Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 10.7 Primary, secondary and tertiary halogenoalkanes-Study Notes - New Syllabus

10.7 Primary, Secondary and Tertiary Halogenoalkanes

Halogenoalkanes can be classified as primary, secondary or tertiary depending on the number of alkyl groups attached to the carbon atom bonded to the halogen. This classification is important because it influences the reactivity and the type of reaction mechanism the compound undergoes.

Classification of Halogenoalkanes

The classification depends on how many carbon atoms are directly attached to the carbon bearing the halogen.

• Primary (1°) halogenoalkane: the carbon bonded to the halogen is attached to only one other carbon atom.
• Secondary (2°) halogenoalkane: the carbon bonded to the halogen is attached to two other carbon atoms.
• Tertiary (3°) halogenoalkane: the carbon bonded to the halogen is attached to three other carbon atoms.

Structural Representation

• Primary: \( \mathrm{R-CH_2-X} \)
• Secondary: \( \mathrm{R-CH(X)-R’} \)
• Tertiary: \( \mathrm{R-C(X)(R’)(R”)} \)

Explanation of the Distinction

• The classification reflects the degree of substitution of the carbon atom bonded to the halogen.
• More substituted carbons (tertiary) have more alkyl groups attached, increasing electron density around the carbon.
• This affects both the stability of intermediates (such as carbocations) and the steric hindrance around the reactive centre.

Effect on Reactivity

• Tertiary halogenoalkanes form more stable carbocations due to the electron-donating effect of alkyl groups.
• Primary halogenoalkanes have less steric hindrance, making nucleophilic attack easier.
• Secondary halogenoalkanes show intermediate behaviour.

Therefore, the classification helps predict whether a compound is more likely to undergo mechanisms such as \( \mathrm{S_N1} \) or \( \mathrm{S_N2} \).