Topic 10 · page 11 of 27

Rates of hydrolysis: primary, secondary, tertiary

With the same halogen, a tertiary halogenoalkane is hydrolysed faster than a secondary one, and a secondary one faster than a primary one. The evidence is the time for a silver halide precipitate to appear.

10 min read10.1010.10(i)10.12

Read first: Water as the nucleophile, Primary, secondary and tertiary halogenoalkanes

Comparing three isomers 10.1010.10(i)10.12

Page 8 showed that the time for a precipitate of silver bromide to appear measures how fast the C–Br bond is hydrolysed. Use that to compare three isomers of C₄H₉Br:

The three have the same formula and the same halogen. Only the structure changes, so any difference in rate is caused by the class of the halogenoalkane. Each is warmed with aqueous silver nitrate in ethanol, and the time for a precipitate to appear is measured.

Three test tubes drawn at the same moment, 30 seconds after mixing. The tube with the tertiary halogenoalkane, 2-bromo-2-methylpropane, is full of cream precipitate. The tube with the secondary halogenoalkane, 2-bromobutane, is mostly cloudy. The tube with the primary halogenoalkane, 1-bromobutane, is only faintly cloudy.
Schematic and illustrative: how cloudy each tube is at the same moment. The tertiary isomer forms the most precipitate and the primary the least.

The illustrative times for the precipitate to appear, in one set of conditions, are:

Halogenoalkane Class Time, t / s Rate ∝ 1000 ÷ t
1-bromobutane primary 90 11
2-bromobutane secondary 35 29
2-bromo-2-methylpropane tertiary 5 200

The same amount of silver bromide must form in each tube before the end point is seen, so the rate is proportional to 1 ÷ time. Multiplying by 1000 only keeps the numbers readable.

Worked exampleHow many times faster?

Using the table, how many times faster is the tertiary isomer than the primary?

  1. Rate ∝ 1 ÷ time. The ratio of rates is the inverse ratio of times.
  2. Rate(tertiary) ÷ rate(primary) = t(primary) ÷ t(tertiary) = 90 ÷ 5 = 18.
  3. The tertiary isomer is hydrolysed 18 times faster (illustrative data).

Check yourself

  1. State the order of rate of hydrolysis for primary, secondary and tertiary halogenoalkanes.
  2. Name an isomer of C₄H₉Br that is hydrolysed faster than 2-bromobutane.
  3. Explain why the same halogen and the same formula make this a fair comparison.
  4. A student's results are: tertiary 5 s, secondary 35 s, primary 90 s. Calculate 1000 ÷ t for each, to 2 significant figures.
  5. How many times faster is the secondary isomer than the primary, using these times?
  6. State what you would observe in the experiment, and what each observation shows.

Answers cover these until you have tried the questions

  1. Tertiary > secondary > primary.
  2. 2-bromo-2-methylpropane, the tertiary isomer.
  3. Only one variable changes, the class of halogenoalkane (the structure). The halogen is the same, so the C–Br bond is the same type in each, and any difference in time is caused by the structure.
  4. Tertiary: 1000 ÷ 5 = 200. Secondary: 1000 ÷ 35 = 28.57 = 29. Primary: 1000 ÷ 90 = 11.11 = 11.
  5. Rate ratio = t(primary) ÷ t(secondary) = 90 ÷ 35 = 2.57 = 2.6 times faster.
  6. A cream precipitate of silver bromide forms. It appears first with the tertiary isomer and last with the primary, which shows that the tertiary isomer is hydrolysed fastest.