CCEA ADVANCED SUBSIDIARY

CHEMISTRY

MODULE 2
 

2.5        Halogenoalkanes

 

General formula : CnH2n+1X     (R¾X)

                                                X= F, Cl, Br, I

Nomenclature

CH3Cl              chloromethane           (methyl chloride)

C2H5Br                        bromoethane              (ethyl bromide)

CH3CHICH3   2-iodopropane           (propyl-2-iodide)

 

Exercise 1

Name the following compounds

1.          CH3CHClCH2CH3                                                                                          

2.         CH3CH2CH(CH3)CH2CHBrCH3                                                                    

3.         CH3CHCl2                                                                                                                  

4.         CH2ClCH2Cl                                                                                                   

5.         (CH3)3CCl                                                                                                      

 

Draw the structural formula of the following compounds

6.                  iodomethane

 

 

 

7.                  2-fluoropropane

 

 

 

8.                  2-bromo-2-methylpropane

 

 

 

9.                  4-bromo-1-iodo-3-methylpentane

 

 

 

Halogenoalkanes are divided into primary (1o), secondary (2o), or tertiary (3o) compounds depending on the number of carbon atoms directly attached to the carbon atom bonded to the halogen atom.

 

Primary           (1o)

                                                         H

                                                          |

CH3Cl           and                     CH3¾C¾Cl                1 carbon atom attached

                                                          |

                                                         H

Secondary      (2o)

                                                           CH3

                                                           |

                                                 CH3¾C¾ Cl              2 carbon atoms attached

                                                           |

                                                           H

Tertiary         (3o)

                                                           CH3

                                                           |

                                                 CH3¾C¾Br               3 carbon atoms attached

                                                           |

                                                           CH3

 

 

Exercise 2

Identify the following as primary, secondary or tertiary halogenoalkanes

1.          CH3CH2CHBrCH3                                                      

2.         CH3CH2CH2CH2CH2Br                                             

3.         CH3CH2C(CH3)2Br                                                                

4.         (CH3)3CCl                                                                  

5.         C2H5I                                                                         

 

 

Physical Properties

Chloromethane (CH3Cl) and bromomethane (CH3Br) are gases.  The rest are oily liquids with a sweet, clinging (cloying) smell.  They are flammable liquids, both denser than, and immiscible with, water.

 

 

 

Preparation of a primary bromoalkane from the corresponding alcohol.

 

 

Preparation of halogenoalkanes

They are generally prepared by substituting the ¾OH group in an alcohol by a halogen atom.

[Know in detail the preparation of 1-bromobutane which includes techniques like

(i)                refluxing 

(ii)    treatment of product to remove solvents and inorganic reagents and

(iii)  purification of product.]

 

 

1.    Reaction with HX

                            reflux

            ROH (l)  +  HX (g)                 RX (l)  +  H2O (l)

The hydrogen halide is prepared 'in situ' by refluxing the sodium salt with the alcohol and excess conc. sulphuric acid.

             reflux                  

e.g.       C4H9OH (l)  +  HBr (g)                      C4H9Br (l)  +  H2O (l)

 

The conc. sulphuric acid serves two purposes

(i)  Generates HBr(g) from NaBr

NaBr (s)  +  H2SO4 (l)    HBr (g)  +   NaHSO4 (s)

 

(ii)  It absorbs water as soon as it forms moving the reaction to the right.

 

PRACTICAL: Preparation of bromoethane                                                     Preparation of 1-bromobutane

 

 

2.  Reaction with PX3 or PX5

 

Cl - use PCl3 or PCl5

Br - use red P + Br2

I - use red P + I2

      reflux

3R¾OH  +  PCl3                                 3R¾Cl  +  H3PO3

      reflux

R¾OH  +  PCl5                               R¾Cl  +  POCl3  +  HCl

 

Reactions with aqueous alkali, ammonia and cyanide ions.

 

 

Nature of the carbon-halogen bond

This is a polar bond all the halogens being more electronegative than carbon

                   ∂+|   ∂-

                 ¾ C¾X

                      |

Bond polarity decreases in the order C¾F > C¾Cl > C¾Br > C¾I

 

Nucleophilic substitution

The reactions of halogenoalkanes are generally the result of the polar nature of the carbon-halogen bond.  This makes the carbon susceptible to nucleophilic attack.  Therefore the most common type of reaction is NUCLEOPHILIC SUBSTITUTION (SN) where the halogen atom is replaced by other functional groups.

