Problem 36 - Basic Principles of Organic Chemistry

Question

tert-Butyl hypobromite is a radical brominating agent that is similar to tert- butyl hypochlorite (Exercise 4 \(18^{*}\) ), but is less easily prepared than the hypochlorite. A good substitute, provided radical bromination is possible, is a mixture of \(\mathrm{BrCCl}_{3}\) and \(\mathrm{ce}\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COCl}\) Thus, bromination of cyclohexene results if a high ratio of bromotrichloromethane to hypochlorite is used. O=C(O)CCC1CCCCC1Br Suggest how this reaction is initiated and propagated, and explain why it is necessary to have an excess of bromotrichloromethane.

Step-by-Step Solution

Answer

The reaction is initiated by forming \( \text{Br} \bullet \) from \( \mathrm{BrCCl_3} \) and propagated by bromine radicals reacting with cyclohexene. An excess of \( \mathrm{BrCCl_3} \) ensures a continuous supply of bromine radicals.

Step 1: Identify components and context

We're given that the reaction involves tert-butyl hypobromite, a radical brominating agent, and we're also given alternatives including \( \mathrm{BrCCl_3} \) and \( \mathrm{(CH_3)_3COCl} \). The main task is to understand how radical bromination occurs using these reagents with cyclohexene and why an excess of bromotrichloromethane is required.

Step 2: Understand the initiation of the reaction

The initiation step typically involves the creation of a bromine radical. In this case, \( \mathrm{BrCCl_3} \) can be photochemically or thermally dissociated to form \[ \text{Br} \bullet \] radicals. This is done under conditions that allow the carbon-bromine bond in \( \mathrm{BrCCl_3} \) to break, producing \( \text{Br} \bullet \) and \( \mathrm{CCl_3} \bullet \).

Step 3: Describe the propagation phase

Once the \( \text{Br} \bullet \) radicals are formed, they can add to the double bond of cyclohexene, resulting in a new radical species: \( \mathrm{C_6H_{10}Br} \bullet \). This radical can then react with another \( \mathrm{BrCCl_3} \), abstracting a bromine atom to form the brominated product and regenerating \( \mathrm{CCl_3} \bullet \), thus continuing the chain reaction.

Step 4: Explain the role of bromotrichloromethane

Bromotrichloromethane serves as a reservoir for the bromine radicals needed to sustain the radical chain reaction. An excess of \( \mathrm{BrCCl_3} \) ensures a steady supply of bromine radicals and helps prevent unwanted side reactions by minimizing competition from other potential radical scavengers. This also ensures that the \( \mathrm{CCl_3} \bullet \) radicals are available to perpetuate the chain reaction.

Step 5: Draw conclusions on the necessity of the excess

Having an excess of bromotrichloromethane helps maintain the propagation step, ensuring efficient bromination of cyclohexene. The excess ensures sufficient radical availability to overcome the typically low concentration of radicals, which can otherwise terminate the chain reactions prematurely.

Key Concepts

tert-butyl hypobromite

Tert-butyl hypobromite is an important radical brominating agent. Its primary function is to facilitate the bromination process by providing bromine radicals. These radicals are highly reactive species that can initiate chain reactions in organic compounds. Just like tert-butyl hypochlorite, tert-butyl hypobromite can introduce bromine into various chemical structures, making it highly valuable in organic synthesis. However, it's worth noting that tert-butyl hypobromite is less easily prepared than its hypochlorite counterpart. This difficulty in preparation sometimes necessitates alternative methods or reagents to achieve the desired bromination.

bromotrichloromethane

Bromotrichloromethane, denoted as \( \text{BrCCl}_3 \), plays a pivotal role in radical bromination reactions. It serves as a source of bromine radicals when subjected to specific conditions such as heat or light. The carbon-bromine bond in bromotrichloromethane can be broken to produce bromine radicals \( \text{Br} \bullet \) and trichloromethyl radicals \( \text{CCl}_3 \bullet \). This reaction is a crucial initiation step in radical bromination. By maintaining a high ratio of bromotrichloromethane to other reagents, it ensures a continuous supply of bromine radicals. This is vital to sustaining the radical chain process and minimizing the risks of side reactions.

cyclohexene bromination

Bromination of cyclohexene involves introducing a bromine atom into the cyclohexene molecule. This process starts when a bromine radical \( \text{Br} \bullet \) adds to the double bond within cyclohexene. The result is the formation of a new radical species \( \text{C}_6\text{H}_{10}\text{Br} \bullet \). This newly generated radical can subsequently react with bromotrichloromethane, abstracting a bromine atom and producing the final brominated product. By using this method, we achieve selective bromination of cyclohexene with reduced side reactions, attributed to the high concentration of bromotrichloromethane that prevents unwanted radical interactions.

radical chain reaction

Radical chain reactions are characterized by initiation, propagation, and termination steps. Initiation involves the generation of reactive radicals, often initiated by light or heat. In the case of our reaction, bromotrichloromethane provides bromine radicals under thermal or photochemical conditions. These radicals propagate the chain reaction by reacting with substances like cyclohexene to form new radicals. Each propagation step can form more bromine radicals, sustaining the chain reaction efficiently. Finally, the termination step occurs when radicals recombine to form stable products. Maintaining an excess of bromine radical sources like bromotrichloromethane ensures the chain reaction continues smoothly, leading to efficient conversion without premature termination.