Haloarenes (aromatic halogen compounds)

 9. Haloarenes

Introduction, Nomenclature and Isomerism

Halogen derivatives of arenes where halo group is directly bonded to aromatic ring are called haloarenes or aryl halides. It may be represented by Ar-X; where Ar is an aryl group and X is halogen (F, Cl, Br or I).   

1.     Write the name of following compound according to IUPAC rule.

              

Preparation

1.      From benzene (by halogenation)

(electrophilic aromatic substitution of benzene)

2.      From benzene diazonium chloride

Sand-Meyer reaction

When benzene diazonium chloride is mixed with cuprous chloride or cuprous bromide dissolved in the corresponding halogen acid at room temperature or warmed, diazo group is replaced by –Cl and –Br respectively. This is called Sand Meyer reaction.

2.     Give an example of Sand-Meyer reaction.                                                           1

3.     Predict the major products of the following reactions:

                                         1

Gatterman reaction

Benzene diazonium chloride when warmed with copper powder along with hydrochloric acid or hydrobromic acid, gives halobenzene. This is called Gatterman reaction.

4.     What products would you expect when benzene diazonium chloride is heated with copper powder in presence in HCl?                                                                                                                           1

5.     Write any three methods of preparation of Chlorobenzene.                  3

Physical Properties

1.       These are generally oily liquids or crystalline solids.

2.       Chlorobenzene is a colourless liquid having aromatic odour.

3.       These are insoluble in water but soluble in organic solvents.

 

 

Chemical Properties

1.      Nucleophilic substitution reaction

Unlike haloalkanes, haloarenes undergo nucleophilic substitution reaction with extreme difficulty. Such low reactivity can be explained as:

a.       Carbon to halogen bond in haloarenes are strong: Resonance effect

Haloarene is resonance hybrid of following structures, which indicates that carbon to halogen bond has partial double bond character. A double bond is shorter and stronger than the corresponding single bond. Hence it is difficult to break down the carbon to halogen bond.

Fig: resonating structure and hybrid of Chlorobenzene

b.      Carbon to halogen bond in haloarenes is less polar: Hybridization effect

In haloarenes halogen atom is bonded to sp² hybridized carbon whereas in Haloalkane halogen atom is bonded to sp³ carbon. (33.33% s- character) is more electronegative than sp³ carbon (25% s-character). Hence carbon to halogen bond in haloarene is less polar than that of haloalkanes.

For example, Chlorobenzene reacts with aqueous NaOH only at 360⁰C and 300 atm producing phenol. (Dow process)

6.     Why is nucleophilic substitution difficult in haloarenes?                      3

7.     Why is haloarene less reactive towards nucleophilic substitution reaction than haloalkane?     2

8.     Why is nucleophilic substitution reaction in Chlorobenzene difficult as compared to Chloroethane?      2

9.     Give reactions why haloalkanes are more reactive than haloarenes in nucleophilic substitution reactions.       2

10. What happens when Chlorobenzene is heated with alcoholic KOH?  

2.      Reduction of Chlorobenzene(with Ni-Al alloy, NaOH, ∆ or LiAlH₄)(preparation of benzene)

11. What happens when Chlorobenzene is treated with Ni-Al in alkaline medium?           1

12. How would you obtain benzene from Chlorobenzene?                                      2

13.  Identify A and B in the following reaction giving pertinent equations.

3.      Action with Na

                        Fittig reaction

Aryl halides when heated with sodium in dry ether as a solvent, biphenyl compounds are obtained like in Wurtz reaction, this is called Fittig reaction.

                        Wurtz-Fittig reaction

Aryl halides when heated with alkyl halides in presence of sodium metal and dry ether as a solvent, alkyl benzene is obtained, this is called Wurtz- Fittig reaction.

14. How would you convert Chlorobenzene into Toluene?                                     1

4.      Action with chloral

15. What happens when Chlorobenzene is treated with chloral in acidic medium?

16. How does Chlorobenzene react with chloral?                                                      1

17. What products would you expect when Chlorobenzene is heated with 2,2,2-trichloroehtanal?  1

18. How would you convert Chlorobenzene into DDT?                                         1

5.      Electrophilic substitution reaction

a.      nitration

Similarly sulphonation, halogenation, Friedel-Craft's acylation and alkylation also occurs slowly in either ortho or para positions.

19. Why does Chlorobenzene give ortho and para products during electrophilic substitution?^#       2

20. Why is haloarene less reactive than benzene in electrophilic substitution reaction?^#   2

_{# refer to class XI}

Uses of Chlorobenzene

1.       As starting material to prepare Chlorotoluene, Aniline, Phenol etc.

2.       To prepare DDT.

MCQs

1.       Which of the following has highest boiling point?

a.       C₆H₅F

b.       C₆H₅Cl

c.        C₆H₅Br

d.       C₆H₅I

2.       When benzene diazonium chloride is heated with the cuprous chloride in corresponding halogen acid, Chlorobenzene is formed. The reaction is

a.       Sand-meyer reaction

b.       Gattermann reaction

c.        Grignard's reaction

d.       Fittig reaction

3.       When benzene diazonium chloride is heated with the Copper and HCl, Chlorobenzene is formed, the reaction is

a.       Sandmeyer reaction

b.       Gattermann reaction

c.        Fittig reaction

d.       All of the above

4.       Bromobenzene and methyl bromide with sodium metal in presence of dry ether give toluene. This reaction is called

a.       Fittig reaction

b.       Wurtz reaction

c.        Wurtz- Fittig reaction

d.       Ullman reaction

5.       C-Cl bond of chlorobenzene in comparison to C-Cl bond in methyl chloride is

a.       Longer and weaker                     c. shorter and weaker

b.       Shorter and stronger  d. longer and stronger

6.       The name of the reaction when halobenzene reacts with sodium metal in the presence of dry ether is

a.       Wurtz reaction

b.       Friedel Craft's reaction

c.        Fittig reaction

d.       Cannizaro's reaction

7.       Biphenyl  is formed from Chlorobenzene by

a.       Fittig reaction

b.       Wurtz reaction

c.        Wurtz-Fittig reaction

d.       Grignard reagent

8.       When Chlorobenzene heated with chloral in the presence of conc. H₂SO_4, the compound formed is

a.       BHC

b.       DDT

c.        Chloropicrin

d.       Chloretone

9.       Chloral is needed for the preparation of

a.       Mischler's Ketone

b.       DDT

c.        Aldrin

d.       Gammexane

10.    DDT is used as

a.       Analgesic

b.       Antipyretic

c.        Insecticide

d.       Antiseptic

11.    Which of the following is o,p-director but deactivating group towards electrophilic substitution reaction.

a.       -CH₃

b.       -Cl

c.        -NO₂

d.       -COOH

 

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