Hydrolysis of Haloalkanes

The hydrolysis of haloalkanes is a nucleophilic substitution in which the nucleophile is water (or hydroxide ion). The importance of this reaction at A-Level lies not in the product — which is just an alcohol we could have made other ways — but in the rates. By comparing how fast different haloalkanes hydrolyse, we can see directly how C–X bond strength controls reactivity across the halogen group, and use the result to confirm the identity of an unknown haloalkane.

This lesson covers the OCR A-Level Chemistry A (H432) specification point 4.2.2 (c)–(d): hydrolysis of haloalkanes with water and silver nitrate; the relationship between rate of hydrolysis and carbon–halogen bond enthalpy.

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1. Hydrolysis — What It Is

Hydrolysis literally means "splitting with water". For a haloalkane, water (or more commonly hydroxide) attacks the δ+ carbon, the C–X bond breaks, and an alcohol is formed along with HX:

$R\text{-}X + H_2O \rightarrow R\text{-}OH + HX$R-X+H2​O→R-OH+HX

This is a slower version of the substitution with NaOH we saw in Lesson 4 — water is a much weaker nucleophile than OH⁻ because the lone pair on O is not negatively charged. But it still works, especially if we add a little heat and silver nitrate to drive the reaction.

Why Silver Nitrate?

Silver ions (Ag⁺) abstract the leaving group X⁻ as soon as the C–X bond starts to break, forming a precipitate of silver halide. This has two effects:

1. It pulls the equilibrium to the right (Le Chatelier).
2. The colour and appearance of the silver halide precipitate reveals which halide was in the original haloalkane (and how fast the substitution happened, because the precipitate appears when hydrolysis occurs).

Silver halide	Colour	Solubility in NH₃(aq)
AgF	— (soluble in water, no precipitate)	n/a
AgCl	White	Soluble in dilute NH₃
AgBr	Cream	Soluble in concentrated NH₃
AgI	Yellow	Insoluble in concentrated NH₃

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2. The Standard Experiment — Comparing Hydrolysis Rates

A classic A-Level practical compares the rates of hydrolysis of three haloalkanes with the same R group but different halogens — e.g. 1-chlorobutane, 1-bromobutane, 1-iodobutane.

2.1 Method

1. Place three test tubes in a water bath at 50 °C.
2. Add 1 cm³ of ethanol and 1 cm³ of aqueous silver nitrate to each tube (ethanol is needed because haloalkanes are not soluble in water alone).
3. Allow to reach 50 °C.
4. Add 3 drops of the haloalkane to each tube simultaneously.
5. Start a stopwatch and record the time for a precipitate to first appear.

2.2 Expected Results

Haloalkane	Precipitate	Time to appear	C–X bond enthalpy / kJ mol⁻¹
1-Fluorobutane	None (or extremely slow)	—	484
1-Chlorobutane	White (AgCl)	~5 min	338
1-Bromobutane	Cream (AgBr)	~1 min	276
1-Iodobutane	Yellow (AgI)	seconds	238

Order of rate of hydrolysis: R–I > R–Br > R–Cl >> R–F.

This is the reverse of the order of halogen electronegativity (which would put R–F as fastest) and the reverse of polarity. The conclusion is clear: rate of hydrolysis depends on bond strength, not bond polarity.

graph TD
  A[Tube at 50 C<br/>ethanol + AgNO3 aq] --> B[Add haloalkane]
  B --> C{Hydrolysis}
  C -->|Fast: R-I| D[Yellow AgI precipitate<br/>in seconds]
  C -->|Medium: R-Br| E[Cream AgBr precipitate<br/>in 1 min]
  C -->|Slow: R-Cl| F[White AgCl precipitate<br/>in 5 min]
  C -->|Very slow: R-F| G[Essentially no precipitate<br/>in lesson time]

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3. Why Bond Enthalpy — Not Polarity — Controls the Rate

The rate-determining step of the hydrolysis involves breaking the C–X bond. The energy required to break it is, to a very good approximation, the bond enthalpy of C–X:

Bond	Mean bond enthalpy / kJ mol⁻¹
C–F	484
C–Cl	338
C–Br	276
C–I	238

The C–I bond is the weakest (longest, poorest orbital overlap), so it breaks easiest and hydrolysis is fastest. The C–F bond is the strongest, so it hardly breaks at all under lab conditions.

What about polarity? The C–F bond is most polar (δ+ biggest on C), so you might expect R–F to be attacked fastest by a nucleophile. But the activation energy — the energy cost of reaching the transition state — is dominated by the breaking of C–X, not by the electrostatic attraction between the nucleophile and C. A strong bond means a high activation energy means a slow reaction.

graph LR
  A[Rate limit: breaking C-X bond] --> B{Which bond is weakest?}
  B -->|C-I 238 kJ/mol| C[Fastest hydrolysis]
  B -->|C-Br 276| D[Medium]
  B -->|C-Cl 338| E[Slow]
  B -->|C-F 484| F[Extremely slow]

Link to Down-Group Trends