Classifying and naming halo compounds

When a hydrogen atom of a hydrocarbon is replaced by a halogen (F, Cl, Br or I), we get a halo compound. There are two kinds:

• Haloalkanes (alkyl halides): the halogen is on an sp³ carbon of a chain, as in chloroethane, CH₃CH₂Cl.
• Haloarenes (aryl halides): the halogen is directly on a benzene ring, as in chlorobenzene, C₆H₅Cl.

Haloalkanes are sorted by the carbon that carries the halogen, just like alcohols: primary (1°) if that carbon is joined to one other carbon, secondary (2°) if to two, tertiary (3°) if to three.

Four special positions, worth learning by picture:

• Allylic: the halogen is on the carbon next to a C=C double bond.
• Benzylic: the halogen is on the carbon next to a benzene ring (C₆H₅CH₂Cl).
• Vinylic: the halogen is on a carbon of the C=C itself.
• Aryl: the halogen is on a ring carbon.

Allylic and benzylic halides behave like ordinary haloalkanes (the carbon is sp³), while vinylic and aryl halides are bonded to an sp² carbon and behave very differently.

The C–X bond. The halogen is more electronegative than carbon, so it pulls the shared electrons towards itself: the carbon gets a partial positive charge (δ+) and the halogen a partial negative one (δ–). Going down the group from F to I, the halogen atom gets bigger, so:

• bond length increases: C–F < C–Cl < C–Br < C–I;
• bond enthalpy (the energy to break the bond) decreases: C–F > C–Cl > C–Br > C–I. Think of a student standing right beside the teacher: it takes a lot of persuading to call them away. A student at the back of the room slips out easily. Short bonds hold tight; long bonds break easily.
• dipole moment mostly falls with electronegativity, with one exception to remember: CH₃Cl has a slightly larger dipole moment than CH₃F, because its longer bond makes up for the smaller charge.

IUPAC names: treat the halogen as a prefix on the alkane: fluoro-, chloro-, bromo-, iodo-. Number the longest chain so the substituents get the lowest numbers, and list prefixes alphabetically: 2-chlorobutane, 1-bromo-2,2-dimethylpropane. For rings with two halogens, use numbers or o-, m-, p-: 1,4-dichlorobenzene is p-dichlorobenzene.