Simple rate measurements and halogenoalkane hydrolysis

What you’ll learn

• Compare rates using a common visible endpoint or a graph gradient.
• Explain controls and chemical trends in halogenoalkane hydrolysis.

Understand the idea

Core practical 5 compares the appearance of silver halide as halide ions are released during hydrolysis. For otherwise comparable samples, reaching the same small amount of precipitate sooner indicates a faster reaction. The quantity $1/t$ is a relative rate measure only when the endpoint represents the same reaction progress and the initial conditions are controlled. It is not automatically a rate in $\mathrm{mol\,dm^{-3}\,s^{-1}}$.

For halogenoalkanes with the same carbon skeleton, the carbon–iodine bond is weaker than the carbon–bromine bond, which is weaker than the carbon–chlorine bond. The iodo compound therefore normally hydrolyses more readily in the stated comparison. This explanation uses carbon–halogen bond breaking; the most polar bond does not necessarily react fastest. Comparing different carbon skeletons introduces another factor and needs a separate explanation.

Temperature, reactant concentrations, solvent composition, total volume and the definition of visible cloudiness should be held constant. A water bath can provide a common temperature, while repeats reveal variation in judging a visual endpoint. For a continuous concentration or gas-volume record, an initial tangent provides an initial rate; an average slope over a long interval answers a different question. The supplied solvent and halogenoalkane hazards determine the supervised ventilation and ignition controls.

Key formulae

$$\mathrm{R{-}X}+\mathrm{H_2O}\rightarrow\mathrm{R{-}OH}+\mathrm{H^+}+\mathrm{X^-}$$

$$\mathrm{Ag^+(aq)}+\mathrm{X^-(aq)}\rightarrow\mathrm{AgX(s)}$$

$$\mathrm{relative\ rate}\propto\frac1t$$

$$\mathrm{initial\ rate}=\left|\frac{d[\mathrm{reactant}]}{dt}\right|_{t=0}$$

Worked example

Equal-concentration samples with the same carbon skeleton reach a common precipitate endpoint after $150\ \mathrm{s}$ for the chloro compound, $60\ \mathrm{s}$ for the bromo compound and $24\ \mathrm{s}$ for the iodo compound. Other conditions are equal. Rank the rates and compare the iodo rate with the bromo rate.

1. For a common endpoint, the shorter time corresponds to the larger value of $1/t$.
2. The order is iodo faster than bromo faster than chloro.
3. The iodo-to-bromo relative rate ratio is $(1/24)/(1/60)=60/24=2.5$. Weaker carbon–iodine bonding supports the observed trend.

Answer: Iodo > bromo > chloro; the iodo sample has 2.5 times the relative rate of the bromo sample.