SUBSTITUTION OF HALOGENOALKANES
Introduction
In halogenoalkanes, the halogen-carbon bond is polarised (due to the high electronegativity of the halogens 4.0
for F down to 2.7 for I, carbon is 2.2) with a δ+ charge on the carbon and a δ– charge on the halogen; this
makes it susceptible to attack by nucleophiles.
The general equation for these reactions is:
Nu- + CR3X  CR3Nu + XWhere:
Nu- is the nucleophile
X is the halogen atom
R is hydrogen or an alkyl group
In this experiment, we will be looking at the effect of the nature of the halogen atom, the nucleophile and the
position of the halogen atom on the ease of the substitution reaction.
As you can see from the equation the reaction produces a halide ion. This is useful as it means we can use silver
nitrate solution to follow the reaction as silver forms a silver halide precipitate in the presence of halide ions.
Experiment 1: The effect of the halogen atom
1. Set up a hot water bath at about 50 to 60OC.
2. Put about 1cm3 of ethanol into three separate test tubes.
3. Add 2–3 drops of 1-chlorobutane to the first tube, 1-bromobutane to the second and 1-iodobutane to the
third, labelling each tube.
4. Place the tubes in the hot water bath.
5. Half fill a fourth test tube with 0.1M silver nitrate solution and stand this in the hot water bath too. Leave
the tubes for about 10 minutes to reach the temperature of the bath.
6. Remove the tubes from the bath and place in a test tube rack.
7. Quickly add a teat pipette full of the silver nitrate solution to each tube, shake to mix the contents and start
the stop clock. Observe the tubes closely over the next few minutes and note the time when a precipitate
appears in each tube.
Source: Wellington College, intranet.wellingtoncollege.org.uk/resource.aspx?id=266342
Experiment 2: The effect of halogen position
1. Put about 1cm3 of ethanol into three separate test tubes.
2. Add 2–3 drops of 1-bromobutane to the first tube, 2-bromobutane to the second tube and 2-bromo, 2methylpropane to the third, labelling each tube.
3. Quickly add a teat pipette full of silver nitrate solution to each tube, shake to mix the contents and start the
stop clock. Observe the tubes closely over the next few minutes and note the time when a precipitate
appears in each tube.
Experiment 3: The effect of the nucleophile
1. Put about 1cm3 of ethanol and 2–3 drops of 1-bromobutane into each of two test tubes.
2. In two further test-tube prepare silver nitrate/nucleophile mixtures comprising 1 cm3 silver nitrate and 1
cm3 water in one and 1 cm3 silver nitrate and 1 cm3 sodium hydroxide in the other.
3. Quickly add a teat pipette full of silver nitrate solution to each tube, shake to mix the contents and start the
stop clock. Observe the tubes closely over the next few minutes and note the time when a precipitate
appears in each tube.
Analysis
1. Write equations to describe each substitution reaction you have performed and state whether it happened
by SN1, SN2 or a mixture.
2. What is the effect of the halogen atom on the rate of substitution? Why does this happen?
3. What is the effect of the halogen position on the rate of substituion? Why does this happen?
4. What is the effect of the nucleophile on the rate of substitution? Why does this happen?
5. Draw one SN1 reaction mechanism and one SN2 from the reactions you have performed.
6. Suggest some modifications to the experiment that would allow us to get quantitative data suitable for an
internal assessment.
Source: Wellington College, intranet.wellingtoncollege.org.uk/resource.aspx?id=266342