Alkyl Halide

Chemical compounds that are mostly derived from alkanes containing one or more halogens are alkyl halides, also called haloalkanes. We may also assume that alkyl halides are a subset of the halocarbon general class.

Alkyl halides or haloalkanes are formed by substituting halogen atoms for hydrogen atoms in aliphatic hydrocarbons (Fluorine, chlorine, bromine, or iodine). Any organic precursors such as alkanes, alkenes, or alcohols and carboxylic acids may also be derived from them. Alkyl halides typically contain hydrogen atoms bound to the alkyl groups of sp3 hybridized carbon atoms. Generally, the alkyl halide formula is R-X.

Alkyl Halide Definition

When one or more hydrogen atoms in an alkane are replaced by halogen atoms in a compound, it is known as an alkyl halide. Halogen atoms include chlorine, fluorine, iodine, or bromine. Alkyl halides can also be referred to as haloalkanes. The classification of alkyl halides can be done on the basis of the different classes they fall into depending on the positioning of the halogen atom on the chain of carbon atoms. CH₃CH₂I which is also known as iodoethane, is an example of alkyl halide.

Primary Alkyl Halides

When the carbon atom is only attached to one other alkyl group, which is bonded to the halogen atom, it is known as primary haloalkane (1° haloalkane). The complicated structure of the attached alkyl group does not matter in the case of primary alkyl halides since there is only one linkage to an alkyl group from the carbon group which holds the halogen. Examples of primary alkyl halide include CH₃CH₂Br.

Secondary Alkyl Halides

When the carbon atom is attached to two other alkyl groups which can either be the same or different, which are bonded to the halogen atom, it is known as secondary haloalkane (2° haloalkane). Examples of secondary alkyl halide include CH₃CHBrCH₃.

Tertiary Alkyl Halides

When the carbon atom is attached to three other alkyl groups which may include any combination of the same or different, which is bonded to the halogen atom, it is known as tertiary haloalkane (3° haloalkane). Examples of tertiary alkyl halides include CH₃CH₃CBrCH₃.

Allylic Halides 

The halogen is bonded to sp3 carbon atoms adjacent to a carbon-carbon double bond.

Vinylic Halides 

The halogen is attached to an sp2 hybridized carbon atom of a carbon-carbon double atom.

Nature of C-X Bond

Carbon halogen bond is polarized in nature as halogen atoms are electronegative due to which carbon attains a partial positive charge and halogen attains a partial negative charge.

Physical Properties of Alkyl Halide

In their pure form, alkyl halides are colorless in nature. On exposure to light, however, bromides and iodides produce color. The decomposition of halogens in the presence of light is the explanation for the production of color. Many of the volatile-nature halogen compounds have a sweet smell.

• Odor- Alkyl Halides, in the pure state, have a pleasant odor while all the higher alkyl halides do not have any odor.

• Color- Alkyl Halides, in its pure state, is colorless. The exception is Iodoalkanes and bromoalkanes when exposed to light after storing for a long period develop color.

• Melting Point- The melting point is dependent on the strength of a compound's lattice structure. There are almost identical boiling points in isomeric di halobenzene, but the difference can be seen in the melting points. Therefore, in the crystal lattice, greater numbers of molecules are compactly packed. Therefore, greater energy is needed to split the lattice structure, thereby increasing the temperature of the compound's melting point.

• Boiling Points- Alkyl Halides possess polarity and strong dipole-dipole attraction between their molecules and have a greater magnitude of the van der Waals forces which makes their boiling points high.

• The boiling points of the same alkyl group of the haloalkanes are in the order RCl<RBr<RI. It is so because as there is an increase in the size of the halogen atom, the magnitude of the van der Waals forces increases.

• The boiling points of isomeric alkyl halides decrease as there is no increase in the branching in the alkyl group. It is so because a spherical shape having a less surface area is attained as there is a branching of the molecule because of which the interparticle forces get weak. The order is 1°>2°>3°.

• If there are the same halogens then the boiling point depends on the molecular mass and increases as the molecular mass increases as an increase in the size of the alkyl group, the magnitude of van der Waals forces increases. The order is R-X<R-CH₂-X<R-CH₂-CH₂-X

• If there is an increase in the number of halogen atoms, there is an increase in the boiling point of the compound as well since the van der Waal force of attraction increases.

• Density- Density is directly proportional to any compound's mass. Therefore, the density increases as the mass grows down the homologous sequence. Thus, fluorine derivatives are less dense than chlorine derivatives and chlorine derivatives are less dense than bromine derivatives. Alkyl bromides and alkyl iodides are heavier than water while alkyl chlorides are lighter. The order is RI>RBr>RCl.

• Solubility- In water, alkyl halides are slightly soluble. For the dissolution of a compound and the breaking of the attractive forces between the halogen and the carbon atom, a comparatively greater amount of energy is needed. The nature of alkyl halides is polar because of which they do not form hydrogen bonds with water molecules and therefore they are sparingly soluble in water. The solvents they are soluble in are organic solvents such as ethers, alcohols, and benzene. 

Preparation of Alkyl Halides

There are 4 different types of preparation techniques for Haloalkanes. They include Preparation of Haloalkanes from:

1. Alcohols-  Replace the hydroxyl group of alcohol with the halogen atom which is attached to the other compound involved. In this reaction, a catalyst is required but only for primary and secondary alcohols.

2. Alkenes by addition of hydrogen halides and halogens- When hydrogen halides are added to alkenes, the Makyknokov’s rule or exhibit Kharash rule is followed. The reactions following Markovnikov’s rule in which all the electrophilic addition reactions are followed are called Markovnikov addition reactions.

