3.3 Halogenoalkanes

A student investigates the rate of reaction of five halogenoalkanes in the method outlined below:

Step 1
The student mixes ethanol with six drops of halogenoalkane and warms the mixture in a water bath at 50 °C.

Step 2
They then added silver nitrate solution.

Step 3
The time taken for the precipitate to form is given in Table 1.

Table 1

Use the information in Table 1 given in part (a) to describe and explain the factors that influence the rate of this type of reaction in halogenoalkanes.

When a fresh sample of aqueous 1-bromopentane reacts with ethanol a compound that contains bromine will be produced. Give the name of the compound containing bromine.

1-chloropentane, which has a boiling point of 108 C, was prepared in the laboratory through the reaction of an alcohol and thionyl chloride, SOCl₂, which has a boiling point of 74.6 C. Two additional products are also produced in this reaction.

2-bromo-2-methylpropane is refluxed with ethanolic potassium hydroxide, KOH. The result is a mixture of products containing roughly a 4 : 1 ratio of alkene to alcohol.

Suggest why there is a mixture of an alkene and an alcohol produced in the reaction outlined in part (a).

An iodoalkane can be prepared by the reaction between red phosphorus, P, and iodine, I₂, to produce phosphorus (III) iodide, PI₃.

This then is reacted with an alcohol to produce the iodoalkane and phosphonic acid, H₃PO₃.

Give the equation for the reaction between butan-2-ol and phosphorus (III) iodide. You do not have to include state symbols.

During the series of reactions to produce a bromoalkane a 50 : 50 mix of sulfuric acid and water is reacted with potassium bromide to produce hydrogen bromide. The mixture is kept very cool.

Explain why sulfuric acid can not be used in the preparation of an iodoalkane

Compound G is a chloroalkane that can undergo two different reactions as outlined in Figure 1.

Figure 1

Compound H is an alkene which does not exhibit geometric isomerism.

Name and outline the mechanism for reaction B, shown in part (a).

Air conditioners manufactured before 1995 commonly used CFCs as a refrigerant. Equipment manufactured before 2010 used hydrochlorofluorocarbons (HCFC) as a refrigerant. New equipment will now use hydrofluorocarbons (HFC) which contain no chlorine.

2-chloropropane can be formed from propane through the mechanism of free radical substitution. The mechanism requires three steps; initiation, propagation and termination.

Figure 2

The reaction pathway outlined in Figure 1 shows two different reactions of a cyclic iodoalkane to produce compound A and compound B.

Compound A reacts with bromine water to give a colourless solution.

Use your knowledge of halogenoalkane reactions to name and outline a possible mechanism of reaction 1 and draw the structure of the compound produced.

Compound B can be oxidised to produce a ketone.

Figure 1

Compound B is also produced in the reaction scheme given in part (a).

1,5-dibromooctane is reacted with ammonia to produce a cyclic compound known as coniine which is a poisonous compound. 

A reaction pathway to produce a carboxylate salt, compound M, which contains three carbon atoms is shown in Figure 2.

Figure 2

Give the correct reagents to form compound L from 1-bromoethane.

A chemist has a sample of 1-chloropropane, which he wants to convert to a molecule with a carbon chain length of four.

Name and draw the reaction mechanism for the reaction which the chemist would use, and suggest suitable reagents and conditions for this conversion to occur.

Hydroxide ions will react with halogenoalkanes in different ways, depending on the reaction conditions. The hydroxide ions either act as a base or as a nucleophile, and different products are formed by two different reaction mechanisms.

Table 1

The reaction of 1-chloropropane with hot, ethanolic potassium hydroxide results in the formation of a product with a molecular weight of 42.

Draw the mechanism for this reaction and state the IUPAC name of the product formed. 

Whether the hydroxide ion acts as a nucleophile or base when reacted with halogenoalkanes does not only depend on the reaction conditions.

Butylamine is an organic compound used as an ingredient in the manufacture of pesticides.

Figure 1 below shows the overall reaction equation for the formation of butylamine from 1-chlorobutane.

Figure 1

Draw the reaction mechanism for the reaction that occurs in Step 1 and label compound X in your mechanism.

Include all necessary partial and full charges.

Compound X undergoes another reaction in Step 2 before butylamine is formed.

A chemist can form but-1-ene either by thermally cracking octane or by reacting 1-chlorobutane with hydroxide ions. The incorrect reaction mechanism for this elimination reaction is shown in Figure 2.

Figure 2

Identify the errors shown in the reaction mechanism in Figure 2.

Suggest which of the methods stated above for synthesising but-1-ene would give a higher atom economy of the product. 

CFCs were initially introduced in the 1930s by the American engineer Midgley for use in refrigerators. They were especially popular due to their non-toxicity and non-flammability.

However, later research showed that CFCs were involved in the depletion of the ozone layer as they release radicals in the upper atmosphere.

An example of a CFC is chlorotrifluoromethane.

Due to their damaging effect on the ozone layer, chemists have now developed alternative compounds to CFCs which can be used as solvents and refrigerants.

An example of such a compound is shown in Figure 1.

Figure 1

Bromotrifluoromethane is another halogenated organic compound which is used in fire extinguishers in aircrafts.

The overall equation for the formation of bromotrifluoromethane from trifluoromethane is as follows:

CHF₃ + Br₂ → CBrF₃ + HBr

The bond enthalpies of some carbon-halogen bonds are shown in Table 1.

Table 1 

Explain, using the data in the table above, whether the environmental impact of CBrF₃ on the ozone layer is more or less severe than that of CClF₃.

The halogenoalkanes have a number of properties and undergo a number of different reactions.

Draw the structure of 1-bromo-2-methylpropane and explain why it is not soluble in water.

A chemist reacts 2-chlorobutane with potassium hydroxide to form product A and product B.            

Table 1 summarises some properties of the two products formed in the two different reactions.

Table 1

State the type of reaction that has occurred to give each product and draw the skeletal formulae of both products.

When 2-chlorobutane reacts with hot ethanolic potassium hydroxide, two structural Isomers are formed. One of these structural isomers also displays stereoisomerism.

Predict whether isomers are formed in the reaction of 2-chlorobutane with cold, aqueous potassium hydroxide.

State the IUPAC name of any products formed during this reaction.