Haloalkanes

1.0What are Haloalkanes? 

Haloalkanes, also known as alkyl halides, are compounds derived from alkanes by replacing one or more hydrogen atoms with halogen atoms. They have a general formula of CₙH₂ₙ₊₁X, where X represents fluorine (F), chlorine (Cl), bromine (Br), or iodine (I).

When a hydrogen atom(s) in an aliphatic hydrocarbon is substituted by a halogen atom(s), it leads to the creation of alkyl halides (also known as haloalkanes). Alkyl halides feature halogen atom(s) bonded to the sp³ hybridized carbon atom of an alkyl group. 

2.0Haloalkanes Classification  

Based on the Number of Halogen Atoms

• Based on the number of halogen atoms, haloalkanes can be mono-, di-, or polyhalogen compounds.        

• They're categorized as primary, secondary, or tertiary depending on the carbon directly bonded to the halogen: one carbon for primary, two for secondary, and three for tertiary.

3.0Nomenclature of Haloalkanes

IUPAC and Common Nomenclature

When only one hydrogen atom is replaced, forming an alkyl halide (or haloalkane), the naming convention closely mirrors that of alkanes. Common names for alkyl halides comprise two elements: the name of the alkyl group alongside the stem of the halogen's name, suffixed with -ide.

• Common Name Format: Alkyl Name + Halide Name

Alkyl halides are named by combining the alkyl group name with the halide name, using prefixes like fluoro-, chloro-, bromo-, or iodo- to indicate the halogen and a number to specify its position. For instance, CH₃CH₂Cl is ethyl chloride in common terms and chloroethane in the IUPAC system.

• IUPAC Name Format: Locator # + Halo Prefix + Parent Alkane 

4.0Nature of C-X Bond

Halogen atoms exhibit higher electronegativity than carbon, resulting in the polarization of the carbon-halogen bond in alkyl halides. Consequently, the carbon atom carries a partial positive charge, while the halogen atom carries a partial negative charge.

Moreover, as we move down the group in the periodic table, the size of the halogen atom increases. Fluorine is the smallest, while iodine is the largest. Consequently, the carbon-halogen bond length also increases from C-F to C-I.

5.0Methods of Preparation of Haloalkanes

Haloalkanes and haloarenes are manufactured through three main methods:

Alcohol conversion

When alcohols react with concentrated halogen acids, phosphorus halides, or thionyl chloride, the hydroxyl group is substituted by a halogen atom. Thionyl chloride is favoured because it generates alkyl halides and SO₂ and HCl gases. Since these gases can escape easily, the reaction produces pure alkyl halides. ZnCl₂   catalyst is necessary for reacting primary and secondary alcohols with HCl.

Hydrocarbon Processing

From alkanes by free radical halogenation

When alkanes undergo free radical halogenation, they produce a complex mixture of isomeric mono- and poly haloalkanes. Separating these into pure compounds is challenging, leading to very low yields of any single compound.

From alkenes

Addition of hydrogen halides:

Hydrogen halides can convert an alkene into the corresponding alkyl halide. This transformation is accomplished by reacting with hydrogen chloride, hydrogen bromide, or hydrogen iodide. 

In the case of propene, two products are formed, but one predominates according to Markovnikov's rule.

Addition of Halogens:

When bromine dissolved in carbon tetrachloride is added to an alkene in the lab, the bromine solution changes to reddish-brown, vital to detecting double bonds in molecules. This reaction produces colorless vic-dibromides.

Halogen Exchange Reaction

Alkyl iodides are commonly produced by reacting alkyl chlorides/bromides with NaI in dry acetone, known as the Finkelstein reaction. The resulting NaCl or NaBr precipitates in dry acetone, facilitating the forward reaction as per Le Chatelier's Principle.       

For synthesizing alkyl fluorides, the preferred method involves heating an alkyl chloride/bromide in the presence of a metallic fluoride such as AgF, Hg₂F₂, CoF₂, or SbF₃. This process is referred to as the Swarts reaction.

6.0Physical Properties of Haloalkanes 

Alkyl halides, especially bromides and iodides, are usually colourless but can develop colour upon light exposure. They often have a sweet smell. Physical properties include varied states (gas, liquid, solid), boiling points increasing with molecular weight, melting points influenced by structure and intermolecular forces, insolubility in water but solubility in organic solvents, and typically denser than water.

7.0Chemical Reactions 

The reactions of haloalkanes can be categorized as follows:

Nucleophilic Substitution

• Nucleophiles attack electron-deficient areas of substrate molecules. In nucleophilic substitution reactions, a nucleophile replaces an existing group in a molecule, often with haloalkanes as substrates.
• These reactions happen when the carbon bonded to the halogen in the haloalkane has a partial positive charge. The nucleophile starts the reaction, causing the departure of the halogen atom, known as the leaving group, as a halide ion.

Nucleophilic substitution is a crucial organic reaction for alkyl halides, involving halogens bonded to sp³ hybridized carbon atoms.

This reaction has been found to proceed by two different mechanisms given below:

Substitution nucleophilic bimolecular (SN₂)

In SN₂ reactions, bulky substituents near the leaving group hinder the reaction. Methyl halides react fastest due to fewer hindrances. Tertiary halides are the least reactive because bulky groups obstruct nucleophiles.

•  Reactivity order: Primary > Secondary > Tertiary halide.

Substitution nucleophilic unimolecular (SN₁)

SN₁ reactions happen in polar protic solvents like water, alcohol, and acetic acid. They occur in two steps: 

• Slow cleavage of the C—Br bond forms a carbocation and bromide ion, then 
•  a nucleophile attacks the carbocation to complete the substitution.