3. Halogen exchange reaction- The free radical bromination and chlorination of alkanes give a complex mixture of isomeric mono- and poly haloalkanes.

4. Hydrocarbons

From Alcohols

As the main element, an organic compound derivative of the alcohol reacts with halogen acid (H-X) to form haloalkanes.

R-OH+H-X → R-X+H2O

Alcohol Reaction with Halides of Phosphorus (PX₅ or PX₃) 

In the formation of chloroalkanes, Bromo alkanes, and Iodo alkanes, this reaction helps. Phosphorus halides exchange the functional alcohol group (-OH) with the corresponding halides in this reaction. The reaction takes place as follows:

ROH + PCl₅ → RCl + POCL₃ + HCl

Alcohol Reaction with Thionyl Chloride as the Required Reagent 

Among the three alcohol reactions, this reagent is the most favored and appropriate one. To form alkyl chlorides, alcohol reacts with Thionyl chloride (SOCl2). The by-products produced in this reaction, however, are of a gaseous nature. The by-products can also easily escape into the atmosphere, leaving the alkyl halide pure. This technique assists in the processing of pure alkyl halide.

R-OH+SOCl2 → R-Cl+SO2+HCl

1. Preparation of Haloalkanes from Hydrocarbons

Free radical halogenation - The free radical halogenation reaction makes alkyl bromides and alkyl chloride formation possible. Radicals are, however, very non-selective in nature. In addition, radicals are non-specific and highly reactive intermediates that lead to product mixture formation. 

For example, free radical bromination or chlorination results in the formation of a number of haloalkanes. The isolation of a single product creates difficulties. It is therefore not the preferred approach for haloalkane preparation.

2. Preparation of Alkyl Halide from alkenes 

The preparation of haloalkanes is possible by the addition of halogens (X2) across the alkene double bond. It is also possible to add halides of hydrogen to (HX). Chlorine, bromine, or even iodine may be found in this halogen. 

a) Addition of HX 

Alkene can be converted into an electrophilic addition reaction to haloalkane. To form R-X, Alkene reacts with HX. The order of halide reactivity with regard to alkenes follows the order of HI > HBr > HCl > HF. 

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b) Anti- Markovnikov Addition

There is another possibility where the reaction contradicts Markovnikov’s rule. This effect is known as the Peroxide effect/ Kharash effect/ Anti-Markovnikov’s rule. In this reaction, alkene in the presence of peroxide reacts with HBr. The Br- or negative component of the reagent is added to the carbon with more hydrogen atoms. For instance, in the presence of peroxide, Prop-1-ene reacts with hydrogen bromide to form 1-bromopropane as a major product.

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3. Haloalkanes and Haloarenes from Halogen Exchange Reaction

a. Finkelstein Reaction

The last form of haloalkane preparation is the halogen exchange reaction. Alkyl chloride or alkyl bromide reacts with sodium iodide in acetone to form alkyl iodides in this reaction.

R-X+NaI → R-I+NaX

b. Swartz Reaction

The formation of alkyl fluorides in this reaction is possible by heating RBr/RCl alkyl fluorides. In the presence of metallic fluorides, such as SbF₃, Hg₂F₂, AgF, CoF₂, the reaction is carried out.

R-Br+AgF → R-F+AgBr

Chemical Properties of Alkyl Halide

The chemical reaction of haloalkanes is divided into three categories:

1. Nucleophilic substitution reaction

2. Elimination reaction

3. Reaction with metals

1. Nucleophilic Substitution Reaction

A nucleophile interacts with haloalkane in this form of reaction, which has a partial positive charge on the carbon atom that is bound to the halogen. A replacement response takes place and leaves a halogen atom (called the leaving group is a halide ion). Since a nucleophile initiates the substitution reaction, it is called the nucleophilic substitution reaction.

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2. Elimination Reaction

If a haloalkane with a hydrogen atom is heated with potassium hydroxide alcoholic solution, it can lead to the removal of the hydrogen atom from the β-carbon atom and the halogen atom from the alpha-carbon atom.  As the β-hydrogen atom is involved in elimination, it is also referred to as the β-removal reaction. Alkene is formed as one of the products.

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3. Reaction with Metals

In order to provide compounds containing carbon-metal bonds, most organic chlorides, bromides and iodides react with certain metals. These compounds are classified as compounds that are organometallic. A substance formed by the dry ether reaction of haloalkanes to magnesium metal. 

In the meantime, Grignard reagents appear to react actively and can respond to any proton source that leads to hydrocarbon formation. Therefore, avoiding Grignard reagents is important. Otherwise, this would be known as one of the hydrocarbon modifications.

Uses of Alkyl Halide

• They are used as solvents for relatively non-polar compounds and as starting materials for the synthesis of a wide range of organic compounds. 

• Chlorine-containing antibiotic, chloramphenicol, produced by soil microorganisms is very effective for the treatment of typhoid fever. 

• In surgery, some entirely fluorinated compounds are known as possible blood substitutes. 

• They are used in organic synthesis as synthon-equivalents. 

• Used as refrigerants and propellants earlier. 

• They are also used in extinguishers for burning.

Conclusion

Alkyl halides are also known as halogen halides which are represented by R-X. They are divided into primary, secondary, and tertiary halides. The bond between carbon and halogen is polar in nature. Alkyl halides are prepared from alcohol, alkene, and hydrocarbons. Alkyl halide undergoes various chemical reactions such as nucleophilic substitution reaction, elimination reaction, and also the reaction with metals.