Meet the Halogen Compounds

When one or more hydrogen atoms of a hydrocarbon are replaced by halogen atoms (F, Cl, Br, I), we get a halogen compound. These are everywhere — from the anaesthetic chloroform and the refrigerant freons to the insecticide DDT and the solvent in your dry cleaner.

Two big families:

  • Haloalkanes (alkyl halides): the halogen is attached to an sp3^3-hybridised carbon of an open-chain (aliphatic) framework. General formula C(n)H(2n+1)X, written R-X.
  • Haloarenes (aryl halides): the halogen is attached directly to an sp2^2-hybridised carbon of an aromatic ring (like chlorobenzene).

This single difference — whether the C-X carbon is sp3^3 or sp2^2 — controls almost all the chemistry in this chapter.

Classification by Number of Halogens and by C-X Carbon

By the number of halogen atoms:

  • Monohalogen — one halogen (e.g. CH3_3Cl, chloromethane).
  • Dihalogen — two halogens (e.g. CH2_2Cl2_2, dichloromethane).
  • Polyhalogen — three or more (e.g. CHCl3_3 chloroform, CCl4_4 carbon tetrachloride).

By the hybridisation of the carbon bearing the halogen:

  • Compounds with sp3^3 C-X (haloalkanes and derivatives):
  • Alkyl halides — X on an sp3^3 C of an alkyl group (R-X).
  • Allylic halides — X on an sp3^3 C adjacent to a C=C double bond (e.g. CH2_2=CH-CH2_2-X).
  • Benzylic halides — X on an sp3^3 C attached to a benzene ring (C6_6H5_5-CH2_2-X).
  • Compounds with sp2^2 C-X:
  • Vinylic halides — X on an sp2^2 C of a C=C double bond (e.g. CH2_2=CH-X).
  • Aryl halides — X directly on an sp2^2 C of an aromatic ring (e.g. C6_6H5_5-X, chlorobenzene).

Classification tree of halides by halogen count and carbon hybridisation

Primary, Secondary and Tertiary Alkyl Halides

Alkyl halides are further classified by how many carbon atoms are attached to the carbon bearing the halogen:

  • Primary (1°): the C-X carbon is attached to one other carbon (e.g. CH3_3CH2_2-Br, bromoethane). Methyl halide (CH3_3X) is also counted here.
  • Secondary (2°): the C-X carbon is attached to two other carbons (e.g. (CH3_3)2_2CH-Br, 2-bromopropane).
  • Tertiary (3°): the C-X carbon is attached to three other carbons (e.g. (CH3_3)3_3C-Br, 2-bromo-2-methylpropane).

This 1°/2°/3° classification is crucial — it decides whether an alkyl halide reacts by the SN1 or SN2 mechanism (Section 5) and how easily it undergoes elimination.

Key Point: Two classifications matter most: (a) sp3^3 (haloalkane) vs sp2^2 (haloarene/vinyl) C-X — controls overall reactivity; and (b) 1°/2°/3° for alkyl halides — controls the substitution mechanism. Master both.

Solved Examples

Example 1: Identify the type of halide

Classify CH2_2=CH-CH2_2-Cl and CH2_2=CH-Cl by the type of C-X carbon.

Solution:

  • CH2_2=CH-CH2_2-Cl: the C-Cl carbon is sp3^3 and adjacent to a C=C — an allylic halide.
  • CH2_2=CH-Cl: the C-Cl carbon is sp2^2 (part of the C=C) — a vinylic halide.

Example 2: Primary, secondary or tertiary?

Classify (CH3_3)3_3C-Br as 1°, 2° or 3°.

Solution: The carbon bearing Br is attached to three other carbons (three CH3_3 groups), so it is a tertiary (3°) alkyl halide.

Example 3: Benzylic vs aryl halide

Distinguish C6_6H5_5-CH2_2-Cl from C6_6H5_5-Cl.

Solution:

  • C6_6H5_5-CH2_2-Cl (benzyl chloride): Cl on an sp3^3 carbon attached to the ring — a benzylic halide.
  • C6_6H5_5-Cl (chlorobenzene): Cl directly on an sp2^2 ring carbon — an aryl halide (haloarene).

Example 4: Number of halogens

Classify CHCl3_3 and CH2_2Cl2_2 by the number of halogen atoms.

Solution: CHCl3_3 (chloroform) has three halogens → polyhalogen (trihalogen); CH2_2Cl2_2 has two halogens → dihalogen.

Example 5: Classify a secondary halide

Classify CH3_3-CHBr-CH3_3 as 1°, 2° or 3°.

Solution: The carbon bearing Br is attached to two other carbons (two CH3_3), so it is a secondary (2°) alkyl halide (2-bromopropane).

Example 6: sp2^2 vs sp3^3 C-X

In which of bromobenzene and bromoethane is the C-Br carbon sp2^2 hybridised?

Solution: In bromobenzene (C6_6H5_5-Br) the C-Br carbon is part of the aromatic ring, so it is sp2^2. In bromoethane (CH3_3CH2_2-Br) it is sp3^3.

Example 7: Allylic halide recognition

Why is 3-chloroprop-1-ene (CH2_2=CH-CH2_2-Cl) called an allylic halide?

Solution: Because the chlorine is on an sp3^3 carbon that is directly attached to a carbon-carbon double bond (the allylic position). The adjacent C=C stabilises the intermediate, making allylic halides quite reactive.

Example 8: Identify a tertiary halide among isomers

Among the isomeric bromides of formula C4_4H9_9Br, which is tertiary?

Solution: (CH3_3)3_3C-Br (2-bromo-2-methylpropane, tert-butyl bromide) — its C-Br carbon is bonded to three carbons, so it is the tertiary isomer.

Example 9: Mono/di/poly

Classify CH3_3CH2_2Cl, ClCH2_2CH2_2Cl and CCl4_4 by the number of halogens.

Solution: CH3_3CH2_2Cl — monohalogen; ClCH2_2CH2_2Cl — dihalogen; CCl4_4 — polyhalogen (tetrahalogen).

Example 10: Aryl vs alkyl reactivity preview

Which is expected to be far less reactive toward nucleophilic substitution: chlorobenzene or chloroethane?

Solution: Chlorobenzene (an aryl halide, sp2^2 C-Cl) is much less reactive — the C-Cl bond has partial double-bond character (resonance), making substitution very difficult, unlike the sp3^3 C-Cl of chloroethane.