Question

\(\mathrm{R}-\mathrm{CH}=\mathrm{CH}_{2}+\mathrm{CCl}_{4}\) Peroxide \(\longrightarrow\) A. Here, \(\mathrm{A}\) is (a) [R]C(Cl)C(Cl)(Cl)Cl (b) [R]C(Cl)CCl (c) Both are correct (d) Reaction does not take place

Short answer

The correct answer is (b).

Step-by-step solution

Identify the Reaction Type

The reaction given is of an alkene with carbon tetrachloride (Cl_4) under peroxide conditions. This typically indicates a free radical addition reaction.

Understand Free Radical Addition

In the presence of peroxides, alkenes undergo free radical chlorination rather than electrophilic addition. Peroxides initiate the free radical chain reaction by generating chlorine radicals.

Analyze the Radical Formation

The chlorine radicals (Cl·) add to the least substituted carbon of the double bond, based on the 'anti-Markovnikov' rule common in radical reactions. This means the addition happens across the double bond with the chlorine attaching to the less substituted carbon.

Product Formation Identification

The chlorine radical adds to the alkene forming a secondary radical, which can further react with another Cl_2 molecule to give a dichloro compound. Thus, from R-CH=CH_2, the end product is R-CHCl-CHCl_2.

Match with Options

Compare the derived product R-CHCl-CHCl_2 with the given options. The structure [R]C(Cl)CCl corresponds to R-CHCl-CHCl_2, matching option (b). Thus, (b) is the correct answer.

Key concepts

Anti-Markovnikov Rule

In the world of organic chemistry, the anti-Markovnikov rule is quite significant, especially when considering reactions involving free radicals. When a hydrogen halide adds across the carbon-carbon double bond of an alkene, where a peroxide is present, the addition often deviates from Markovnikov's rule.
The anti-Markovnikov rule states that the halogen will add to the less substituted carbon atom, not the more substituted one. This outcome contrasts with typical electrophilic additions which follow Markovnikov's rule, placing the hydrogen atom on the less substituted carbon.

• The presence of peroxides is crucial because they decompose to generate radical species.
• These radicals cause the formation of an alternate reaction pathway, leading to the anti-Markovnikov product.

Understanding this rule is key to predicting the structure of the product in radical chlorination reactions.

Alkene Reactions

Alkenes, characterized by their carbon-carbon double bonds, are highly reactive and can undergo a variety of chemical reactions. An important type of alkene reaction is the addition reaction, in which atoms are added to the carbons of the double bond, converting it to a single bond.
Alkene reactions include:

• Electrophilic addition, where the alkene pi-bond acts as a nucleophile.
• Free radical addition, which can proceed under the influence of peroxides.
• Syn and anti addition, describing the stereochemistry of the product.

In our specific case, due to the influence of peroxides, the radical mechanism prevails, leading to unique product outcomes that may not be predicted by classical electrophilic pathways.
Recognizing the differing conditions and mechanisms helps differentiate which alkene reaction pathway will occur.

Peroxides in Organic Chemistry

Peroxides serve a unique role in organic chemistry due to their ability to initiate radical chain reactions.
During decomposition, peroxides break down into free radicals, which are highly reactive species with unpaired electrons. These radicals are capable of starting a domino effect that propels radical reactions, like the chlorination seen in alkene reactions under peroxidation conditions.

• Common peroxides used in organic reactions include hydrogen peroxide (H₂O₂) and benzoyl peroxide.
• Radicals formed from peroxides can either initiate a reaction or propagate it further.

The role of peroxides is pivotal—they set the stage for alternative reaction mechanisms that defy expectation, paving the way for the anti-Markovnikov additions in alkene chemistry.

Radical Chain Reactions

Radical chain reactions are fascinating processes that involve a stepwise mechanism divided into initiation, propagation, and termination phases.

• **Initiation**: Radicals are first generated. In our discussion involving peroxides, the peroxide itself decomposes to form the initial radical species.
• **Propagation**: Radicals add to alkenes, generating new radicals which can continue the reaction chain.
• **Termination**: Radicals combine, concluding the chain reaction.

This mechanism allows the transformation of simple reactants into more complex products, and notably the creation of multiple products if not controlled.
The unique nature of radical chain reactions lies in their potential to occur under mild conditions, and their capacity to drive reactions to completion efficiently. Mastery of their mechanism allows for detailed predictions and manipulations of organic synthesis pathways.