Methods of Preparation - Haloalkanes and Haloarenes for JEE

Today, there are over 1600 halogenated organic compounds. There are several ways to prepare haloalkanes and haloarenes. The existence of haloalkanes can be traced back to the 15^th century. The first haloalkane produced was chloroethane. Then, in the 19^th century, such compounds were manufactured and synthesised to understand the organic chemistry and structure of alkanes.

There are four main types of manufacturing processes for haloalkanes and haloarenes. These include the production of haloalkanes and haloarenes from:

• Alcohol.

• Hydrocarbons.

• Hydrogen halide and halogen.

• Halogen exchange reaction.

Preparation from Alcohol (Haloalkanes)

The most convenient way to prepare haloalkanes is from alcohol. When R-OH reacts with the appropriate reagent, the reaction forms R-X. Suitable reagents to aid the reaction are concentrated halogen acid (HX), phosphorus halide (PX₅ or PX₃), and thionyl chloride (SOCl₂).

• The Reaction of Alcohol with Halogen Acid

Organic alcohol derivatives react with halogen acid (H-X) to form haloalkanes as the main product.

$ROH+HX \xrightarrow[]{catalyst}RX+H_{2}O$

Preparation

Preparation Example of Chloroalkane:

The preparation of chloroalkane is an example of the production of haloalkane by the reaction of alcohol and halogen acid. In this case, the primary and secondary alcohols react with the hydrochloric acid gas to form haloalkanes in the presence of anhydrous ZnCl₂, which acts as a catalyst for this reaction.

$(CH_{3})_{3}C-OH+HCL\to(CH_{3})_{3}C-O^{+}H_{2} $

$(CH_{3})_{3}C^{+}\xrightarrow[]{Cl^{-}}(CH_{3})_{3}C-Cl$

Manufacture of Haloalkanes and Haloarenes from Hydrocarbons

The manufacture of haloalkanes and haloarenes from hydrocarbons is possible using three different methods. They are radical halogenations of haloalkanes electrophilic substitution reaction.

• Sandmeyer Reaction

i) Radical halogenation

Formation of alkyl bromide and alkyl chloride is possible by radical halogenation reaction. However, radicals are inherently very non-selective. In addition, radicals are non-specific and highly reactive intermediates that form a mixture of products.

For example, bromination or chlorination of free radicals leads to the formation of various haloalkanes. This makes it difficult to separate a single product. Therefore, it is not the preferred method for preparing haloalkanes. 

$CH_{3}CH_{2}CH_{2}CH_{3}\xrightarrow[]{Cl_{2}/H\nu}CH_{3}CH_{2}CH_{2}CH_{2}Cl+CH_{3}CH_{2}CH(Cl)CH_{3}$

ii) Electrophilic Substitution Reaction

This method is useful for the preparation of haloarenes such as aryl bromide and aryl chloride. Electrophilic substitution uses halogens such as chlorine and bromine in the presence of Lewis acid to form aryl bromide and aryl chloride. However, the reaction must follow certain conditions to produce the appropriate electrophile.

For example, the reaction should be carried out in the presence of Lewis acid. In addition, the reaction must take place in the dark. The reaction to obtain an electrophile is as follows:

$C_{6}H_{5}-CH_{3}+X_{2}\xrightarrow[]{Fe/Dark}CH_{3}-C_{6}H_{5}-X(para)+CH_{3}-C_{6}H_{5}-X(ortho)$

Preparation:

The electrophiles for the above reaction are Cl⁺ and Br⁺, and HCl and HBr are by-products of the reaction. Therefore, the electrophilic substitution mechanism is used for producing aryl bromide and aryl chloride.

• Sandmeyer Reaction Mechanism

It is two-step method of Diazonium salt formation and Diazonium salt reaction with a cuprous halide (Cu₂X₂)

• Primary aromatic amines react with sodium nitrite in the presence of cold mineral acids to form diazonium salts. In this case, HNO₂ is produced in the reaction by treating sodium nitrite with HX at a temperature of 273 to 278 K.

$C_{6}H_{5}-NH_{2}\xrightarrow[0.5^{\circ}C]{NaNO_{2}+HCl}N^{+}_{2}Cl^{-}-C_{6}H_{5}\xrightarrow[]{CuCl/HCl}C_{6}H_{5}-Cl$

Haloalkanes and Haloarenes from Alkenes

Haloalkanes and haloarenes can be prepared by adding halogen (X2) to the double bond of the alkene. It is also possible to add hydrogen halide (HX). This halogen can be chlorine, bromine, or iodine.

