Alkyl Halide

Alkyl Halide Definition - 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 Structures

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Primary, Secondary and Tertiary Alkyl Halide 

1.  If the halogen atom attached to a carbon atom which is further attached to one carbon atom is said to be primary alkyl halide. Example-C2H5Cl

2. If the halogen atom attached to a carbon atom which is further attached to two carbon atoms is said to be a secondary alkyl halide.

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3. If the halogen atom attached to a carbon atom which is further attached to three carbon atoms is said to be a tertiary alkyl halide.

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Allylic Halides 

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

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Vinylic Halides 

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

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Nature of C-X Bond

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

Physical Properties of Alkyl Halide 

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

1. Melting Point

The melting point is dependent on the strength of a compound's lattice structure. There are almost identical boiling points in isomeric dihalobenzene, 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.

2. Boiling Point

The stronger intermolecular attraction forces are due to the dipole-dipole and van der Waals interaction. On the intermolecular forces of attraction, the boiling point of haloalkanes depends. The boiling points of chloride, bromide, and iodide derivatives are also comparatively higher for hydrocarbons of equal molecular mass. As we pass down the group in the homologous chain, the size and molecular mass of halogen members increase, thus creating stronger attraction forces. As we pass down the group in the homologous series, the boiling point therefore increases. 

The boiling point order for alkyl halides is RI > RBr > RCl > RF

3. Density

Density is directly proportional to any compound's mass. Therefore, the density increases as the mass grow down the homologous sequence. Thus, fluorine derivatives are less dense than chlorine derivatives and chlorine derivatives are less dense than bromine derivatives.

4. Solubility

In water, alkyl halides are slightly soluble. Since they are polar compounds, haloalkanes are immiscible with water. 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.

Preparation of Alkyl Halides

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

1. Alcohols

2. Hydrocarbons

3. Alkenes by addition of hydrogen halides and halogens

4. Halogen exchange reaction.

1. 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 favoured 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

2. 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.

3. 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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4. 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

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

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

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

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

5. Used as refrigerants and propellants earlier. 

6. 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.