The Chemistry of the Haloalkanes

This lesson covers: 

  1. How to name halogenoalkanes
  2. Polarity of the carbon-halogen bond
  3. Trend in hydrolysis rates of halogenoalkanes
  4. 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A halogenoalkane is a type of chemical compound where one or more hydrogen atoms in an alkane have been replaced by halogen atoms (like fluorine, chlorine, bromine, or iodine).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.

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.