Alkyl Halides are compounds where a halogen (fluorine, chlorine, bromine, or iodine) is attached to a carbon chain. Another name of Alkyl Halide is Haloalkanes, Halogenoalkanes. They play essential roles in the pharmaceuticals, plastics, and agriculture industries, contributing to various applications like medicine synthesis, plastic production, and pesticide formulation.

In this article, we will learn about Alkyl Halides, the Classification of Alkyl Halides, Uses, and others in detail.

What Are Alkyl Halides?

Alkyl halides are a type of organic compound composed of carbon, hydrogen, and halogen atoms. they are a subset of a general class of hydrocarbons. They consist of alkyl groups, which are hydrocarbon chains, bonded to one or more halogen atoms (fluorine, chlorine, bromine, or iodine). The general structure of alkyl halides involves a carbon atom from the alkyl group directly linked to a halogen atom.

The image added below shows some alkyl halides.

Alkyl Halide

These compounds play a significant role in organic chemistry and various industrial processes. The classification of alkyl halides is often based on factors such as the number of halogen atoms present or the position of the halogen atom along the carbon chain. Alkyl halides exhibit diverse chemical properties and reactivities, making them important in the synthesis of various organic compounds.

Learn more about, Haloalkanes and Haloarenes

Alkyl Halide Formula

As Alkyl halides are monohalogen derivatives of alkanes the general formula for alkyl halide is

C_nH_2n+1X or R-X

where,

• R is Alkyl Group
• X is Halogen Atom

Alkyl Halide Functional Group

The functional group in Alkyl Halide is Haloges. (Chlorine, Bromine, Iodine)

Classification of Alkyl Halide

Alkyl halides are categorized as primary, secondary, or tertiary based on the number of carbon atoms directly bonded to the halogen. This classification reflects the degree of substitution and reactivity of the alkyl halide in various chemical reactions.

Based on Number of Halogen Atoms

Alkyl halides can be classified based on the number of halogen atoms present in the molecule. On this basis the halogen atoms are,

• Mono-Halogenated Alkyl Halides (Containing one Halogen Atom)
• Poly-Halogenated Alkyl Halides (Containing More than One Halogen Atom)

Based on Position of Halogen Atom along the Chain of Carbon Atom

Based on position of halogen atom along the chain of carbon atom Alkyl halides are of three types that are,

• Primary Alkyl Halide
• Secondary Alkyl Halide
• Tertiary Alkyl Halide

Primary Alkyl Halide

In a primary alkyl halide, the carbon atom bonded to the halogen is connected to only one other alkyl group. It doesn’t matter if this alkyl group is small or bulky. An example of a primary alkyl halide is Chloromethane (CH₃Cl), where the carbon bonded to chlorine is attached to only one methyl group. A primary alkyl halide would prefer to undergo SN2 reactions.

Secondary Alkyl Halide

For secondary alkyl halides, the carbon atom linked to the halogen is directly attached to two other alkyl groups. These two alkyl groups can either be the same or different. An example is 2-bromopropane (CH₃CHBrCH₃), where the carbon bonded to bromine is connected to two methyl groups.

Tertiary Alkyl Halide

Tertiary alkyl halides have the carbon carrying the halogen bonded to three alkyl groups. Again, these alkyl groups may be the same or different. An illustration is 2-chloro-2-methylpropane (CH₃C(CH₃)₂Cl), where the carbon bonded to chlorine is attached to three methyl groups.

Properties of Alkyl Halide

Various properties of alkyl halide are,

• Alkyl halides are colorless in their pure form, but bromides and iodides can develop color when exposed to light.
• Alkyl Halide have a sweet smell.
• Methyl chloride, methyl bromide, ethyl chloride, and certain chlorofluoromethanes exist as gases at room temperature, while higher members are either liquids or solids.
• Boiling points of chlorides, bromides, and iodides are significantly higher than those of hydrocarbons with the same molecular mass.
• Boiling points decrease in the order RI > RBr > RCl > RF as the size and number of electrons increase.
• Bromo-derivatives, iodo-derivatives, and polychloro-derivatives of hydrocarbons are heavier than water.
• Haloalkanes are less soluble in water due to the need for energy to overcome attractions between the haloalkane molecule and break hydrogen bonds in water.
• Solubility in water is limited, but haloalkanes dissolve more readily in organic solvents due to a complex interaction between the haloalkanes and the solvent molecules.

