Planning an investigation to determine the reactivity of HalogenoAlkanes

In this plan, I will devise an experiment that will enable me to determine which of 3 halogenoalkanes is the most reactive. A halogenoalkane is a halogen containing alkane; in this investigation I shall be using 1-bromobutane, 1-chlorobutane and 1-iodobutane and an aqueous solution of Silver Nitrate to test the reactivity of each.

Examining bond polarity, halogens are more electronegative than carbon; a polar bond will form, where the halogen as delta negative in charge.

The order of polarity is Cl>Br>I, with Chlorine has the highest electronegativity and thus forms the most polar bond with Carbon of the three. Similarly, Iodine has the lowest and forms the least polar bond. As the polarity of the bond increases, the repulsion of opposite charges increases, so this would suggest that a C-Cl bond would result in the highest polarity and repulsion, requiring the least energy to be applied to be broken of the three C-Hal bonds. Therefore, of the three halogenoalkanes, that containing Chlorine should be the most reactive.

There is, however, a contradicting theory. When looking at bond strength, C-I needs to be applied with the lowest amount of energy in comparison to the other two for it to be broken. Understandably, this greatly contradicts the previously given theory according to bond polarity. Within any reaction that occurs, the reactants must collide with sufficient energy to react, which is known as activation energy. There are more likely to be more molecules with the required activation energy to break the C-I bond than the C-Cl at a given state, meaning that a halogenoalkane containing Iodine is the most reactive.

Taking this into consideration, it is difficult to make a prediction as to which of the three is the most reactive. Whether it is the halogenoalkane that contains Iodine or Chlorine remains unclear. The theories are not comparable, because the value of the data is not the same, and due to this I am unable to make a satisfactory prediction.

The polar nature of the C-Hal bond makes it susceptible to attack from a nucleophile (an electron lone pair donor); in this instance, where I will be using an aqueous solution of Silver Nitrate, H2O will act as ...

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In a substitution reaction, the halogen atom will leave as a halide, meaning the atom or group of atoms replacing the halogen atom must possess a lone-pair of electrons. The lone-pair is donated to the δ+ carbon atom, forming a new covalent bond. Ref 1

Although the free halide ion may loosely associate with free Hydrogen ions, it will mainly bond with Silver ions, forming an opaque precipitate within the solution. It is this precipitate, and the time taken for it to form, that I will use to measure the reactivity when hydrolysing the different halogenoalkanes.

The colour of the precipitate that forms depends on the halide reacting:

Silver Chloride forms white precipitate
Silver Bromide forms cream precipitate
Silver Iodide forms yellow precipitate. Ref 2

The equations for the reactions that will take place are identical, excluding a change in halogen, of course, are as follows for Bromine:

CH3CH2CH2CH2Br + HO CH2CH2CH2CH2OH + H+ + Br- (HBr)

Ag+ (aq) + Br- (aq) AgBr (s)

By simply replacing “Br” with either ‘Cl’ or ‘I’, the equation remains the same. Drawn beneath the first equation is the mechanism of the nucleophilic substitution that occurs.

Halogenoalkane properties

Because halogenoalkanes are typically volatile liquids that do not mix with water, I shall be using a cosolvent that will dissolve both, ethanol. The ethanolic Silver Nitrate will allow the halogenoalkane to react with the water, and the halide ion to leave. It is it then able to bond with free Silver ions.

The halogenoalkanes have fairly low boiling points, and I do not want for them to be partially evaporating as the experiment occurs, as it would allow for anomalies to occur. As the lowest boiling point is for 1-chlorobutane, which is 79°C, I will ensure that as a constant, the temperature is kept well below for every test. This constant remains a fair test, and will prevent the evaporation that I want to avoid.

Previously, I measured out 1 cm3 of water, and counted out how many drops it formed. By doing this, I can determine the volume of one drop of water, or of any liquid, as they will all be the same. Volume of 1 drop is 0.4166....cm3.

I shall be using 1 cm3 of both ethanol and of Silver Nitrate (0.05M) and I need to work out what volume of the halogenoalkanes I should use, so as to maintain a fair test. As it 0.05M dm-3, I will need to divide through by 1000 to get the desired volumes of halogenoalkane required in each case.

n = m / Mr v = m / d

Using 1-chlorobutane as an example, I will work out the volume required.

m = 0.00005 x 92.6= 4.63 x10-3 g

v = (4.63 x10-3) / 0.886 = 5.225733634 cm3 x10-3 v = 5.23 cm3 (3 S.F)

The smallest volume I can measure is one drop, and that is far greater than any of the volumes of halogenoalkanes required. This means that each of them will be in great excess, which more or less eliminates the error by having each in the same excessive situation.

Method

Measure out 1 cm3 of ethanol and Silver Nitrate solution into a test tube, and mark a cross on the opposite side. Place in a beaker of water allow to reach the temperature of 45°C.
Start a timer when adding one drop of 1-chlorobutane.
Record the time taken for a precipitate to form and obscure the cross marked on the other side.
Repeat twice.
Repeat steps 1-4 for both 1-bromobutane and 1-iodobutane.

By having each halide reacting three times, I can derive an average time taken for the cross to obscured. The faster it occurs, the more reactive it is. I can then deduce which of the three is the most reactive.

Whilst doing the experiment, it is vital to beware of the safety precautions. Silver Nitrate is corrosive and causes burns and can form explosive silver fulminates with ethanol. Goggles should be warn to prevent damage to eyes, and care when handling to prevent any contact with skin.

The halogenoalkanes are all highly flammable, but 1-bromo and 1-iodobutane are irritants to the eyes and skin, and can cause the respiratory system harm, so all reactions should take place in a well ventilated fume cupboard. Ref 3

References

Ref 1 – information extracted from textbook “Chemistry 1”, page 138

Ref 2 – information extracted from textbook “Advanced Chemistry for you” page 124

Ref 3 – healthy and safety information from “Hazcards”

Apparatus required

Tony Lewis