6.3 halogenoalkanes, alcohols & organic techniques

Question 1

what is a halogenoalkane?

Answer 1

a halogenoalkane is an alkane where at least one hydrogen atom is replaced by a halogen atom.

Question 2

describe the three types of halogenoalkane.

Answer 2

• primary - two hydrogen atoms and one alkyl group.
• secondary - one hydrogen atom and two alkyl groups.
• tertiary - no hydrogen atoms and three alkyl groups.

Question 3

give the reaction mechanism by which halogenoalkanes can be hydrolysed to form alcohols.

Answer 3

nucleophilic substitution.

Question 4

give an example of a nucleophile which readily reacts with a halogenoalkane.

Answer 4

OH⁻

Question 5

during nucleophilic substitution, the C-X bond breaks heterolytically. describe what is meant by heterolytic bond fission.

Answer 5

heterolytic bond fission refers to the process by which both electrons from the bond are taken by the halogen.

Question 6

one way in which halogenoalkanes can be hydrolysed to form alcohols is using water. give one other reagent which could be used in this reaction, and the conditions required for hydrolysis to occur.

Answer 6

• reagent: warm aqueous potassium hydroxide.
• conditions: heating under reflux.

Question 7

give the general equation for the hydrolysis of a halogenoalkane to an alcohol.

Answer 7

R-X + KOH → ROH + KX

Question 8

give the reagent and conditions required to form a nitrile from a halogenoalkane.

Answer 8

• reagent: potassium cyanide in ethanol.
• conditions: heating under reflux.

Question 9

give the general equation which represents the formation of a nitrile from a halogenoalkane.

Answer 9

R-X + CN⁻ → R-C≡N + X⁻

Question 10

halogenoalkanes react with excess ethanolic ammonia to form which group of organic compounds?

Answer 10

amines.

Question 11

halogenoalkanes can also undergo elimination reactions to form alkenes. give the reagent and conditions required for this reaction to occur.

Answer 11

• reagent: warm alkali dissolved in ethanol.
• conditions: heating under reflux.

Question 12

describe a test which could be used to compare the reactivities of halogenoalkanes.

Answer 12

• add silver nitrate solution to three test tubes containing a reaction mixture of a halogenoalkane and ethanol.
• the silver ions in the silver nitrate solution will react with the halide ions to give a silver halide precipitate.
• time how long it takes for a precipitate to form in each test tube - the more quickly a precipitate forms, the faster the rate of hydrolysis, and the more reactive the halogenoalkane.

Question 13

explain why the rate of hydrolysis of halogenoalkanes increases as bond enthalpy decreases.

Answer 13

• the bond enthalpy of the carbon-halogen bond of a halogenoalkane depends on the size of the halogen functional group.
• as the size of the halogen increases, the bond enthalpy decreases, meaning that less energy is required to break the bond.
• as a result, the rate of hydrolysis increases as bond enthalpy decreases.

Question 14

give the order of reactivity of the first four C-X bonds.

Answer 14

most reactive → least reactive:

C-I; C-Br; C-Cl; C-F

Question 15

give the general formula for the alcohol homologous series.

Answer 15

CₙH₂ₙ₊₁OH

Question 16

alcohols can react to form halogenoalkanes using which reaction mechanism?

Answer 16

a substitution reaction.

Question 17

give the reagent required to produce a chloroalkane from an alcohol.

Answer 17

HCl.

Question 18

give an example of a reagent and catalyst which can be used to form a bromoalkane from an alcohol.

Answer 18

• reagent: potassium bromide.
• catalyst: 50% concentrated sulphuric acid.

Question 19

alcohols can be dehydrated to form alkenes via an elimination reaction. give an example of the reagent and the conditions required for this reaction to occur.

Answer 19

• reagent: an acid catalyst, such as phosphoric acid.
• conditions: heating of the reaction mixture.

Question 20

explain why two different alkene products can be formed from one alcohol elimination reaction.

Answer 20

• when an alcohol is dehydrated, water is eliminated from the alcohol.
• the water molecule is made up from the hydroxyl group and a hydrogen atom bonded to the adjacent carbon atom.
• as a result, two possible alkene products can be formed depending on which side of the hydroxyl group the hydrogen atom is eliminated from.

Question 21

give the reagent required to oxidise an alcohol.

Answer 21

acidified potassium dichromate (VI)

Question 22

state the products formed from the oxidisation of the following alcohols:

• primary alcohols.
• secondary alcohols.
• tertiary alcohols.

Answer 22

• primary alcohols - oxidised to aldehydes, and further oxidised to carboxylic acids.
• secondary alcohols - oxidised to ketones.
• tertiary alcohols - cannot be oxidised.

Question 23

give the general formula for the aldehyde and ketone (C=O) functional group.

Answer 23

CₙH₂ₙO

Question 24

Fehling’s solution is an example of a reagent which can be used to differentiate between an aldehyde and a ketone. give one other reagent which can be used to carry out this test, including the results you would expect to obtain.

Answer 24

• reagent: Tollen’s reagent.

expected results

• presence of a ketone: the solution remains clear.
• presence of an aldehyde: the reagent is reduced to solid silver, and forms a ‘silver mirror’ on the surface of the solution.