Learn more about, Properties of Haloalkanes and Haloarenes

Nomenclature of Alkyl Halides

• Name Order: When naming alkyl halides, the name is typically written in the order of the alkyl group followed by the halogen.
• Locating the Carbon Chain: Identify the longest continuous chain of carbon atoms in the molecule. This chain gives the base name, and the carbon atoms are numbered.
• Numbering: Assign a number to each carbon in the chain, starting from the end nearest to the halogen. The goal is to give the halogen the lowest possible number.
• Prefix for Halogen: Add the prefix “fluoro,” “chloro,” “bromo,” or “iodo” to indicate the type of halogen present.
• Alphabetical Order: If there are multiple halogen substituents, list them in alphabetical order, using their prefixes.
• Isomers: Consider different structural isomers and select the one with the lowest numbering for the halogen.

Examples

• Chloromethane for CH₃Cl
• 1-Bromopropane for CH₃CH₂CH₂Br
• 2-Iodo-3-methylbutane for CH₃CH(CH₃)CH(CH₃)CH₂I

Chemical Reactions of Haloalkanes

Some reactions of Haloalkanes are,

Nucleophilic Substitution Reaction

In this reaction type, a nucleophile interacts with a haloalkane, where the carbon atom bonded to the halogen carries a partial positive charge. This leads to a substitution reaction, where the leaving group, the halogen atom, departs as a halide ion. This reaction, initiated by a nucleophile, is termed a nucleophilic substitution reaction. It stands out as one of the most practical organic reactions involving an alkyl halide with a halogen bonded to sp³ hybridized carbon.

For example: When a nucleophile reacts with methyl chloride, the chlorine atom (leaving group) is replaced by the nucleophile. The reaction can be represented as follows:

CH₃Cl + Nuchleophile → CH₃Nuchleophile + Cl^–

Elimination Reaction

When a haloalkane containing a hydrogen atom is heated with an alcoholic solution of potassium hydroxide, it undergoes an elimination reaction. This process involves the removal of a hydrogen atom from the β-carbon atom and a halogen atom from the α-carbon atom. Consequently, one of the products formed is an alkene. This type of reaction, where the elimination involves the β-hydrogen atom, is commonly referred to as a β-elimination reaction. In instances where multiple β-hydrogen atoms are present, typically one alkene is produced as the primary product.

CH₃X + KOH(alc) → C₂H₆ + KX + H₂O

Reaction with Metals

Many organic chlorides, bromides, and iodides can react with specific metals, resulting in compounds containing carbon-metal bonds. These compounds are referred to as organometallic compounds. The formation of these products occurs when haloalkanes react with magnesium metal in dry ether.

Some important name reactions containing Alkyl Halides are,

Wurtz Fittig Reaction

Wurtz Fittig reaction involves the coupling of two alkyl halides to form a carbon-carbon bond. This reaction is typically conducted in the presence of sodium or another alkali metal. The alkali metal serves as a mediator, facilitating the formation of the carbon-carbon bond between the alkyl groups.

For example, the reaction between two alkyl halides, bromomethane (CH3Br) and bromoethane (C2H5Br), in the presence of metallic sodium (Na). the sodium atoms facilitate the combination of the methyl and ethyl groups, forming neopentane. The reaction can be represented as follows:

2CH₃Br + 2C₂H₂Br + 2Na → (CH₃)₃CCH₂CH₃ + 2NaBr

Ulmanns Reaction

Ullmann’s reaction involves the coupling of aryl halides (halides attached to aromatic rings) to form biaryl compounds. This reaction is commonly catalyzed by copper, either in the form of copper powder or copper salts. It is a valuable tool in the synthesis of complex organic molecules containing aromatic rings.

