Alkyl Halides

Do you know what is Alkyl halide?

Alkyl halide meaning belongs to compounds derived from alkanes through the substitution of one or more hydrogen atoms with halogen atoms such as fluorine, chlorine, bromine, or iodine. They belong to the broader category of halocarbons.

examples of alkyl halides containing various halogen atoms like:

• Methyl Chloride (Chloromethane): CH₃Cl
• Ethyl Bromide (Bromoethane): C₂H₅Br

1.0Introduction

Definition of alkyl halide involves an organic compound where a halogen atom (like fluorine, chlorine, bromine, or iodine) replaces hydrogen atoms in an alkane, resulting in a carbon-halogen bond. These compounds are versatile and find use in many fields due to their reactivity and diverse applications.

The structure of alkyl halides involves the replacement of hydrogen atoms in an aliphatic hydrocarbon with halogen atoms. These compounds can be synthesized from various organic precursors, including alkanes, alkenes, alcohols, and carboxylic acids. Typically, alkyl halides consist of hydrogen atoms bonded to the sp³ hybridized carbon atoms within alkyl groups.

2.0Classification of Alkyl halide- 

Alkyl halides, also referred to as haloalkanes or halogenoalkanes, can be classified in several ways based on their structure, the type of halogen atom present, or their reactivity. Here's a breakdown of their classifications:

1. On the basis of number of halogen(X) atoms:

      Haloalkanes and haloarenes can be classified as follows:

2. On the basis of the Number of Halogen Atoms Attached:

Primary Alkyl Halides (1°)	Primary alkyl halides have a halogen atom directly bonded to a carbon atom that is connected to only one other carbon atom.
Secondary Alkyl Halides (2°)	These have a halogen atom attached to a carbon atom bonded to two other carbon atoms.
Tertiary Alkyl Halides(3°)	These compounds feature one halogen atom bonded to a carbon atom that is bonded to three other carbon atoms.

3. Allylic halides

             These are the compounds in which the halogen atom is bonded to an sp³ hybridised carbon atom next to carbon-carbon double bond.

4. Benzylic halides

These compounds have a halogen atom bonded to an sp³ hybridized carbon atom adjacent to an aromatic ring.

3.0Alkyl Halide Properties

In this section we will discuss about physical and chemical properties of Alkyl halide

Physical Properties of Alkyl Halide

• State at Room Temperature: Alkyl halides can exist as gases, liquids, or solids at room temperature depending on their molecular weight and structure. For instance, low-molecular-weight alkyl halides are often gases (e.g., chlorofluorocarbons), while higher-molecular-weight ones can be liquids or solids.
• Solubility: Their solubility in water varies depending on the size of the alkyl group and the type of halogen. 
• Small alkyl halides are more soluble in water due to their ability to form hydrogen bonds, whereas larger alkyl halides are less soluble.
• Boiling and Melting Points: Generally, alkyl halides have higher boiling and melting points compared to their corresponding alkanes due to dipole-dipole interactions and London dispersion forces caused by the polar covalent bonds between the halogen and carbon.

Chemical Properties of Alkyl Halide

Reactivity: Alkyl halides are often used as substrates in various organic reactions such as nucleophilic substitution, elimination, and in some cases, as intermediates in organic synthesis.

1. Nucleophilic Substitution: They are prone to nucleophilic substitution reactions due to the polarity of the carbon-halogen bond, wherein the halogen atom is replaced by a nucleophile. 

$\mathrm{R}-\mathrm{X}+\stackrel{\ominus }{\mathrm{N}}\mathrm{u}⟶\mathrm{R}-\mathrm{Nu}+\stackrel{\ominus }{\mathrm{X}}$

Example of NSR                       

${\mathrm{CH}}_3-{\mathrm{CH}}_2-\mathrm{Cl}\stackrel{\text{ aq. }\mathrm{NaOH}}{\to }{\mathrm{CH}}_3-{\mathrm{CH}}_2-\mathrm{OH}$

2. Elimination Reactions: Alkyl halides can undergo elimination reactions (like E₁ and E₂ mechanisms) to form alkenes or alkyne, particularly when treated with a strong base.

3. Reactions with Metal: One of the common reactions is the nucleophilic substitution reaction when haloalkanes react with certain metals, particularly metals like lithium, sodium, and magnesium. These reactions typically result in the formation of organometallic compounds.

