Question

How are the haloalkanes prepared from alkanes and alkenes? What type of mechanism is involved in these preparations?

Short answer

Haloalkanes are prepared from alkanes by free radical halogenation and from alkenes by electrophilic addition.

Step-by-step solution

Preparation of Haloalkanes from Alkanes

Haloalkanes can be prepared from alkanes through a process known as free radical halogenation. In this reaction, an alkane reacts with a halogen, typically chlorine or bromine, in the presence of UV light or heat. The reaction proceeds via a free radical mechanism, which involves three main steps: initiation, propagation, and termination. \[ \text{RH} + \text{X}_2 \xrightarrow{\text{UV light}} \text{RX} + \text{HX} \]1. **Initiation:** Halogen molecules dissociate into free radicals. \[ \text{X}_2 \rightarrow 2\text{X}^\cdot \]2. **Propagation:** A hydrogen atom from the alkane is substituted, forming a haloalkane and another radical. \[ \text{RH} + \text{X}^\cdot \rightarrow \text{R}^\cdot + \text{HX} \] \[ \text{R}^\cdot + \text{X}_2 \rightarrow \text{RX} + \text{X}^\cdot \]3. **Termination:** Free radicals combine to form stable molecules, halting the reaction.

Preparation of Haloalkanes from Alkenes

Haloalkanes can be prepared from alkenes through electrophilic addition. Alkenes react with halogens (such as Cl\(_2\) or Br\(_2\)) or hydrogen halides (like HCl or HBr) across the double bond.1. **Addition of Halogens (Halogen Addition):** This involves adding a halogen molecule across the double bond to form a dihaloalkane: \[ \text{RCH=CHR} + \text{X}_2 \rightarrow \text{RCHX-CHXR} \]2. **Addition of Hydrogen Halides (Hydrohalogenation):** This involves adding a hydrogen halide across the double bond following Markovnikov's rule, which results in the more substituted haloalkane being the major product: \[ \text{RCH=CHR} + \text{HX} \rightarrow \text{RCHX-CH_2R} \]

Types of Mechanisms Involved

In preparing haloalkanes from alkanes, the mechanism involved is free radical substitution. This process involves the formation of radicals and their reactions. In preparing haloalkanes from alkenes, the mechanism is electrophilic addition. This mechanism involves the electrophile attacking the electron-rich double bond, leading to the addition across the bond.

Key concepts

Free Radical Halogenation

Haloalkanes can be synthesized from alkanes through a process known as free radical halogenation. This involves a reaction between an alkane and a halogen, such as chlorine or bromine. The presence of UV light or heat is essential for this reaction. The process starts with the initiation step, where the halogen molecule splits into two reactive free radicals.

Once these radicals are formed, the reaction enters the propagation phase. Here, a free radical reacts with an alkane, replacing a hydrogen atom with a halogen atom. Another radical is generated, allowing this chain reaction to continue. Finally, in the termination step, free radicals combine to form stable molecules, thus concluding the reaction. This mechanism results in the formation of haloalkanes and demonstrates the stability of radical intermediates that sustain the reaction.

Electrophilic Addition

The preparation of haloalkanes from alkenes typically involves electrophilic addition. Alkenes react with halogens or hydrogen halides in this type of reaction. Being rich in electrons, the double bond of an alkene attracts the electrophile present in the halogen or hydrogen halide.

An interesting part of this process involves alkenes reacting with halogens such as \(Cl_2\) or \(Br_2\), resulting in the formation of dihaloalkanes. Alternatively, when hydrogen halides like \(HCl\) or \(HBr\) are added, the reaction yields haloalkanes following specific rules such as Markovnikov’s rule. This ensures the halogen attaches to the more substituted carbon, optimizing the stability of intermediates and the final haloalkane product.

Alkane

Alkanes are simple hydrocarbons composed only of carbon and hydrogen atoms. They follow the general formula \(C_nH_{2n+2}\), where \(n\) represents the number of carbon atoms. These molecules are saturated, meaning they contain only single bonds between carbon atoms.

The lack of multiple bonds makes alkanes less reactive compared to their unsaturated counterparts. However, under specific conditions, such as the presence of UV light or heat, they can participate in reactions like free radical halogenation. This process substitutes one or more hydrogen atoms in an alkane with halogen atoms, thereby forming haloalkanes.

Alkene

Alkenes are hydrocarbons similar to alkanes but differ as they include at least one carbon-carbon double bond. This double bond is the source of their reactivity because it is a region of high electron density. Alkenes follow the general formula \(C_nH_{2n}\).

Thanks to this double bond, alkenes can readily undergo addition reactions where new atoms or molecules add across the bond. Electrophilic addition is a common reaction for alkenes and plays a crucial role in the conversion to haloalkanes. The double bond's electrons interact with an electrophilic species, aiding the addition and influencing the final product type.

Markovnikov's Rule

A principle known as Markovnikov's rule guides the outcome of certain addition reactions involving alkenes and hydrogen halides. This rule predicts how a molecule will add across the double bond of an alkene, determining the structure of the final haloalkane.

According to Markovnikov’s rule, in reactions involving unsymmetrical alkenes and hydrogen halides, the hydrogen from the hydrogen halide will attach to the less substituted carbon atom. Meanwhile, the halogen (e.g., Cl or Br) will bond with the more substituted carbon. This approach provides a more stable carbocation intermediate during the reaction, favoring the production of the more substituted and stable haloalkane as the primary product. Understanding this rule helps students predict and comprehend the outcomes of these molecular transformations.