Introduction

• Halogenoalkanes have the general formula C_nH_2n+1X.   X is a halogen.

Naming halogenoalkanes

• When a hydrogen is replaced by a halogen, the prefix fluoro-, chloro-, bromo- and iodo- are used:-

                                  CH₃ – Cl                      Chloromethane

                                  CH₃ – CH₂Br               Bromoethane

• 2 isomers can be derived from a monosubstituted propane:-

                                  CH₃ – CH₂ – CH₂Cl    1 – Chloropropane

                                  CH₃ – CHCl – CH₃     2 – Chloropropane

• Side chains are named in the same way as the alkanes:-

                                  CH₃ – CH – CH₂Cl     1 – Chloro – 2 - methylpropane

                                               CH₃

                                  CH₃ – CCl – CH₃        2 – Chloro – 2 - methylpropane

                                               CH₃

• Multi halogen substituted compounds use di, tri to indicate how many of that halogen is present in the compound:-

                                  CH₂Br – CH₂Br           1,2 - dibromoethane

SAQ 12.1

The classification of halogenoalkanes

SAQ 12.2

EXPT – Reactions of the halogenoalkanes

1)  Miscibility with water

• To a test tube add about 1cm³ of water and 1cm³ of a halogenoalkane.

• Place in a bung and shake well.  Record your results.

2)  Hydrolysis with sodium hydroxide

• To 1cm³ of 20% sodium hydroxide solution in ethanol (CARE).

• Add an equal volume of water followed by followed by 0.5cm³ of 2 – chloro 2 – methylpropane. 

• Shake from side to side for a minute.

• To test for chloride ions add an equal volume of 2M nitric acid to neutralise the sodium hydroxide.

• Spot out onto universal indicator paper to ensure the solution is acidic.

• Add a few drops of 0.02M silver nitrate.  If chloride ions are present a white precipitate of silver chloride will appear.

3)  The relative rates of hydrolysis of 1 – halogenobutanes.

• Arrange 3 test tubes in a row and add 3 drops of halogenoalkane in the order:  1 – chlorobutane, 1 – bromobutane and 1 – iodobutane.

• Add 4cm³ of 0.02 silver nitrate to each of the 3 tubes at the same time.

• Put all 3 test tubes in a water bath at the same time.

• Note the order of precipitation.

Interpretation of the experiments

Physical properties

1)   Miscibility with water

• Halogenoalkanes have dipole – dipole forces of attraction where as water has hydrogen bonding.

• The dipole – dipole forces of attraction are not similar in strength. 

• This means that water and halogenoalkanes are immiscible.

• They are volatile but not as much as alkanes

SAQ 12.3

Nucleophilic Substitution

2)  Hydrolysis with sodium hydroxide

The mechanism:-

• Cl is more electronegative than carbon so it has a d-.  This means the carbon bonded to the chlorine in the halogenoalkane is Cd+.

• It will be an advantage if the attacking group has a negative charge.

• Such species are called Nucleophiles and often have negative charges.

• This is a substitute reaction as the – OH has been substituted for – Cl.

• As halogenoalkanes do not mix with water, it is mixed with ethanol before adding dilute aqueous sodium hydroxide.

• Warming causes nucleophilic substitution and an alcohol is made.  A similar but slower reaction occurs with water:

                            CH₃CH₂Cl_(l)     +       OH^-_(aq)           à     CH₃CH₂OH_(l)   +    Cl^-_(aq)

                            CH₃CH₂Cl_(l)     +       H₂O_(l)             à     CH₃CH₂OH_(l)   +    HCl_(aq)

SAQ 12.4

3)  The relative rates of the hydrolysis of 1 – halogenobutanes

• The rate of precipitation:-

                                  Iodoalkane

                                  Bromoalkane

                                  Chloroalkane

• The bond energies give us a good indication of the reactivity of the halogenoalkanes:-

                                  E(C – Cl)   =  340 KJ mol^-1

                                  E(C – Br)  =  280 KJ mol^-1

                                  E(C – I)      =  240 KJ mol^-1

• The weakest would be expected to break first.

• As this one breaks first it will form a precipitate with Ag⁺ first:

Ag⁺_(aq)           +             Cl^-_(aq)          à         AgCl_(s)

• This also tells us that the carbon halogen bond strength overrides the magnitude of the d+ on the carbon.

• Electronegativity of the halogens decreases as you go down group 7.  This means that the C – Cl bond will be more polar than the C – Br bond which will be more polar than the C – I bond:-

                                  d+   d-                   d+   d-                 d+  d-

                                  C – Cl        >          C – Br        >       C – I

• This means that the carbon atom attached to the chlorine will be the most electron deficient.  However the C – I bond breaks first.

Reaction with ammonia - You need the mechanism for this reaction

• Mixed with ehanolic ammonia and heated under pressure produces amines:

                            CH₃CH₂Cl       +       NH₃               à     CH₃CH₂NH₂    +    HCl

• This is a primary amine.

SAQ 12.5

EXPT - Elimination reactions

• To a round bottom flask add 2cm³ of 20% sodium hydroxide solution in ethanol (CARE).

• Add 1cm³ of 1 Chlorohexane 

• Shake from side to side for a minute, add a few anti bumping granules and reflux gently for 20 minutes.

• Allow to cool and set up for distillation.  Distill off  the liquid +/- 2^oC of the boiling point of the product (work it out).

• Add a few drops of bromine water bung and shake.  Note the results.

Interpretation

• When pure alcoholic alkali (KOH) is used and refluxed instead of aq alkali a different reaction occurs:

Mechanism – you do not need to know this

• In this reaction the halogen and a hydrogen are both extracted from the molecule.

• The OH^- is thought to be such a strong base that it pulls a proton off the molecule.

• The halogen then separates from the molecule.

• This is thought to occur in more than 1 stage.

Summary of the reactions of the halogenoalkanes

1)  Preparation halogenoalkanes – from Ch11 - Alcohols

      Preparation of reagent:-

                            2NaBr_(s)           +       H₂SO_4(aq)      à     2HBr_(aq)     +       Na₂SO_4(aq)

      Reaction

                            CH₃CH₂OH_(l)   +       HBr_(aq)           à     CH₃CH₂Br_(l)    +    H₂O_(l)

      Mechanism -    Protonation, Nucleophilic substitution.

      Conditions -     Sulphuric acid / potassium halide, heat.

      Nucleophile -   Br^-

2)  Rate of hydrolysis then precipitation with silver nitrate, AgNO₃

                            1^st                 Iodo > Bromo > Chloro                     3^rd

3)  Substitution by a aq hydroxyl group, OH^-

      Reaction

                            CH₃CH₂Cl_(l)     +       OH^-_(aq)           à     CH₃CH₂OH_(l)   +    Cl^-_(aq)

      Mechanism -    Nucleophilic substitution.

      Conditions -     aq potassium hydroxide, KOH.

      Nucleophile -   OH^-

4)  Substitution by an amine group, NH₃

      Reaction

                            CH₃CH₂Cl_(l)     +       NH_3(aq)           à     CH₃CH₂NH_2(l) +    Cl^-_(aq)

      Mechanism -    Nucleophilic substitution.

      Conditions -     Alcoholic (ethanol) ammonia, NH₃ / heat / Pressure.

      Nucleophile -   NH₃

5)  Elimination to an alkene

      Reaction

                            CH₃CH₂Cl_(l)     +       OH^-_(aq)           à     CH₂ = CH_2(l)    +    Cl^-_(aq)    +       H₂O_(l)

      Mechanism -    Elimination.

      Conditions -     alcoholic (ethanol) potassium hydroxide, KOH / reflux with heat.