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Reactivity of halogenoalkanesEdexcel International A Level Chemistry: Revision notes

Section 1

Comparing rates of hydrolysis

Halogenoalkanes are hydrolysed by water, which acts as a weak nucleophile and replaces the halogen with OH. In the test, aqueous silver nitrate in ethanol is used: the halide ion released forms a silver halide precipitate, so the time for a precipitate to appear measures the rate. A shorter time means a faster reaction.

R–X + H₂O → R–OH + X⁻ + H⁺ and Ag⁺ + X⁻ → AgX(s)

Precipitates: AgCl white, AgBr cream, AgI yellow. Ethanol is used so that the water-insoluble halogenoalkane and the aqueous reagent mix in one solution.

Key termshydrolysisrate of reaction

Section 2

Effect of the halogen: bond enthalpy

For primary halogenoalkanes with the same carbon skeleton, the rate of hydrolysis increases C–Cl < C–Br < C–I.

The explanation is bond enthalpy. Mean bond enthalpies: C–Cl 346, C–Br 290, C–I 228 kJ mol⁻¹. The C–X bond has to be broken in the reaction, and the C–I bond is the weakest, so less energy is needed and it reacts fastest. C–F (467 kJ mol⁻¹) is so strong that fluoroalkanes are essentially unreactive.

Polarity is not the explanation: C–Cl is the most polar bond (chlorine is the most electronegative), yet chloroalkanes react slowest.

Key termsbond enthalpy
Common mistake

Do not say iodoalkanes are more reactive because C–I is more polar. It is less polar. The reason is the weaker bond.

Section 3

Effect of structure: primary, secondary, tertiary

For halogenoalkanes with the same halogen, such as the isomers of C₄H₉Br, the rate of hydrolysis increases in the order:

primary < secondary < tertiary

So 2-bromo-2-methylpropane (tertiary) gives a cream precipitate within seconds, 2-bromobutane (secondary) more slowly and 1-bromobutane (primary) slowest. You are expected to use observations and data (times) to compare them and state this trend; the reason is explained in Unit 4.

Key termsisomers
Exam tip

Rate is proportional to 1/time. A precipitate in 15 s against 300 s means the reaction is 20 times faster.

Section 4

Core Practical 5: rates of hydrolysis

Method: place equal volumes of aqueous silver nitrate in ethanol in separate test tubes in a water bath (about 60 °C). Add the same small volume (a few drops) of each halogenoalkane at the same time, start a stopwatch and time how long it takes for a precipitate to first appear (for example until a mark beneath the tube is no longer visible).

Control variables: temperature, volumes and concentration of silver nitrate, volume of halogenoalkane and of ethanol. Repeat for reliability.

Safety: eye protection; silver nitrate stains skin; ethanol is flammable so use a water bath, not a naked flame.

Key termscontrol variable

Section 5

Core Practical 6: preparing 2-chloro-2-methylpropane

The tertiary alcohol 2-methylpropan-2-ol reacts with concentrated hydrochloric acid: (CH₃)₃COH + HCl → (CH₃)₃CCl + H₂O.

  1. Shake the reagents in a separating funnel, releasing pressure through the tap.
  2. Let the layers separate. The organic product is the upper layer (less dense). Run off and discard the lower aqueous layer.
  3. Wash with aqueous sodium hydrogencarbonate to remove residual acid (CO₂ is released, so vent the funnel); separate again.
  4. Dry with an anhydrous salt (e.g. anhydrous sodium sulfate or calcium chloride), then decant.
  5. Distil and collect the fraction at about 51 °C (boiling temperature determination).
Key termsseparating funneldistillationanhydrous

Must know

  • Rate of hydrolysis: C–I > C–Br > C–Cl, explained by bond enthalpy (C–I weakest), not polarity
  • Primary < secondary < tertiary for the rate of hydrolysis (same halogen)
  • Precipitates: AgCl white, AgBr cream, AgI yellow; time to precipitate measures the rate
  • Core Practical 5: control temperature, volumes and concentrations
  • Core Practical 6: shake, separate, wash with NaHCO₃, dry, distil at about 51 °C

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Reactivity of halogenoalkanes

  1. A student compares the rates of hydrolysis of 1-chlorobutane, 1-bromobutane and 1-iodobutane. A few drops of each halogenoalkane are added to separate test tubes containing aqueous silver nitrate in ethanol, held in a water bath at 60 °C, and the time for a precipitate to appear is measured.
    Explain the order of the rates of hydrolysis of the three compounds in terms of bond enthalpy.2 marks
  2. A technician compares three isomers of C₄H₉Br: 1-bromobutane, 2-bromobutane and 2-bromo-2-methylpropane. Equal volumes of each are added to separate tubes of aqueous silver nitrate in ethanol at the same temperature, and the time for the first precipitate is recorded.
    The precipitate appeared after 15 s for 2-bromo-2-methylpropane and after 300 s for 1-bromobutane. Calculate how many times faster the tertiary isomer reacted, and state one variable that had to be kept constant.2 marks
  3. A student prepares 2-chloro-2-methylpropane by shaking 2-methylpropan-2-ol with concentrated hydrochloric acid in a separating funnel. The product is a colourless liquid that is immiscible with water. Its density is 0.84 g cm⁻³, the aqueous acid layer has a density of about 1.1 g cm⁻³, and the product boils at 51 °C.
    After shaking and standing, two layers form. State which layer is the organic product, explain how you know, and describe what the student then does to the other layer.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).