Making Halogenoalkanes

This lesson covers: 

  1. How to name halogenoalkanes
  2. How halogenoalkanes are classified as primary, secondary or tertiary
  3. Reactions to produce halogenoalkanes
  4. Polarity of the carbon-halogen bond
  5. Trend in hydrolysis rates of halogenoalkanes
  6. Comparing halogenoalkane reactivity experimentally

Naming halogenoalkanes

A halogenoalkane is a type of chemical compound where one or more hydrogen atoms in an alkane have been replaced by halogen atoms (like fluorine, chlorine, bromine, or iodine).

To name a halogenoalkane, we use prefixes (like fluoro-, chloro-, bromo-, iodo-) to indicate the type and number of halogen atoms.


Here are some examples of halogenoalkanes:

Examples of halogenoalkanes including dichloromethane, iodethane, and 2-bromo-1-fluoropropane with their chemical structures.

Types of halogenoalkanes

Halogenoalkanes with one substituted halogen atom can be categorised based on the groups attached to the carbon with the halogen:

  1. Primary - The halogen is attached to a carbon atom connected to only one alkyl group (or no alkyl groups).
  2. Secondary - The halogen is attached to a carbon atom connected to two alkyl groups.
  3. Tertiary - The halogen is attached to a carbon atom connected to three alkyl groups.
Diagram showing primary secondary and tertiary halogenoalkanes with halogen and alkyl group attachments.

Reactions to produce halogenoalkanes

Halogenoalkanes can be produced via several reactions:

  1. Free-radical substitution of alkanes by Cl2 or Br2 in the presence of UV light. For example, ethane reacts with chlorine under UV light to form chloroethane:

CH3CH3 + Cl2 ➔ CH3CH2Cl + HCl


  1. Electrophilic addition of an alkene with a halogen or hydrogen halide at room temperature. For example, ethene reacts with bromine to form 1,2-dibromoethane:

CH2=CH2 + Br2 ➔ CH2Br-CH2Br


  1. Substitution of an alcohol by reaction with:
  • a hydrogen halide, 
  • KCl and concentrated H2SO4 or concentrated H3PO4,
  • PCl3 with heat,
  • PCl5, or
  • SOCl2.

For example, ethanol reacts with HBr to form bromoethane:

CH3CH2OH + HBr ➔ CH3CH2Br + H2O

Polarity of the carbon-halogen bond

In halogenoalkanes, the carbon-halogen bond is polar because halogen atoms have a higher electronegativity than carbon. This causes an uneven distribution of electrons, making the carbon atom partially positively charged (δ+) and the halogen atom partially negatively charged (δ-).

Diagram showing the polarity of the carbon-bromine bond with partial positive charge on carbon and partial negative charge on bromine.

This polarity in the bond makes the carbon atom a target for nucleophiles (electron pair donors). Common nucleophiles include OH-, CN-, NH3, and H2O.

Trends in hydrolysis rates

Hydrolysis of a halogenoalkane is a reaction where the carbon-halogen bond breaks in the presence of water, forming an alcohol and a hydrogen halide.


For example:

RCl + H2O ➔ ROH + H+ + Cl-


The rate of hydrolysis depends on the bond enthalpy of the carbon-halogen bond. Bond enthalpy measures bond strength quantitatively - it is the energy required to break one mole of bonds between two atoms in the gaseous state.


Carbon-halogen bonds with lower bond enthalpies are weaker and require less energy to break, allowing them to react at a faster rate.


Carbon-halogen bond enthalpy decreases down group 7 because:

  • The atomic radius of the halogen increases.
  • The carbon-halogen bond length increases.
  • The electrostatic attraction between bonding electrons and nuclei decreases.
  • The amount of energy needed to break these longer, weaker bonds decreases.


Therefore, iodoalkanes (with the weakest carbon-halogen bonds) hydrolyse the fastest, while fluoroalkanes (with the strongest bonds) hydrolyse the slowest.

BondBond enthalpy (kJ mol-1)
C–F467
C–Cl346
C–Br290
C–I228

Comparing halogenoalkane reactivity using experiments

To compare the relative reactivity of chloro-, bromo- and iodo-alkanes, a common experiment is performed:

  1. Place a chloroalkane, a bromoalkane, and an iodoalkane in separate test tubes.
  2. Add ethanol to each tube and warm them in a water bath at 50°C.
  3. Add silver nitrate solution to each tube. The water in the solution hydrolyses the halogenoalkane (RX):

RX(aq) + H2O(l) ➔ ROH(aq) + H+(aq) + X-(aq)

  1. The halide ions (X-) produced then react with the silver ions to form a silver halide precipitate:

X-(aq) + Ag+(aq) ➔ AgX(s)

  1. Time how long it takes for a visible precipitate to form after adding silver nitrate.


A quicker formation of precipitate indicates a faster hydrolysis. Generally, iodoalkanes show the fastest precipitate formation, and chloroalkanes the slowest.


The colour of the precipitate also helps to identify the original halogenoalkane:

  • Silver chloride (AgCl) forms a white precipitate.
  • Silver bromide (AgBr) forms a cream precipitate.
  • Silver iodide (AgI) forms a yellow precipitate.