Topic 10 · page 5 of 27

Primary, secondary and tertiary halogenoalkanes

A halogenoalkane is primary, secondary or tertiary according to how many other carbon atoms are bonded to the carbon that carries the halogen. The class decides how fast it reacts.

9 min read10.7

Read first: Naming and drawing halogenoalkanes

Counting carbons on the C–X carbon 10.7

Find the carbon that carries the halogen. Count how many other carbon atoms are bonded to it.

Three isomers of C4H9Br as skeletal formulae. 1-bromobutane: the carbon bonded to Br has one carbon neighbour, so it is primary, shown 1 degree. 2-bromobutane: the carbon bonded to Br has two carbon neighbours, so it is secondary, 2 degrees. 2-bromo-2-methylpropane: the carbon bonded to Br has three carbon neighbours, so it is tertiary, 3 degrees. The C-Br carbon and its carbon neighbours are marked.
The carbon bonded to Br is marked with a ring. Count the carbon atoms joined to it: one, two or three.
Class Carbon neighbours of the C–X carbon Hydrogens on the C–X carbon Example
Primary (1°) 1 2 1-bromobutane, CH₃CH₂CH₂CH₂Br
Secondary (2°) 2 1 2-bromobutane, CH₃CHBrCH₂CH₃
Tertiary (3°) 3 0 2-bromo-2-methylpropane, (CH₃)₃CBr

Bromomethane has no carbon neighbours at all. It is usually treated with the primary halogenoalkanes in reactions.

Isomers have different classes 10.7

Four halogenoalkanes have the formula C₄H₉Br. They are structural isomers: the same molecular formula and different structures.

The four structural isomers of C4H9Br as skeletal formulae: 1-bromobutane, primary; 2-bromobutane, secondary; 1-bromo-2-methylpropane, primary; 2-bromo-2-methylpropane, tertiary.
The four isomers of C4H9Br. Two are primary, one is secondary and one is tertiary.

Different classes react at different rates. Core Practical 5 compares the primary, secondary and tertiary isomers, and the next pages say why.

Worked exampleClassify and name an isomer of C₅H₁₁Br

Classify CH₃CH₂C(CH₃)₂Br and name it.

  1. The carbon bonded to Br is bonded to CH₃, CH₃ and CH₂CH₃: three carbon atoms.
  2. It is tertiary.
  3. The longest chain is CH₃CH₂C–CH₃, four carbons (butane). Br and a methyl group are on carbon 2.
  4. Name: 2-bromo-2-methylbutane.

Check yourself

Classify each halogenoalkane as primary, secondary or tertiary.

  1. 2-chloropropane
  2. 1-iodopropane
  3. 3-bromopentane
  4. 1-bromo-2-methylpropane
  5. 2-bromo-2-methylbutane
  6. Give the names and classes of the two isomers of C₃H₇Br.

Answers cover these until you have tried the questions

  1. Secondary. The C–Cl carbon (carbon 2) is bonded to two other carbons.
  2. Primary. The C–I carbon is an end carbon, bonded to one other carbon.
  3. Secondary. Carbon 3 of pentane is bonded to carbons 2 and 4.
  4. Primary. The C–Br carbon is bonded to one carbon (the CH). The methyl branch is on the next carbon.
  5. Tertiary. Carbon 2 is bonded to carbon 1, carbon 3 and the methyl group.
  6. 1-bromopropane, primary, and 2-bromopropane, secondary.