The addition of HX:

• Alkenes can be converted to haloalkanes by electrophilic addition reaction. Alkenes react with HX to form R-X. 

• The order of reactivity of halides with alkenes follows the order of HI > HBr > HCl > HF.

• The reaction, in this case, is an example of a regioselective reaction. With this type of reaction, the quantity of the product grows steadily. In addition, the reaction determines the major product by addition to the entire alkene double bond according to Markovnikov's addition rules.

• According to Markovnikov's law, in the addition reaction of asymmetric alkenes, the negative part of the reagent or halogen binds to the carbon-containing fewer hydrogen atoms.

For example, prop-1-ene reacts with hydrogen bromide to form 2-bromopropane as the main product.

$C_{3}H_{6}+Br_{2}\xrightarrow[]{CCl_{4}}C_{3}H_{6}Br_{2}$

• Peroxide Effect (Karasch effect) 

There is another possibility that the reaction contradicts Markovnikov's law. This effect is known as the peroxide effect/Karasch effect/anti-Markovnikov law. In this reaction, the alkene reacts with HBr in the presence of peroxide. The Br or negative part of the reagent binds to the carbon with more hydrogen atoms.

$CH_{3}CH=CH_{2}+HBr\xrightarrow[]{peroxide}CH_{3}-CH_{2}-CH_{2}-Br$

Haloalkanes and Haloarenes from Halogen Exchange Reactions

• Finkelstein Reaction

The final method for preparing haloalkanes and haloarenes is the halogen exchange reaction. In this reaction, alkyl chlorides or alkyl bromides react with sodium iodide in acetone to form alkyl iodides.

$R-X+NaI\rightleftharpoons R-I-NaX$

$C_{2}H_{5}-Cl+NaI\rightleftharpoons C_{2}H_{5}-I+NaCl$

Preparation 

Since the reaction is an equilibrium reaction, other products may be produced. Differences in the solubility of alkyl halides in acetone are used to drive the reaction forward. Sodium iodide is soluble in acetone, but NaCl or NaCl is insoluble. Therefore, they precipitate during the reaction and can be easily removed from the reaction mixture.

• Swarts Reaction

In this reaction, alkyl fluoride can be formed by heating the alkyl fluoride RBr/RCl. The reaction is carried out in the presence of metal fluorides such as SbF₃, Hg₂F₂, AgF, CoF₂.

$CH_{3}Br+AgF\rightarrow CH_{3}F+AgBr$

Hunsdiecker Method

• The Hunsdiecker reaction is a chemical reaction in which a silver salt of a carboxylic acid reacts with a halogen to form an unstable intermediate, which is further thermally decarboxylated to form a final product known as an alkyl halide. 

$R-\overset{\begin{matrix} O \\ \parallel   \\\end{matrix}} {\mathop{C}} \, -{{O}^ {-}} A{{g}^ {+}} \xrightarrow [CC{{l}_ {2}}] {B{{r}_ {2}}} R-Br$

• The Hunsdiecker reaction mechanism mainly contains organic radical intermediates.

• Formation of reactive intermediates.

• Generation of decarboxylation to form diradical pairs.

• Rebinding of reactants to the desired product. To further decompose the

The reaction begins by heating silver carboxylate in CCl₄ with bromine. 

• Silver carboxylate is converted to acyl hypobromous acid mainly by the presence of bromine. 

• Next, a stable silver bromide precipitate is formed. 

• The result is a radical chain reaction that homogenised the weaker oxygen-bromine bond. 

• This produces a bromine atom and a carboxyl radical. 

• This carboxyl group is decarboxylated to result in the formation of a diradical pair of hydrocarbyl or alkyl groups, which then recombine to form the desired halide, in this case alkyl bromide.

Summary

The Hunsdiecker reaction is a chemical reaction in which a silver salt of a carboxylic acid reacts with a halogen to form an unstable intermediate, which is then thermally decarboxylated to form a final product known as an alkyl halide. This reaction is also known as the Hunsdiecker-Borodin reaction or the Borodin reaction. This is an example of both a halogenation reaction and a decarboxylation reaction. The various manufacturing processes include the conversion of alcohols to alkyl halides, the addition of halogens to alkenes, and the hydrogen halide of alkenes. Manufacturing technology was so reliable and efficient that it became an inevitable part of industrial chemistry.