Alkyl halides (also called haloalkanes) have the general formula R-X, where X is a halogen (F, Cl, Br, I). They are classified as primary (1°), secondary (2°), or tertiary (3°) according to the carbon bonded to the halogen. Nomenclature follows IUPAC rules: the halogen is named as a prefix (fluoro-, chloro-, bromo-, iodo-) with the longest carbon chain as the parent alkane. Common names use alkyl halide format (ethyl chloride, isopropyl bromide).
Physical Properties and Bond Strength
Boiling points of alkyl halides increase with molecular weight and with halogen size: RI > RBr > RCl > RF. Alkyl halides are generally denser than water, with polyhalogenated compounds (CHCl₃, CH₂Cl₂, CCl₄) being notably dense. The C-X bond strength decreases down the group: C-F (~485 kJ/mol) > C-Cl (~340 kJ/mol) > C-Br (~285 kJ/mol) > C-I (~215 kJ/mol). This bond weakening correlates with increasing leaving group ability, making alkyl iodides the most reactive in substitution reactions.
Nucleophilic Substitution
Alkyl halides undergo two mechanistic pathways for nucleophilic substitution. The SN2 mechanism is a concerted, one-step process involving backside attack by the nucleophile with inversion of configuration. It is favored by primary substrates, strong nucleophiles, and aprotic solvents. The SN1 mechanism proceeds in two steps via a carbocation intermediate, with the leaving group departing before the nucleophile attacks. SN1 reactions show racemization at the stereocenter and are favored by tertiary substrates, weak nucleophiles, and protic solvents. Key factors governing the competition between SN1 and SN2 include substrate structure, nucleophile strength and concentration, leaving group ability, and solvent polarity.
Elimination Reactions
Alkyl halides also undergo elimination to form alkenes. The E2 mechanism is a concerted, one-step process where a base abstracts a β-hydrogen as the leaving group departs, requiring antiperiplanar geometry. E2 follows Zaitsev’s rule (more substituted alkene is major product), unless a bulky base (KOtBu) is used to favor the less substituted Hofmann product. The E1 mechanism proceeds via carbocation formation followed by loss of a β-proton. E1 competes with SN1 under similar conditions (tertiary substrates, weak bases, protic solvents). Substitution and elimination often compete; high temperature and strong bulky bases favor elimination, while good nucleophiles at moderate temperature favor substitution.
Organometallic Reagents from Alkyl Halides
Alkyl halides are precursors to organometallic reagents, which are among the most valuable tools in C-C bond formation. Grignard reagents (RMgX) are prepared by reacting an alkyl halide with magnesium metal in anhydrous ether or THF. Organolithium reagents (RLi) are prepared similarly from lithium metal or by lithium-halogen exchange. These organometallic species react as carbanion equivalents, adding to carbonyl compounds, epoxides, and alkylating agents. The Schlenk equilibrium describes the aggregation state of Grignard reagents in solution.
Applications and Environmental Aspects
Alkyl halides are widely used as solvents (CH₂Cl₂, CHCl₃), refrigerants (CFCs, now phased out), and intermediates in pharmaceutical and agrochemical synthesis. Allylic and benzylic halides are particularly reactive in substitution reactions. Polyhalogenated compounds such as chloroform and carbon tetrachloride have historical importance in anesthesia and dry cleaning. The environmental persistence of chlorofluorocarbons (CFCs) and their role in ozone depletion led to the Montreal Protocol, a landmark environmental treaty for their phase-out.
