Classification, nomenclature and the C–X bond

The essentials

Halogen derivatives are classified by the number of halogen atoms (mono-, di-, polyhalogen) and by the type of carbon bearing the halogen. In sp³ C–X compounds the halogen is on an alkyl carbon: primary (1°), secondary (2°) or tertiary (3°), allylic (next to a C=C) or benzylic (next to a benzene ring). In sp² C–X compounds the halogen is on a double-bonded carbon (vinylic) or directly on a benzene ring (aryl halides).

Key points

• Common names: alkyl halides, e.g. isopropyl chloride for (CH₃)₂CHCl. IUPAC names: halogen as prefix, e.g. 2-chloropropane.
• Dihalides on the same carbon are geminal (gem-dihalides, alkylidene halides); on adjacent carbons they are vicinal (vic-dihalides, alkylene dihalides).
• In disubstituted benzenes, positions 1,2 = ortho, 1,3 = meta, 1,4 = para.
• The C–X bond is polar because halogens are more electronegative than carbon; carbon carries a partial positive charge.
• Bond length increases from C–F to C–I; bond enthalpy decreases from C–F to C–I. So C–I breaks most easily.

Exam tips

• (CH₃)₃CBr is 2-bromo-2-methylpropane, a tertiary halide; CH₂=CHCH₂Cl is an allylic halide (3-chloropropene).

Common mistakes

• Calling CH₂=CHCl allylic; it is vinylic because Cl is on the sp² carbon itself.
• Confusing benzylic (C₆H₅CH₂Cl) with aryl (C₆H₅Cl).