Topic 10 · page 12 of 27

Rates of hydrolysis: chloro, bromo, iodo

For primary halogenoalkanes the order of reactivity is iodo > bromo > chloro. Bond polarity predicts the opposite order, so the explanation is bond enthalpy: the weaker the C–X bond, the faster it breaks.

12 min read10.1010.10(ii)10.13

Read first: Rates of hydrolysis: primary, secondary, tertiary

Same skeleton, different halogen 10.1010.10(ii)

Now keep the class the same and change the halogen. Compare three primary halogenoalkanes with the same carbon chain: 1-chlorobutane, 1-bromobutane and 1-iodobutane. Each is warmed with aqueous silver nitrate in ethanol, and the precipitate also names the halogen: white AgCl, cream AgBr, yellow AgI.

Three test tubes drawn at the same moment, 30 seconds after mixing. The tube with the iodoalkane is full of yellow precipitate. The tube with the bromoalkane has a thin cream cloud. The tube with the chloroalkane is almost clear, with only a faint white cloud.
Schematic and illustrative: how much precipitate each tube holds at the same moment. Iodo is fastest and chloro slowest.
Halogenoalkane Precipitate Time, t / s Rate ∝ 1000 ÷ t
1-chlorobutane AgCl, white 300 3.3
1-bromobutane AgBr, cream 90 11
1-iodobutane AgI, yellow 14 71

These times are illustrative. The order is what real experiments show:

Why: bond enthalpy, not polarity 10.13

The slow part of each reaction is the breaking of the carbon–halogen bond. Two things might control how easily it breaks.

Polarity. The more polar the bond, the more δ+ the carbon, so the stronger the attraction to the nucleophile. Electronegativity differences from carbon (2.5): C–Cl 0.5, C–Br 0.3, C–I 0.0. Polarity falls from chloro to iodo, so it predicts that chloroalkanes should be fastest. That is wrong.

Bond enthalpy. The mean bond enthalpy of C–X falls down the group: C–Cl 346, C–Br 290, C–I 228 kJ mol⁻¹. A lower bond enthalpy means a weaker bond and less energy needed to break it. Bond enthalpy predicts iodoalkanes should be fastest. That matches the experiment.

Two bar charts side by side. Left: the electronegativity difference from carbon falls from 0.5 for C-Cl to 0.3 for C-Br to 0.0 for C-I, so polarity predicts chloro fastest. Right: the mean bond enthalpy falls from 346 for C-Cl to 290 for C-Br to 228 kJ per mole for C-I, so bond enthalpy predicts iodo fastest. A line at the bottom says the observed order is iodo, then bromo, then chloro, which matches bond enthalpy.
Polarity gives the wrong order. Bond enthalpy gives the right one.

Worked exampleExplain the order of hydrolysis

Explain why 1-iodobutane gives a precipitate with silver nitrate before 1-chlorobutane does, although C–Cl is the more polar bond.

  1. Hydrolysis needs the C–X bond to break.
  2. The mean bond enthalpy of C–I (228 kJ mol⁻¹) is lower than that of C–Cl (346 kJ mol⁻¹), so the C–I bond is weaker.
  3. Less energy is needed to break C–I, so it breaks more easily and the reaction is faster.
  4. The rate is controlled by bond breaking, not by polarity, so the more polar bond is not the faster one.

Check yourself

  1. State the order of rate of hydrolysis for primary chloro-, bromo- and iodoalkanes.
  2. Give the colour of the precipitate for each.
  3. Explain the order using bond enthalpy values.
  4. Explain why polarity gives the wrong order of reactivity.
  5. 1-bromobutane takes 90 s and 1-chlorobutane takes 300 s. How many times faster is the bromoalkane?
  6. A student says "the iodoalkane reacts fastest because C–I is the most polar bond". Correct this.

Answers cover these until you have tried the questions

  1. Iodo > bromo > chloro.
  2. Chloro: white (AgCl). Bromo: cream (AgBr). Iodo: yellow (AgI).
  3. The C–X bond must break. The mean bond enthalpies are C–Cl 346, C–Br 290, C–I 228 kJ mol⁻¹. C–I is the weakest, so it needs the least energy to break and reacts fastest. C–Cl is the strongest and is slowest.
  4. Chlorine is the most electronegative of the three, so C–Cl is the most polar bond and polarity would predict chloroalkane fastest. The experiment shows it is the slowest.
  5. Ratio = 300 ÷ 90 = 3.33, so 3.3 times faster.
  6. C–I is the least polar of the three bonds (an electronegativity difference of 0.0). It reacts fastest because it has the lowest bond enthalpy, so it is the weakest and easiest to break.