lab24

TitleNucleophilic substitution reactions of halogenoalkanes

Data Collection, Data Processing and Presentation

Experiment 1: Effect of the halogen atom on the rate of hydrolysisTable 1: Tubes for experiment 1.

Tube ContentTime noted for precipitation, (t)

*Rate of hydrolysis, (s-1)

Observation

11 cm3 ethanol + 2 drops 1-chlorobutane

- Approaching 0After 1 minute, the mixture changed to yellow precipitate

21 cm3 ethanol + 2 drops 1-bromobutane

237.00±0.21% 0.004±0.211%After 4 minutes, the

mixture slightly changed to pale yellow solution

31 cm3 ethanol + 2 drops 1-iodobutane


mixture remains unchanged

*Rate of hydrolysis can be calculated using this formula.

Rate of hydrolysis =

1. The fastest hydrolysis of halogenoalkane is 1-iodobutane and the slowest is 1-chlorobutane.

2. Halogenoalkane that has the most polar carbon-halogen bond is 1-iodobutane.3. Yes. The differing polarity is the reason for the different rates of hydrolysis. As

the polarity of the carbon-halogen bond changes from chlorine to iodine, the rate of hydrolysis increases.

Table 2: Bond energies.

Carbon-halogen bond Bond enthalpies at 298 KC-Cl 338C-Br 276C-I 238

* August 2001 International Baccalaureate Organization Chemistry Data Booklet.4. According to Table 2, as the carbon-halogen bond changes from chlorine to

iodine, the bond enthalpies at 298 K decrease. This is because the strength of C-Cl bond is higher compared to C-I. As a result, less energy required to break C-Cl bond whereas for C-I, more energy is required. Thus, the rate of hydrolysis of 1-iodobutane is faster compared to the rate of hydrolysis of 1-bromobutane and 1-chlorobutane.

Experiment 2: Effect of the structure of the carbon skeleton on the reaction mechanism and reaction rate

Table 3: Tubes for experiment 2.

Tube ContentTime noted for precipitation, (t)

*Rate of hydrolysis, (s-1)

Observation



mixture changed to pale cloudy white precipitate


181.00±0.28% 0.006±0.276%

After 3 minutes, the mixture slightly

changed to cloudy precipitate

3

1 cm3 ethanol + 2 drops 2-

bromo-2-methylpropane

30.00±1.67% 0.033±1.667%After 30 seconds, the mixture changed to a very white precipitate

*Rate of hydrolysis can be calculated using this formula.

Rate of hydrolysis =

1. The fastest bromoalkane being hydrolysed is 2-bromo-2-methylpropane and the slowest is 1-bromobutane.

2. In this experiment, the solvent used is a 1:1 mixture of ethanol and water which means the mechanism that likely to be favoured is SN1 mechanism. This is because the polar solvent, water, will hydrate ions and stabilise the carbocation. Thus, under this circumstance where the carbocation is stable, SN1 mechanism is likely to occur.

3. According to the proposed mechanism above, 2-bromo-2-methylpropane would be the most favoured and 1-bromobutane would be the least favoured. This is because 2-bromo-2-methylpropane is a tertiary carbocation and is more stable compared to 1-bromobutane which is primary carbocation, due to the inductive effect of the alkyl group. Thus, SN1 mechanism is likely to occur if the reaction goes via a tertiary carbocation.

4. From 1-bromobutane to 2-bromo-2-methylpropane, the relative rates of hydrolysis increase. This is because the rates depend on the inductive effect of the alkyl group where as the number of alkyl group increase from 1-bromobutane to 2-bromo-2-methylpropane, the inductive effect also increases. As a result, the stability of carbocation increases from 1-bromobutane to 2-bromo-2-methylpropane. Thus, the rate of hydrolysis also increases.

Experiment 3: Effect of the solvent on the reaction mechanism and reaction rate (the Finkelstein reaction)

Table 4: Tubes for experiment 3.Tube Content Time elapsed, (t) Observation

1

5 cm3

Finkelstein’s reagent + 8 drops 1-bromobutane

20 minutes

The colourless mixture changed to a little amount of

precipitate

2

5 cm3

Finkelstein’s reagent + 8 drops 2-bromobutane

The colourless mixture slightly changed to yellow

solution

3

5 cm3

Finkelstein’s reagent + 8 drops

2-bromo-2-methylpropane

The colourless mixture changed to pale yellow

solution

1. The fastest bromoalkane that undergoes nucleophilic substitution by iodide ions is 1-bromobutane and the slowest is 2-bromo-2-methylpropane.

2. In this experiment, the solvent used is propanone which means the mechanism that likely to be favoured is SN2 mechanism. This is because the solvent used is non-polar solvent, propanone. Thus, under this circumstance SN2 mechanism is likely to occur.

3. According to the proposed mechanism above, 1-bromobutane would be the most favoured and 2-bromo-2-methylpropane would be the least favoured. This is because 1-bromobutane is a primary carbocation that has a single alkyl group attached to its carbon of carbon-halogen. As a result, less inductive effect causes the formation of unstable carbocation. Thus, SN2 mechanism is likely to occur.

4. From 2-bromo-2-methylpropane to1-bromobutane, the relative rates of hydrolysis increase. This is because the rates depend on the inductive effect of the alkyl group where as the number of alkyl group decrease from 2-bromo-2-methylpropane to1-bromobutane, the inductive effect also decreases. As a result, the stability of carbocation decreases from 2-bromo-2-methylpropane to1-bromobutane. Thus, the rate of hydrolysis also increases.

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