 

                H                                               H                                  H             

                |∂+   ∂-                                      |                                    |

Nu- + R¾ C ¾X          Nu-        +     R¾ C¾ X                      R¾C¾Nu +  X-

                |                                                 |                                    |

                H                                               H                                  H

 

 

A nucleophile is a negative ion or neutral molecule with a lone pair of electrons

(e.g. OH-, Cl-, CN-, :NH3, H2O).

 

1.    Reaction with aqueous alkali

Halogenoalkanes react when refluxed with aqueous hydroxide ions forming an alcohol.

                                 NaOH (aq)


C2H5Br  +  OH-                                   C2H5OH  +  Br-

                                    reflux

2.    Reaction with ammonia

Halogenoalkanes react when refluxed with an ethanolic solution of ammonia forming an amine.

                              NH3/ethanol


C3H7Cl  +  NH3                                   C3H7NH2  +  HCl

                                    reflux               amine

a large excess of ammonia prevents formation of 2o and 3o amines.

Note that the HCl formed reacts with the ammonia.


NH3  +  HCl                             NH4Cl

 

 


3.    Reaction with cyanide ions

Refluxing an ethanolic solution of potassium cyanide with a halogenoalkane forms a nitrile.

                        KCN/ethanol


C2H5I  +  CN-                          C2H5CN  +  I-

                            reflux           ethanitrile

 

This is a method of introducing an extra carbon atom into the compound.

 

 PRACTICAL: Reactions of 1-bromobutane                                       

 

Substitution with hydroxide ions.  Mechanisms of the reactions of primary and tertiary bromoalkanes with hydroxide ions, viewed as nucleophilic substitution.

 

 

Hydrolysis

Halogenoalkanes undergo slow hydrolysis with water and more quickly with hydroxide ions forming an alcohol.

                                      reflux

            R¾X  +  OH-                             R¾OH  +  X-

Kinetic studies of this type of reaction indicate that there are two possible mechanisms depending on whether the compound is a primary or tertiary halogenoalkane.

 

Tertiary(3o)       SN1 mechanism

 

(CH3)3C¾Br  +  OH-                            (CH3)3C¾OH  +  Br-

2-bromo-2-methylpropane               2-methylpropan-2-ol

 

This reaction is first order with respect to (CH3)3C¾Br and zero order with respect to OH-.

This suggests that the slow rate determining step is unimolecular and the following mechanism has been proposed for this reaction.

(i)                  CH3                                                                   CH3

                      |                        slow                               |

            CH3¾ C¾ Br                                                   CH3¾ C+       +  Br-

                      |          rate determining step                    |

                      CH3                                                                   CH3

The positive charge on the carbocation (carbonium ion) is reduced by the positive inductive (i.e. electron donating) effect of the three methyl groups which spreads the positive charge thus stabilising the ion.

(ii)  The carbonium ion is attacked by the nucleophile, OH-.

                       CH3                                                                  CH3

                       |                                  fast                                 |

            CH3¾ C +     +   :OH-                                        CH3¾ C ¾ OH

                       |                                                                        |

                       CH3                                                                  CH3

The SN1 mechanism is favoured by tertiary halogenoalkanes.

Primary(1o) SN2 mechanism

 The rate equation for these reactions is given by

Rate = k [nucleophile] [halogenoalkane]

In this case the reaction is first order with respect to both the nucleophile and the halogenoalkane.  This means that the slow rate determining step is bimolecular.  The mechanism is a single step reaction involving a transition state.

CH3__CH2__Br  +  OH-    CH3__CH2__OH  +  Br-

bromoethane                                         ethanol

 

The negative charge of the nucleophile is attracted to the ∂+ charge on the carbon atom.

                        CH3                               CH3          -                                        CH3

                     ∂+|     ∂-                           |                                                         |

  HO:-  +          C¾ Br              HO-----C-----Br                                 HO ¾ C    +    Br-

                                                                                                 

 

                H          H                       H          H                                          H         H

                                                 transition state

 

The planar transition state rapidly converts to the alcohol by expelling the hydroxide ion.

In practise the halogen is replaced with __OH by refluxing with aqueous sodium hydroxide

            reflux

CH3CH2Br  +  NaOH (aq)                              CH3CH2OH  +  NaBr

 

The presence of bromide ions, formed in the reaction, can be shown by acidifying the solution with dilute nitric acid and adding silver nitrate solution. 

A cream coloured precipitate of AgBr forms which is soluble in concentrated ammonia solution.

Ag+(aq)  +  Br-(aq)      AgBr (s)

 

 

PRACTICAL: Effect of halogen atom on the ease of hydrolysis              

 

Relative strength and reactivity of carbon-halogen bonds.  Comparative ease of hydrolysis of 1-chl