Haloalkanes: Classification and Nomenclature

• Haloalkane (alkyl halide) definition: a chemical compound derived from an alkane by substituting one or more hydrogen atoms with halogen atoms, such as fluorine, chlorine, bromine, or iodine
• Haloalkanes see widespread use because of their valuable chemical properties
• General formula: RX, where R is an alkyl group and X is a halogen atom
• Haloalkanes are classified by the carbon atom to which the halogen is attached

Classification of Haloalkanes

• Primary (1°) haloalkanes: The halogen atom attaches to a primary carbon atom, which bonds to only one other carbon atom
• Secondary (2°) haloalkanes: The halogen atom attaches to a secondary carbon atom, bonding to two other carbon atoms
• Tertiary (3°) haloalkanes: The halogen atom attaches a tertiary carbon atom, bonding to three other carbon atoms

Nomenclature

• IUPAC nomenclature is used
• Steps:
  • Identify the parent alkane chain
  • Number the carbon atoms in the parent chain, giving the carbon atom bonded to the halogen the lowest possible number
  • Name the halogen substituent with the prefix fluoro-, chloro-, bromo-, or iodo-
  • List the substituents in alphabetical order with their corresponding carbon numbers

Physical Properties

• Boiling Points: Haloalkanes generally have higher boiling points than their corresponding alkanes because of greater molecular weight and increased van der Waals forces
• Boiling points increase with increasing molecular weight and size of the halogen atom
• Solubility: Haloalkanes are generally insoluble in water but soluble in organic solvents
• Density increases with the number and atomic mass of the halogen atoms

Preparation of Haloalkanes

• Halogenation of Alkanes: Alkanes react with halogens in the presence of ultraviolet light or heat to form haloalkanes via a free radical mechanism
• Addition of Hydrogen Halides to Alkenes: Alkenes react with hydrogen halides (HCl, HBr, HI) to form haloalkanes, following Markovnikov's rule, where the hydrogen atom attaches to the carbon with more hydrogen atoms, and the halogen attaches to the carbon with fewer hydrogen atoms
• From Alcohols: Alcohols react with hydrogen halides (HX), phosphorus halides (PX3, PX5), or thionyl chloride (SOCl2)
• Halide Exchange (Finkelstein Reaction): Haloalkanes react with metal halides to exchange one halogen for another; it is particularly useful for preparing iodoalkanes

Chemical Reactions

• Nucleophilic Substitution Reactions: Haloalkanes undergo nucleophilic substitution reactions, where a nucleophile replaces the halogen atom
  • SN1 reactions: Unimolecular nucleophilic substitution reactions occur in two steps, involving the formation of a carbocation intermediate; they are favored by tertiary haloalkanes in polar protic solvents
  • SN2 reactions: Bimolecular nucleophilic substitution reactions occur in one step, where the nucleophile attacks the carbon atom bonded to the halogen from the backside; they are favored by primary haloalkanes in polar aprotic solvents
• Elimination Reactions: Haloalkanes undergo elimination reactions (dehydrohalogenation) in the presence of a strong base, resulting in the formation of an alkene
  • Zaitsev's rule says that in elimination reactions, the major product is the more substituted alkene
• Reaction with Metals: Haloalkanes react with certain metals like magnesium and lithium to form Grignard reagents and organolithium compounds, respectively; these are important reagents in organic synthesis
• Reduction: Haloalkanes can be reduced to alkanes using reducing agents like metal hydrides (e.g., LiAlH4 or NaBH4)

Uses of Haloalkanes

• Solvents: Used as solvents for nonpolar substances
• Refrigerants: Certain haloalkanes (e.g., freons) were used as refrigerants, but their use has been phased out due to their ozone-depleting properties
• Anesthetics: Some haloalkanes (e.g., halothane) have been used as general anesthetics
• Synthesis of Organic Compounds: Haloalkanes are versatile intermediates in organic synthesis, used for the preparation of various organic compounds

Environmental Concerns

• Ozone Depletion: Chlorofluorocarbons (CFCs) were widely used as refrigerants and aerosol propellants but are now known to deplete the ozone layer
• Global Warming: Some haloalkanes are potent greenhouse gases and contribute to global warming
• Toxicity: Some haloalkanes are toxic and can pose health hazards

Stability

• The stability of haloalkanes depends on the halogen atom and the structure of the alkyl group
• The bond dissociation energy decreases in the order of: C-F > C-Cl > C-Br > C-I
• Tertiary haloalkanes are more prone to elimination reactions due to the stability of the resulting alkene

Grignard Reagents

• Grignard reagents are organomagnesium halides with the formula RMgX, where R is an alkyl or aryl group and X is a halogen
• They are prepared by reacting a haloalkane with magnesium metal in anhydrous ether solvent
• These reagents are highly reactive and are used to form carbon-carbon bonds by reacting with carbonyl compounds like aldehydes and ketones

Organolithium Reagents

• Organolithium reagents have the formula RLi, where R is an alkyl or aryl group
• Prepared by reacting a haloalkane with lithium metal in an aprotic solvent
• They are more reactive than Grignard reagents and are also used to form carbon-carbon bonds

Elimination Reactions (E1 and E2)

• E1 reactions: Unimolecular elimination reactions occur in two steps, involving the formation of a carbocation intermediate; they are favored by tertiary haloalkanes in polar protic solvents
• E2 reactions: Bimolecular elimination reactions occur in one step, where the base removes a proton and the leaving group departs simultaneously; they are favored by strong bases and polar aprotic solvents