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Haloalkanes (alkyl halides) are saturated hydrocarbons where one or more hydrogens are replaced by halogens (F, Cl, Br, I). They are classified as primary, secondary, or tertiary based on whether the halogen-bearing carbon is bonded to one, two, or three carbons, respectively. This classification is critical for predicting their reactivity in substitution and elimination reactions.
You already know how to name alkanes using IUPAC conventions and understand their structural features — chains of sp³-hybridized carbons with tetrahedral geometry. Haloalkanes (also called alkyl halides) are simply alkanes in which one or more hydrogen atoms have been replaced by a halogen atom: fluorine, chlorine, bromine, or iodine. In IUPAC nomenclature, halogens are treated as substituents (prefixed as fluoro-, chloro-, bromo-, or iodo-) on the parent alkane chain. For example, CH₃CH₂Cl is chloroethane, and CH₃CHBrCH₃ is 2-bromopropane. The numbering follows the same lowest-locant rule you learned for alkane substituents.
The most important structural feature of a haloalkane is the classification of the carbon bearing the halogen as primary (1°), secondary (2°), or tertiary (3°). This classification counts how many other carbon atoms are directly bonded to the halogen-bearing carbon. In CH₃CH₂Br (bromoethane), the carbon holding the bromine is bonded to one other carbon — it is primary. In (CH₃)₂CHCl (2-chloropropane), that carbon is bonded to two other carbons — secondary. In (CH₃)₃CBr (2-bromo-2-methylpropane), it is bonded to three — tertiary. This seemingly simple distinction turns out to be the single most important predictor of how a haloalkane will react.
The reason the classification matters so much is steric and electronic. A primary carbon is relatively unhindered — a nucleophile can approach the backside of the C-X bond without bumping into bulky groups, which favors the SN2 mechanism you will study next. A tertiary carbon is heavily shielded by three alkyl groups, making backside attack almost impossible, but those same alkyl groups stabilize a carbocation through hyperconjugation and inductive effects, favoring SN1 and elimination pathways instead. Secondary haloalkanes sit in the middle — they can react by multiple mechanisms depending on the other reaction conditions.
The carbon-halogen bond itself deserves attention. Halogens are more electronegative than carbon, so the C-X bond is polar, with a partial positive charge (δ+) on carbon and a partial negative charge (δ−) on the halogen. This polarity makes the carbon electrophilic — attractive to nucleophiles. Bond strength decreases going down the periodic table (C-F > C-Cl > C-Br > C-I), while leaving group ability follows the opposite trend (I⁻ > Br⁻ > Cl⁻ > F⁻), because larger halide ions better stabilize the negative charge. This is why alkyl iodides and bromides are the most common substrates in substitution reactions, while alkyl fluorides are generally unreactive under standard conditions.