Emily Bullas Reaction Portfolio Assignment
Reaction 1: Preparing an Alcohol from a Halogenoalkane
This practical aimed to prepare an alcohol from a halogenoalkane. The reaction occurs when
halogenoalkanes are mixed with ethanol, water and silver nitrate, producing a halide ion precipitate.
When this reaction is carried out under the same conditions for different halogenoalkanes, the time
taken for the precipitate to form indicates the reactivity of the halogenoalkane. The formation of the
halide ion precipitate varies depending on the degree of the halogenoalkane – primary, secondary or
tertiary. In primary halogenoalkanes, the reaction occurs mostly between the halogenoalkane and
water, producing an alcohol, hydrogen ions and halide ions. The reaction equation is: R-Hal + H₂O
→ R-OH + H⁺ + Hal⁻ (1). The reaction mechanism for this reaction is made up of two steps, the first
being a SN2 nucleophilic substitution reaction, shown in figure 1 below, using 1-bromoethane as an
example.
Figure 1: Primary Halogenoalkanes Mechanism Step 1
This reaction is slow because water is a poor nucleophile, as it has a weak charge. The next step
involves a water molecule removing one of the hydrogens which are attached to the oxygen,
producing an alcohol and a hydroxonium ion, as shown in figure 2 below (2).
Figure 2: Primary Halogenoalkanes Mechanism Step 2
Tertiary halogenoalkanes ionise of their own accord to a small extent, with the reversible reaction:
R-Hal ⇌ R⁺ + Hal⁻. For secondary halogenoalkanes, a mixture of the two processes is taking place (1).
The reaction mechanism for this reaction begins with the isolation of the halogenoalkane, shown in
figure 3.
Figure 3: Tertiary Halogenoalkanes Mechanism Step 1
, This is followed by water making a very rapid attack on the carbocation formed, shown in figure 4
below.
Figure 4: Tertiary Halogenoalkanes Mechanism Step 2
This stage of the reaction is a SN1 nucleophilic substitution reaction. The final stage involves a
molecule removing one of the hydrogens attached to the oxygen, giving an alcohol and a
hydroxonium ion, as in the final step of the primary halogenoalkane mechanism. This step is shown
below in figure 5.
Figure 5: Tertiary Halogenoalkanes Mechanism Step 3
For this reaction, water’s poor nucleophilic affects do not control the overall rate of reaction, but
rather how quickly the halogenoalkane ionises (2).
For secondary halogenoalkanes, a mixture of the two processes is taking place (1).
The halogenoalkanes used in this practical were 1-iodobutane, 1-bromobutane, 1-chlorobutane, 2-
bromobutane and 2-bromo-2methylpropane. Each of these chemicals differ in their reactivity, which
was to be shown in the results of this practical.
Method:
To carry out this practical, each of the halogenoalkanes were placed in test tubes with water and
ethanol, heated to around 50°C and then mixed with silver nitrate solution. The time taken to
produce a precipitate was recorded, as well as visual observations of the precipitate. The full method
can be found in Appendix I: Conversion of a Halogenoalkane to an Alcohol. The conditions used in
this reaction were 50°C temperature, and 1 atmosphere of pressure. It also involved the presence of
water.
