Hydrolysis of halogenoalkanes

Introduction

The study of the nucleophilic substitution reactions of halogenoalkanes cuts right across the Core and the Additional Higher Level material on the syllabus. In sub-topic 10.2 Standard Level students need to be able to describe with equations the substitution reactions of halogenoalkanes with aqueous sodium hydroxide and they also need to understand what a nucleophile is and the concept of nucleophilic substitution. In sub-topic 20.1 Higher Level students need to be able to explain these substitution reactions in terms of SN1 and SN2 mechanisms. This means that HL students need to understand the concept of ‘curly arrows’. Higher Level students also need to be able to explain the effect of changing the nucleophile1, changing the alcohol from primary to secondary to tertiary and changing the halogen from chlorine to bromine to iodine on the rate of nucleophilic substitution. Through this practical students investigate all these effects; it therefore covers almost all the HL material as well as the SL. However, SL students should be able to understand much of the HL material especially the use of bond enthalpies to explain why iodoalkanes react faster than chloroalkanes. In fact this theory can then be extended to explain why fluorohydrocarbons make good fire extinguishers. All the explanations and mechanisms for these reactions can be found in Chapter 10 of the IB Study Guide.

Footnote

1 A student once said to me that she understood why the hydroxide ion is a better nucleophile than water as it is a negative ion and much more highly charged. She then went on to comment that she could not see that the nature of the nucleophile would make any difference with tertiary halogenoalkanes as these proceed by an SN1 mechanism. In other words, the nature of the nucleophile should have no effect on the rate of the reaction as the nucleophile does not appear in the rate equation. What would your response be to this?

Teacher's notes

The first set of experiments are actually a revision (UK) review (USA) of the reactions of halide ions with silver nitrate which students have already seen in the practical on Group 17. This is to emphasise that silver ions only form a precipitate with halide ions. When the halogen atom is covalently bonded to a carbon atom then no precipitation with silver ions will occur.

In one sense this practical does not fit sub-topic 10.2 as the nucleophile is not sodium hydroxide. The hydroxide ion is too good a nucleophile for this reaction and no obvious difference would be observed between 1-chlorobutane and 1-iodobutane as they would both react very fast. For this reason a deliberately poor nucleophile is used so that the changes can easily be seen. In fact the iodoalkane will react immediately, the bromoalkane will form a precipitate within five minutes and the chloroalkane will still not have reacted by the end of the five minute period. It will also be seen that the tertiary bromoalkane reacts much faster than the primary bromoalkane with the secondary bromoalkane intermediate.

There are two good explanations as to why bromobenzene is so unreactive towards nucleophilic attack. One is that the delocalised pi electrons in the benzene ring repel the nucleophile and the other is that the pi electrons in the ring delocalise with a non-bonding pair of electrons on the bromine atom making the carbon to bromine bond (C−Br) much stronger and therefore more difficult to break.

This set of reactions has been designed to work well. It perhaps typifies a really good Chemistry experiment which demonstrates the theory and asks students searching questions to ensure that they fully understand the underlying Chemistry. It can also be used to exemplify some social and ethical issues as one famous chloro-organic compound is dioxin. Many students will have heard of dioxin. It was infamously used in the Vietnam War during the 1960 and 1970s. The Americans used it in a compound called ‘Agent Orange’. It was not chemical warfare as such because it was used as a defoliant rather than specifically targeted against people but it had the unfortunate affect of causing many birth deformities. I took the photograph when I visited the War Remnant Museum in Ho Chi Minh City (formerly known as Saigon).

Standard Level Higher Level Student worksheet

HYDROLYSIS OF HALOGENOALKANES

Halogenoalkanes react with an aqueous solution of hydroxide ions according to the equation:

RX + OH− → ROH + X− ( X = Cl, Br or I)

The following experiments examine the effect on the rate of hydrolysis when (a) the halogen is changed and (b) primary, secondary and tertiary halogenoalkanes and a halogenoarene are used. Ethanol is used both as a good solvent and as a rather poor hydrolysis agent in place of hydroxide ions which tend to react too quickly for comparisons to be made.

ENVIRONMENTAL CARE:

The aqueous residues will contain the heavy metal ions of Ag+ and the organic residues will contain halogenated hydrocarbons so it important not to dispose of any waste down the sink. Place all residues in the marked beaker in the fume cupboard. At the end of the practical the two immiscible layers in the combined waste can be separated and the aqueous layer transferred to the 'heavy metals waste' container and the remainder to the 'Organic Waste' container.

SAFETY:

As only small quantities are being used there are no particular hazards associated with this practical. However it is worth noting that some chlorinated organic compounds e.g. dioxin, 2,4,5-trichlorophenoxyethanoic acid (Agent Orange) and polychlorinated biphenyls (PCB's) can be extremely poisonous.

PROCEDURE:

(a) Add a few drops of silver nitrate solution with a concentration of 0.05 mol dm-3 to separate aqueous solutions of NaCl, NaBr and NaI.

(b) Repeat the above experiment by adding silver nitrate to a few drops of 1-bromobutane. Do not confuse a water/oil emulsion with a solid precipitate.

(c) This is a more quantitative experiment to compare the rates of hydrolysis. In each of three test-tubes place 1 cm3 of ethanol. Using separate teat pipettes place 2 drops of 1-chlorobutane in the first test-tube, 2 drops of 1-bromobutane in the second and 2 drops of 1-iodobutane in the third. Stand all three test-tubes in a beaker of water at about 60-65 oC and place another test-tube containing about 5 cm3 of 0.05 mol dm-3 silver nitrate solution in the warm water. Wait until the contents of all the test-tubes have reached approximately 60 oC and then place 1 cm3 of the silver nitrate solution into each of the other test-tubes, and quickly shake each tube to mix the contents. Note and time carefully what you observe throughout the next five minutes.

(d) Repeat the above experiment using bromobenzene, 2-bromobutane and 2-bromo-2-methylpropane.

QUESTIONS

1. What types of reagents (electrophiles or nucleophiles) react with halogenoalkanes? Explain your answer in terms of the polarity of the carbon-halogen bond.

 
2. Explain the significance of experiments (a) and (b).

3. Does the higher temperature have an effect on the reaction with 1-bromobutane ? Explain.

4. Does changing the halogen atom affect the rate of hydrolysis? If so, explain why.

5. Place the different bromo- compounds in increasing order of their rates of hydrolysis and try to explain the order.

This worksheet can also be downloaded from:

Standard Level Higher Level Hydrolysis of halogenoalkanes

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