Haloalkanes / Alkyl Halides

• Haloalkanes are also known as alkyl halides
• Haloalkanes are a class of organic compounds that contain a halogen atom bonded to an sp3 hybridized carbon atom
• The general formula for haloalkanes is RX, where R is an alkyl or substituted alkyl group and X is a halogen (F, Cl, Br, I)
• Haloalkanes are widely used as solvents, refrigerants, pharmaceuticals, and chemical intermediates

Nomenclature

• Naming haloalkanes involves identifying the parent alkane chain and naming the halogen substituents
• The halogen is named as a prefix: fluoro-, chloro-, bromo-, iodo-
• The position of the halogen on the chain is indicated by a number, chosen to give the lowest possible number for the substituents
• CH3CH2Cl is chloroethane
• CH3CHBrCH3 is 2-bromopropane

Physical Properties

• Haloalkanes are generally denser than their corresponding alkanes, because the halogens have a higher atomic mass
• Boiling points of haloalkanes increase along with molecular weight and halogen size
• The carbon-halogen bond is polar, leading to dipole-dipole interactions between haloalkane molecules, increasing the boiling point compared to alkanes of similar molecular weight
• Haloalkanes are generally insoluble in water, but soluble in organic solvents

Preparation of Haloalkanes

• Alkanes react with halogens (Cl2, Br2) in the presence of UV light or heat to form haloalkanes via a radical chain mechanism via radical halogenation of alkanes; this reaction isn't very selective, leading to a mixture of products
• Alkenes react with hydrogen halides (HCl, HBr, HI) to form haloalkanes via the addition of hydrogen halides to alkenes; this reaction follows Markovnikov's rule, where the hydrogen atom adds to the carbon with more hydrogen atoms already, and the halogen adds to the carbon with fewer hydrogen atoms
• Alcohols react with hydrogen halides (HCl, HBr, HI) to form haloalkanes, typically with a catalyst such as ZnCl2 present, via the reaction of alcohols with hydrogen halides
• Alcohols can be converted to haloalkanes using reagents like thionyl chloride (SOCl2) or phosphorus halides (PCl5, PBr3) via the reaction of alcohols with thionyl chloride or phosphorus halides; these reactions are generally more selective and give better yields than using hydrogen halides directly

Reactions of Haloalkanes

• Nucleophilic Substitution (SN1 and SN2)
• Undergo nucleophilic substitution reactions, where the halogen atom is replaced by a nucleophile
• SN1 (substitution nucleophilic unimolecular) and SN2 (substitution nucleophilic bimolecular) are the two main types of nucleophilic substitution reactions
• SN2 reactions are concerted, where bond breaking and bond forming occur simultaneously, are favored by strong nucleophiles and primary haloalkanes, and result in inversion of stereochemistry at the carbon center
• SN1 reactions occur in two steps involving ionization of the haloalkane to form a carbocation, followed by attack of the nucleophile on the carbocation, are favored by tertiary haloalkanes, weak nucleophiles, and polar protic solvents, and result in racemization at the carbon center
• Elimination Reactions (E1 and E2)
• Can also undergo elimination reactions, where a hydrogen atom and the halogen atom are removed from adjacent carbon atoms, forming an alkene
• E1 (elimination unimolecular) and E2 (elimination bimolecular) are the two main types of elimination reactions
• E2 reactions are concerted, favored by strong bases and heat, and follow Zaitsev's rule, where the major product is the more substituted alkene
• E1 reactions occur in two steps and are favored by weak bases, tertiary haloalkanes, and polar protic solvents, while also following Zaitsev's rule
• React with metals like magnesium (Mg) and lithium (Li) to form organometallic reagents, such as Grignard reagents (RMgX) and organolithium reagents (RLi)
• Reaction with metals creates reagents that are strong nucleophiles and bases and are widely used in organic synthesis
• Can be reduced to alkanes using reducing agents like metal hydrides (e.g., LiAlH4, NaBH4) or by catalytic hydrogenation via reduction

Factors Affecting SN1 and SN2 Reactions

• SN2 reactions are favored by primary haloalkanes, while SN1 reactions are favored by tertiary haloalkanes in terms of substrate structure
• Strong nucleophiles favor SN2 reactions, while weak nucleophiles favor SN1 reactions based on nucleophile strength
• Good leaving groups (e.g., I-, Br-, Cl-) favor both SN1 and SN2 reactions based on leaving group ability
• Polar aprotic solvents (e.g., acetone, DMSO) favor SN2 reactions, while polar protic solvents (e.g., water, alcohols) favor SN1 reactions based on solvent effects
• Higher temperatures generally favor elimination reactions (E1 and E2) over substitution reactions (SN1 and SN2) dependent on temperature

Common Haloalkanes

• Chloromethane (CH3Cl) is used as a refrigerant and in silicone production
• Chloroethane (CH3CH2Cl) is used as a topical anesthetic and in ethylcellulose production
• Dichloromethane (CH2Cl2) is a common solvent used in many chemical reactions and industrial processes
• Chloroform (CHCl3) was historically used as an anesthetic and is now primarily used as a solvent and reagent in chemical synthesis
• Carbon Tetrachloride (CCl4) was formerly used as a solvent and cleaning agent, now restricted because of its toxicity and ozone-depleting properties
• Bromoethane (CH3CH2Br) is used in organic synthesis
• Iodoethane (CH3CH2I) is used in organic synthesis