Consider the reaction between bromobenzene (C₆H₅Br) and iodobenzene (C₆H₅I) catalyzed by copper. The reaction is represented as,

C₆H₅Br + C₆H₅I + 2Cu → C₆H₅-C₆H₅ + 2CuI

Hunsdeiker Reaction

The Hunsdiecker reaction involves the conversion of a silver salt of a carboxylic acid into the corresponding alkyl halide. This reaction is often used for the preparation of alkyl halides from carboxylic acids. It typically involves the use of a halogenating agent, such as bromine or chlorine.

consider the reaction of silver benzoate (C₆H₅COOAg) with bromine (Br₂). The reaction is represented as,

C₆H₅COOAg + Br₂ → C₆H₅Br + COOAgBr

Learn more about, Chemical Reactions of Haloalkanes and Haloarenes

Prepration of Alkyl Halides

• Alkyl halides, or halogenated hydrocarbons, can be synthesized through various methods. One common approach involves the reaction between hydrocarbons and halogenating agents.
• In this synthesis, a hydrocarbon, typically an alkane or alkene, reacts with a halogen, such as chlorine or bromine. The reaction leads to the substitution of hydrogen atoms with halogen atoms in the hydrocarbon molecule.
• For example, in the chlorination of methane, one hydrogen atom in methane is replaced by a chlorine atom, resulting in the formation of chloromethane:

CH₄ + Cl₂ → CH₃Cl + HCl

• Similarly, in the bromination of ethane, one hydrogen atom in ethane is substituted with a bromine atom, yielding bromoethane:

C₂H₆ + Br₂ → C₂H₅Br + HBr

• This type of synthesis is widely employed in the preparation of alkyl halides, which serve as essential intermediates in various organic reactions and the production of diverse chemical compounds.

Learn more about, Methods of Preparation of Haloalkanes and Haloarenes

Uses of Alkyl Halides

Alkyl Halides are useful in various way, some of them are listed below:

• Cleaning Processes: Alkyl halides, with their halogenated solvent properties, are widely utilized for effective cleaning in various industrial processes. Their ability to dissolve contaminants makes them valuable in tasks like degreasing and surface preparation.
• Medicinal Applications: Alkyl halides play a crucial role in the synthesis of pharmaceuticals. They are used to create specific chemical structures that are essential for the development of various medicines, contributing to advancements in healthcare.
• Plastics Production: In the realm of materials, alkyl halides are employed in the synthesis of plastics. Their incorporation helps in shaping and molding materials, contributing to the versatile range of plastic products in our daily lives.
• Refrigerants: Some alkyl halides function as refrigerants in cooling systems. They undergo phase transitions easily, allowing them to absorb and release heat efficiently, making them integral to the functioning of refrigeration and air conditioning systems.
• Dye Synthesis: Alkyl halides are utilized in the preparation of dyes. Their chemical properties enable the creation of vibrant and lasting colors in textiles and other materials, contributing to the textile and fashion industries.
• Pesticides: In agriculture, alkyl halides are incorporated into the production of pesticides. Their pesticidal properties assist in protecting crops from harmful insects and pests, contributing to enhanced agricultural yields.
• Perfume Synthesis: The fragrance industry benefits from alkyl halides, which are involved in the synthesis of perfumes. These compounds contribute to the creation of diverse and appealing scents.
• Synthetic Rubber: Alkyl halides are integral in the production of synthetic rubber. Their inclusion in polymerization processes contributes to the development of rubber with specific properties, used in various applications, including tire manufacturing.
• Fire Extinguishers: Certain alkyl halides find application in fire extinguishers. Their chemical properties make them effective in suppressing fires by disrupting the chemical chain reactions that sustain combustion.
• Electronic Components: Alkyl halides are used in the manufacturing of electronic components. Their role in specific chemical reactions contributes to the production of materials used in the electronics industry, supporting the creation of various devices and technologies.

Difference Between Alkyl, Allyl, and Vinyl Halides

The difference between Alkyl, Allylic and Vinylic Halides are added in the table below,

Markownikov and Anti Markownikov Rule

Two rules that are useful in reactions involving Alkyl Halides are,

• Markownikov Rule
• Anti Markownikov Rule

Markovnikov’s Rule

In certain chemical reactions, like the addition of hydrogen halides (HX) to unsymmetrical alkenes, Markovnikov’s rule dictates that the hydrogen atom tends to attach to the carbon atom with more existing hydrogen atoms, while the halogen attaches to the carbon with fewer hydrogen atoms. This rule is particularly relevant in electrophilic additions to alkenes and is based on the stability of the resulting carbocation intermediate.