Elimination Reactions 

Heating a haloalkane with a β-hydrogen atom in an alcoholic potassium hydroxide solution eliminates a hydrogen atom from the β-carbon and a halogen atom from the α-carbon, forming an alkene. This process, involving the removal of a β-hydrogen atom, is commonly referred to as β-elimination.

In dehydrohalogenation reactions, the Zaitsev rule states that the preferred product is the alkene with more alkyl groups attached to the doubly bonded carbon atoms.

Reaction with Metals

Organic chlorides, bromides, and iodides can react with certain metals to form compounds containing carbon-metal bonds, known as organo-metallic compounds. A significant type, discovered by Victor Grignard in 1900, is alkyl magnesium halide, RMgX, commonly called Grignard reagents. These reagents are obtained by reacting haloalkanes with magnesium metal in dry ether.

• When reacted with sodium in dry ether, alkyl halides undergo the Wurtz reaction, producing hydrocarbons with double the number of carbon atoms in the halide.

8.0Polyhalogen Compounds 

Polyhalogen compounds refer to carbon compounds containing more than one halogen atom. These chemicals play essential roles in industry and agriculture. Their utilization and environmental impacts are examined in detail.

• Dichloromethane (Methylene chloride)CH₂Cl₂

Methylene chloride is toxic to the human central nervous system, causing minor hearing and visual loss at low concentrations and dizziness, nausea, tingling, and numbness at high doses. It also induces intense stinging and minor reddening upon skin contact and may cause corneal burns if it contacts the eyes. In animal studies, exposure to methylene chloride vapours led to corneal injury.

• Dichloromethane is used as a paint remover, metal cleaner, extraction solvent in pharmaceuticals and food manufacturing, propellant in aerosols, and as a dewaxing agent and refrigerant.

• Chloroform (Trichloromethane) CHCl₃

Chloroform, with the chemical formula CHCl₃, is also known as trichloromethane in IUPAC nomenclature. The molecule is formed by overlapping a carbon atom's hybrid sp³ orbital with the 1^s orbital of a hydrogen atom and three carbon atoms' hybrid sp³ orbitals with the partially filled p-orbitals of three chlorine atoms. The structure of CHCl₃ is tetrahedral.

Uses of Chloroform:

• Chloroform is a solvent used in various industries, particularly as a chloroform (though alternatives are more common due to toxicity), a laboratory reagent in chemical synthesis, pharmaceuticals, and refrigerants like R-22.

Environmental Effects of Chloroform:

• Inhaling chloroform vapours can cause dizziness, fatigue, and headaches, even at low concentrations. Long-term exposure harms the liver and kidneys as chloroform converts to phosgene. Direct contact irritates the chloroform of the skin. When exposed to light, chloroform can produce toxic phosgene gas, which poses severe health risks.

• Iodoform (Tri-iodomethane) CHI₃

Iodoform, with the chemical formula CHI₃ and the IUPAC name triiodomethane, forms through the interaction of one sp³ hybrid orbital of the carbon atom with the 1s orbital of hydrogen. Meanwhile, the other three sp³ hybrid orbitals of the carbon atom overlap with the partly filled p-orbitals of iodine during iodoform's production.

Properties of Iodoform:

Iodoform appears as a light yellow crystalline solid with a distinctive odor and a melting point of 392 K. It is insoluble in water but readily dissolves in ethyl alcohol and ether. Its antibacterial effect stems from the release of free iodine.

Uses of Iodoform:

While the iodine released from iodoform is employed as an antiseptic, the compound itself has fallen out of favor due to its strong odor. However, it is still used in pharmaceutical synthesis.

• Carbon Tetrachloride (Tetrachloromethane) CCl4

Carbon tetrachloride is a colourless, oily liquid with a pungent odour. It is insoluble in water but soluble in organic solvents such as ether and alcohol. Its boiling point is 350 K, and it is combustible.

• Carbon tetrachloride is used in manufacturing aerosol can refrigerants, propellants, chlorofluorocarbons (CFCs), and as a solvent in medication production. It was historically used in dry cleaning and as a fire extinguishing agent. However, exposure can cause liver cancer, dizziness, nausea, nerve damage, coma, or death. It can also lead to erratic heartbeats and eye irritation. 
• Environmentally, it depletes the ozone layer, increasing UV radiation exposure, raising risks of skin cancer, eye disorders, and weakened immune systems.

Freons 

• Freons, the chlorofluorocarbon compounds of methane and ethane, are stable, non-reactive, non-toxic, non-corrosive gases. Freon 12 (CCl₂F₂) is a common industrial freon, produced from tetrachloromethane via the Swarts reaction. 
• Used in aerosol propellants, refrigeration, and air conditioning, global freon production reached 2 billion pounds annually by 1974. Despite their industrial uses, freons eventually enter the atmosphere, where they can disrupt the ozone balance in the stratosphere through radical chain reactions.

p,p’-Dichlorodiphenyltrichloroethane(DDT)

• DDT, the first chlorinated organic insecticide, was discovered by Paul Muller of Geigy Pharmaceuticals in 1939. Its effectiveness against malaria-spreading mosquitoes and typhus-carrying lice led to widespread global use after World War II. However, by the late 1940s, resistance in insects and toxicity to fish became evident.

• DDT's chemical stability and fat solubility exacerbated the issue, as it accumulates in fatty tissues and persists in the environment. Consequently, the United States banned its use in 1973, although it remains in use in some regions worldwide.