sequence of chemical reactions required to transform a starting molecule into a target compound. It serves as a visual representation of the synthetic strategy, laying out the reagents, intermediates, and products that appear during the transformation. In the case of converting cyclohexane into racemic 3-bromocyclohexene, our reaction roadmap consists of two major steps. First, an elimination reaction to form cyclohexene from cyclohexane, and second, a bromination reaction to introduce a bromine atom at the 3-position of the cyclohexene ring.

By visualizing these steps in a roadmap, students can better understand the logical progression of the synthesis and the role each reagent plays. This tool is not only practical for planning and executing organic syntheses but also for educational purposes where it aids in presenting a clearer picture of complex chemical processes.

Elimination Reaction

An elimination reaction is a type of organic reaction in which two atoms or groups are removed from a molecule, forming a double or triple bond. This is a key transformation in organic synthesis, particularly when creating unsaturated compounds from saturated precursors. In our exercise, to transform cyclohexane into cyclohexene, we use a strong base, potassium tert-butoxide (t-BuOK).

The base abstracts a proton (H+) from a carbon atom adjacent to the one holding the leaving group, which in this case is a hydrogen atom. This deprotonation step is crucial as it initiates the formation of the double bond by the removal of two hydrogen atoms from the cyclohexane (one as a proton and the other as a hydride), resulting in cyclohexene.

Bromination Reaction

The bromination reaction is a subset of halogenation reactions where a bromine atom is introduced into an organic compound. This is often done to add functionality or to modify the reactivity of the molecule. In our exercise, we carry out the bromination of cyclohexene to form 3-bromocyclohexene. We employ N-bromosuccinimide (NBS) as the brominating agent due to its ability to deliver bromine atoms selectively and efficiently.

NBS acts via a free radical mechanism that preferentially targets allylic and benzylic positions in hydrocarbons. The bromine atom from NBS adds to the cyclohexene, creating a bromonium ion intermediate, which is then attacked by a nucleophile to yield the final product, 3-bromocyclohexene. This transformation is critical for the synthesis of our target molecule.

Racemic Mixture

A racemic mixture is a 50:50 mixture of two enantiomers of a chiral molecule. Enantiomers are mirror-image isomers of each other and have identical physical properties except for the way they rotate plane-polarized light, which is why a racemic mixture is optically inactive. In the synthesis of 3-bromocyclohexene from cyclohexene using NBS, the reaction proceeds such that both possible enantiomers are formed in equal amounts.

This occurs because the intermediate bromonium ion formed during the reaction can be attacked from either side, leading to the formation of both enantiomers. Understanding the concept of racemic mixtures is crucial in organic chemistry, as selectivity for one enantiomer over the other can be vital in pharmaceutical applications.

Organic Synthesis

Organic synthesis is the process of constructing organic molecules through a series of chemical reactions. The aim is often to produce a desired chemical compound efficiently and selectively, starting from simpler substances. It is a fundamental aspect of organic chemistry and is instrumental in creating compounds for various applications including pharmaceuticals, materials, and agrochemicals.

In our specific exercise, the organic synthesis involves transforming cyclohexane into racemic 3-bromocyclohexene by an elimination reaction to create cyclohexene followed by a bromination reaction. Such multi-step syntheses require careful planning and execution, often visualized through a reaction roadmap, to obtain the intended product successfully. It also highlights the need to consider factors such as yield, purity, and stereoselectivity in the design of these synthetic routes.