Anti-Markovnikov Rule

Conversely, the Anti-Markovnikov rule is observed in specific reactions, notably in the hydroboration-oxidation of alkenes. In this context, the hydrogen adds to the carbon with fewer existing hydrogen atoms, and the other substituent (often a boron compound) adds to the carbon with more hydrogen atoms. This rule contrasts with Markovnikov’s principle and is associated with reactions that involve boron compounds.

Names of Alkyl Halide

Name of some common Alkyl Halides are added below in the article,

Common or Trivial Names of Alkyl Halide

Trivial Name of some common Alkyl Halides are,

• Methyl Chloride: CH₃Cl
• Methyl Bromide: CH₃Br
• Ethyl Chloride: CH₃CH₂Cl
• Propyl Bromide: CH₃CH₂CH₂Br
• Isopropyl Chloride: (CH₃)₂CHCl
• Butyl Fluoride: CH₃CH₂CH₂CH₂F
• Isobutyl Bromide: (CH₃)₂CHCH₂Br
• Sec-Butyl Iodide: CH₃CH(I)CH₂CH₃

IUPAC Names of Alkyl Halide

IUPAC Names of some common Alkyl Halides are,

• Chloromethane: CH₃Cl
• Bromomethane: CH₃Br
• Chloroethane: CH₃CH₂Cl
• Bromopropane: CH₃CH₂CH₂Br
• 1-Chloropropane: CH₃CH₂CH₂Cl
• Fluorobutane: CH₃CH₂CH₂CH₂F
• 1-Bromopropane: CH₃CH₂CH₂CH₂Br
• 1-Iodo-2-methylpropane: CH₃CHICH(CH₃)CH₃

Nature Of C-X Bond In Alkyl Halides

The nature of C-X bonds in Alkyl Halides is explained in the points added below,

• Covalent Bond: Carbon-halogen (C-X) bond in alkyl halides is primarily covalent, involving the sharing of electrons between carbon and the halogen atom.
• Polarity: C-X bond exhibits polarity, with the halogen being more electronegative than carbon. As a result, there is a partial negative charge on the halogen and a partial positive charge on carbon.
• Dipole Moment: Due to the polarity, alkyl halides possess a dipole moment. The dipole moment points from the carbon atom toward the halogen atom.
• Electron Density: The halogen withdraws electron density from the carbon atom, making the carbon partially electron-deficient. This is known as the inductive effect.
• Strength of C-X Bond: The strength of the C-X bond depends on the nature of the halogen. Generally, as we move down the halogen group (e.g., from fluorine to iodine), the bond strength decreases.
• Reactivity: The reactivity of alkyl halides in various chemical reactions is influenced by the nature of the C-X bond. For instance, the reactivity may increase with bond polarizability, leading to different reaction pathways.

Sample Questions on Alkyl Halides

Q1. Identify the alkyl halide in the given chemical structure:

CH₃— CH₂— CH₂— Br

Q2. Classify the following alkyl halide:

CH₃— CHCl— CH₂— CH₃

Q3. Predict the major product of the reaction:

CH₂ = CH₂ + HBr → ?

Alkyl Halides – FAQs

1. What are Alkyl Halides?

Alkyl halides are compounds where a halogen (fluorine, chlorine, bromine, or iodine) is bonded to a carbon atom.

2. What the examples of Alkyl Halides?

Examples of alkyl halides are,

• Methyl chloride (CH₃Cl)
• Ethyl bromide (C₂H₅Br)
• Isopropyl fluoride (CH₃CHFCH₃)
• Tert-butyl iodide (C(CH₃)₃I)

3. What is Markovnikov’s Rule, and how does it apply to Alkyl Halides?

Markovnikov’s rule is a principle that guides the addition of hydrogen halides to unsymmetrical alkenes. In the context of alkyl halides, it dictates that the hydrogen atom tends to attach to the carbon with more existing hydrogen atoms during such additions, impacting the resulting product’s structure and stability.

4. What are 2 Alkyl Halide examples?

The 2 Alkyl Halides are,

• 2-Chloropropane
• Fluorocyclopentane