What is a halogenoalkane?

An alkane with one or more hydrogen atoms replaced by halogen atoms.


Halogenoalkanes can also be called haloalkanes.

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Why are carbon-halogens bonds polar?

Because halogens are more electronegative than carbon, so they pull the shared electrons towards them, giving the carbon a partial positive charge.

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What is bond enthalpy?

Bond enthalpy is the energy required to break one mole of covalent bonds between two atoms in the gaseous state.

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State and explain the trend in bond enthalpy of the carbon-halogen bond as we descend group 7. 

As we descend group 7, the carbon-halogen bond enthalpy decreases.


This is because the atomic radius of the halogens increases which leads to a longer carbon-halogen bond and a weaker attraction of the shared pair of electrons in the bond to the halogen nucleus.

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Outline 3 types of reaction that can be used to produce halogenoalkanes.

  1. Free radical substitution of alkanes by chlorine or bromine in UV light.
  2. Electrophilic addition of alkenes with a halogen or hydrogen halide at room temperature.
  3. Substitution of alcohols e.g. by reactions of halide ions in the presence of acid.

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Explain how halogenoalkanes can be classified as primary, secondary or tertiary based on their structure.

  • Primary: halogen atom attached to carbon with one other alkyl group (or no alkyl groups).
  • Secondary: halogen atom attached to carbon with two other alkyl groups.
  • Tertiary: halogen atom attached to carbon with three other alkyl groups.

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State and explain the trend in the rate of hydrolysis of halogenoalkanes.

The rate of hydrolysis increases in the order: chloroalkanes < bromoalkanes < iodoalkanes. 


This is due to the decrease in bond enthalpy of the carbon halogen bond in the order C-Cl > C-Br > C-I.


The fastest rate of hydrolysis occurs in iodoalkanes because the C-I bond is the weakest. 

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Outline an experiment to compare the hydrolysis rates of 1-chlorobutane, 1-bromobutane, and 1-iodobutane.

  1. Set up 3 test tubes with ethanol and drops of 1-chlorobutane, 1-bromobutane, and 1-iodobutane.
  2. Heat the test tubes in a water bath at 50°C along with a tube of silver nitrate solution.
  3. Add the silver nitrate to each test tube and start timing.
  4. Observe the time taken for the silver halide precipitate to form in each tube.
  5. 1-Iodobutane forms the precipitate fastest, 1-bromobutane forms the second fastest, and 1-chlorobutane forms the slowest.

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Outline an experiment to identify the halogen (Cl, Br or I) present in halogenoalkanes (RX). Explain your answer with ionic equations.

  1. Carry out a hydrolysis reaction in ethanol with aqueous silver nitrate:

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

  1. The halide ions produced react with silver ions to form a silver halide precipitate:

Ag+(aq) + X-(aq) ➔ AgX(s)

  1. The colour of the precipitate indicates which halogen is present - AgCl is white, AgBr is cream and AgI is yellow.

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