Classification of Haloalkanes (alkyl halides) Chemistry Tutorial

Key Concepts

• Haloalkanes are organic molecules that contain a halogen atom (Group 17 atom) as the functional group.

  Name of Halogen	fluorine	chlorine	bromine	iodine
  Symbol	F	Cl	Br	I

• Haloalkanes are molecules in which a halogen atom (X) has substituted for one of the hydrogen atoms in the parent alkane molecule.
• The carbon atom that is bonded to the halogen atom is referred to as the head carbon.
• Haloalkanes are also known as halogenoalkanes or alkyl halides.⁽¹⁾
• Haloalkanes can be classified as methyl, primary, secondary or tertiary, depending on the location of the halogen atom (X):

  Classification	General Structure	General Formula
  methyl	H |   H− C −X   | H	CH3X
  primary (1°)	H |   R− C −X   | H	R-CH2X
  secondary (2°)	X |   R− C −R'   | H	R-CHX-R'
  tertiary (3°)	X |   R− C −R'   | R"	R-C[R']X-R"

  The head carbon is highlighted.

  R, R' and R" are alkyl chains (chains of CH₂ terminating in a CH₃)

• The location of the halogen atom in a haloalkane determines the reactivity of the molecule.⁽²⁾
• In general, reactions of haloalkanes with OH^- or RO^- are dependent on the location of the halogen atom ⁽³⁾:

  ⚛ Substitution reactions are favoured by halomethanes and primary haloalkanes.

  (see Substitution Reactions of Haloalkanes)

  ⚛ Elimination reactions are favoured by tertiary haloalkanes.

  (see Dehydrohalogenation of Haloalkanes)

  ⚛ Substitution and elimination reactions are equally favoured by secondary haloalkanes.

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Halomethanes (Methyl Halides)

When a halogen atom, X, replaces one of the hydrogen atoms of a methane molecule (CH₄) the result is a haloalkane (CH₃X).

The halomethanes (methyl halides) are given in the table below:

Primary Haloalkanes (Primary Alkyl Halides)

One alkyl group is bonded to the head carbon, that is, the halogen atom (X) is bonded to the last carbon atom of the carbon chain.

The general formula for a primary haloalkane (1° alkyl halide) is R-CH₂X where R is an alkyl chain and X is the halogen atom.

Some examples of primary chloroalkanes (1° alkyl chlorides) are given in the table below:

Secondary Haloalkanes (Secondary Alkyl Halides)

Two alkyl groups are bonded to the head carbon.

The halogen atom is bonded to a carbon atom that is not at the end of the chain, and this carbon atom is also bonded to a hydrogen atom.

The general formula for a secondary haloalkane (2° alkyl halide) is R-CHX-R' where R and R' are alkyl groups and X represents a halogen atom.

Some examples of secondary bromoalkanes (2° alkyl bromides) are given in the table below:

Tertiary Haloalkanes (Tertiary Alkyl Halides)

Three alkyl groups are bonded to the head carbon.

The halogen atom is bonded to a carbon atom that is itself bonded to 3 other carbon atoms.

The general formula for a tertiary haloalkane (3° alkyl halide) is R-C[R']X-C" where R, R' and R" are alkyl groups and X is the halogen atom.

Some examples of tertiary fluoroalkanes (3° alkyl fluorides) are given in the table below: