Theory / Explanation:
Nucleophilic substitution occurs in the hydrolysis of halogenoalkanes in a one step mechanism. Nucleophilic substitution happens when a molecule is attacked by a nucleophile and an atom or group of atom in the molecule is replaced by the nucleophile.
In this reaction, bond enthalpy is very important. The amount of energy required to break the carbon-halogen bonds, (bond enthalpy), provides the most evidence of the rates of hydrolysis. The bond enthalpy decreases down the group.
The decreasing bond enthalpies going down group 7 of the periodic table show that the carbon-halogen bonds are easier to break at the bottom of the group because they are much weaker. The fastest rate of reaction will be given by the compounds containing C-I bonds as its bonds are easily broken, and it’s the most reactive. Compounds containing the C-Br bonds will be the next easiest followed by C-Cl bonds being much harder to break and therefore requiring the longest amount of time to hydrolyse and form a precipitate.
The reason for the decrease in bond enthalpy down the group of halogens is a result of increase in atomic size. Iodine’s atomic size is the greatest and therefore the forces needed to be broken are the smallest. The large atomic size causes the bond enthalpy to be reduced and there is an increase in reactivity due to greater shielding of the outer electrons, and greater distance from the nucleus.
Apparatus:
- 3 test tubes
- 45 ºC water bath
- Burette
- 3 beakers
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Ethanol ( 3cm3 per experiment) x3
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Silver Nitrate ( 3cm3 per experiment) x3
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Iodobutane 3.0 cm3
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Bromobutane 2.0 cm3
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Chlorobutane 0.9 cm3
- Fine pipette
- Thermometer
- Stop clock/ timer
- Piece of paper with an X on it
Method:
The best test for halogenoalkanes is to warm with water and then test the halide using aqueous silver nitrate solution. The experiment is successful because the reactions between the ions can be seen by the formation of a visible silver halide precipitate. The speed with which the mixture goes cloudy is an indication of the speed of the reaction and hence the reactivity of the C-halogen bond.
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Using a burette add 3 cm 3 of silver nitrate into three separate test tubes. Using the burette again measure 3 cm 3 of ethanol to each test tube.
- Place all 3 test tubes in a water bath of 45ºC
(The raised temperature will allow nucleophilic substitution to occur and increase the reaction without the reagents evaporating, and will provide a more controlled environment)
3. Using a fine pipette measure out the measurements of the following halogenoalkanes, using a pipette for better precision.
0.9 cm3 of chlorobutane
2.0 cm3 of bromobutane
3.0 cm3 of Iodobutane
(As each of the above compounds had different densities, you have to make sure that they all have the same number of moles, of equal volumes of molecular mass and density. These were the calculations carried out to work this out.
4. Using a thermometer, check that the test tubes of silver nitrate/ethanol have reached the set temperature of the water bath of 45ºC Once this has been done, the individual halogenoalkane in each beaker can be added to each individual test tube of silver nitrate and ethanol.
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Now using a stop watch, place a piece of paper with an X on it behind the test tube and see how long it takes for the X to disappear. This will give the time needed for the reaction to take place, consequently differing the rates of reaction
- Record results in an appropriate table like this one, and then repeat the experiment.
Hazards and Precautions
Silver nitrate is a toxic compound, and so are halogenoalkanes.
So when using the above chemicals, the experiments should be done in a well ventilated area or in a fume cupboard where possible, as this removes the toxic gases. As ethanol and halogenoalkanes are highly flammable, care should be taken that they are not near naked flames. Eye protection should be worn at all times, as well as protective clothing to prevent injury caused by splash back or entry of any of the reagents or the hot water.
References
Ratcliff, Eccles, Johnson, Nicholson & Raffan – Chemistry 1, 2003, endorsed by OCR- pages 138-139
K.Gadd & S.Gurr – Chemistry 1994, Thomas Nelson & Sons Ltd – page 299
Ritchie, Rob – Revise AS Chemistry- 2004, Letts education, pages 132-133