For example, when a haloalkane (such as an alkyl bromide or chloride) reacts with magnesium in the presence of an ether solvent, it forms a Grignard reagent. Grignard reagents are organomagnesium compounds and are highly reactive intermediates used in various organic synthesis reactions.

The general reaction can be represented as:

$\mathrm{R}-\mathrm{X}+\mathrm{Mg}\stackrel{\text{ Dry ether }}{\to }\mathrm{R}-\mathrm{MgX}$

4.0Important Named Reactions

Some of the important named reactions of alkyl halide are given below:

1. Fittig Reaction- The Fittig reaction is a synthesis method that joins two aryl or vinyl halides using sodium or potassium metal to create biaryl compounds.      

2. Wurtz Fittig Reaction-  The Wurtz-Fittig reaction is a chemical synthesis method that involves combining an alkyl halide and an aryl halide using metallic sodium, resulting in a compound containing both alkyl and aryl groups.

5.0Methods of Preparation of Haloalkanes

From Hydrocarbons

• By halogenation of alkane - Halogenation of alkanes is a primary method to prepare haloalkanes. This reaction involves replacing one or more hydrogen atoms in an alkane molecule with halogen atoms such as chlorine, bromine, or iodine.

${\mathrm{CH}}_3-{\mathrm{CH}}_3+{\mathrm{X}}_2\stackrel{\mathbf{h}\mathbf{v}/\mathbf{\Delta }/\mathbf{UV}}{\to }{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{X}+\mathrm{HX}$

• By hydrohalogenation of alkene- This process involves adding a hydrogen halide (such as HCl, HBr, or HI) across the carbon-carbon double bond of an alkene, resulting in the formation of a haloalkane.

From Alcohol

Haloalkanes can be synthesized from alcohols through a substitution reaction known as nucleophilic substitution.                          

$\mathrm{R}-\mathrm{OH}+{\mathrm{PCl}}_3\to \mathrm{R}-\mathrm{Cl}+{\mathrm{POCl}}_3+\mathrm{HCl}$

$3\mathrm{R}-\mathrm{OH}+{\mathrm{PX}}_3\to 3\mathrm{R}-\mathrm{X}+{\mathrm{H}}_3{\mathrm{PO}}_3$                                                         

Halogen Exchange Reaction

• Finkelstein reaction (Preparation of alkyl iodide)

The Finkelstein reaction is a classic method used to prepare alkyl iodides from alkyl chlorides or bromides by exchanging the halogen atom. It involves the reaction of an alkyl chloride or bromide with a metal iodide, typically sodium iodide (NaI), in an appropriate solvent such as acetone or dimethyl sulfoxide (DMSO).

$\begin{array}{rl}& \mathrm{R}-\mathrm{X}+\mathrm{Nal}\underset{\text{ acetone }}{\overset{\text{ dry }}{\to }}\mathrm{R}-\mathrm{I}+\mathrm{NaX}\\ & \mathrm{X}=\mathrm{Cl},\mathrm{Br}\end{array}$

• Swarts reaction (Preparation of alkyl fluoride)

The Swarts reaction is a method used to prepare alkyl fluorides from alkyl chlorides or bromides by exchanging the halogen atom. It involves the reaction of an alkyl chloride or bromide with a metal fluoride, usually antimony trifluoride (SbF₃), in the presence of a Lewis acid catalyst like hydrogen fluoride (HF).

${\mathrm{CH}}_3-\mathrm{X}+\mathrm{AgF}\to {\mathrm{CH}}_3-\mathrm{F}+\mathrm{AgX}(\mathrm{X}=\mathrm{Cl},\mathrm{Br})$

6.0Application/Uses of Alkyl Halide

Alkyl halides, also known as haloalkanes or alkyl halogen compounds, find diverse applications across several fields due to their unique properties and reactivity. Here are some common uses:

• Organic Synthesis: Valuable starting materials for creating diverse organic compounds.
• Solvents: Chloroform, dichloromethane used as versatile solvents.
• Medicinal Chemistry: Essential in drug synthesis and as functional groups in pharmaceuticals.
• Pesticides and Herbicides: Components in the production of agricultural chemicals.
• Fluorinated Compounds: Used in specialty chemicals and materials due to unique properties.
• Plastics and Polymers: Contribute to making various materials with distinct properties.
• Refrigerants (historically): Previously used as refrigerants and aerosol propellants (e.g., CFCs).
• Lab Reagents: Serve as reagents in organic chemistry experiments and transformations.