Aryl halides | Structure, preparation, properties and facts
CHAPTER 6: Aryl Halides and Aralkyl Halides
B.sc 1st year BookOrganic Chemistry(Page 1)
Introduction: aromatic halogen compounds are of two types- (a) aryl halides and (b) aralkyl halides. The distinction is based on the fact that in the former one the halogen atom is directly attached to the aryl ring, such compounds are less reactive. If the halogen substituent is attached to the carbon in the side chain constitutes aralkyl halides. These are much more reactive compounds.
Methods of preparation of Aryl Halides :
By Direct Halogenation of Arenes:
Chloroarenes, bromoarenes, and iodoarenes can be prepared by direct halogenation of arenes. Chloroarenes and bromoarenes are easily obtained by treating chlorine and bromine in presence of Lewis acids such as anhydrous ferric or aluminum halide as catalysts. For examples :
If an excess halogen is used, disubstituted products (ortho and para) are likely to be formed.
This is because the Chloro and Bromo benzenes formed in reactions I and II are ortho and para directing.
lodination is difficult because the HI formed during the reaction being a powerful reducing agent reduces the aryl iodide to arene and iodine.
However, iodoarenes can be obtained by carrying out the reaction in presence of oxidizing agents such as nitrogen acid, iodic acid, etc. The HI produced is oxidized to iodine and the reaction proceeds in the forward direction.
2HI + HNO3 → 2NO2 + 2H2O
5HI + HIO3 → 3I2 + 3H2O
Similarly, direct fluorination is difficult owing to the high dissociation of F2 molecule and inability the formation of F+ ion thereafter.
From Arylamines (anilines) via Diazonium salts :
(a). By Sandmeyer Reaction:
The reaction between aryl diazonium salt (Ar-N2-X) and cuprous salt (Cu2X2 X = Cl or Br)
in presence of HX (HCl or HBr) yields corresponding chloro or bromo arenes.
Nucleophiles such as Cl– or Br– replace the diazonium group. This reaction is known as the “Sandmeyer reaction“. An iodo substituent will replace the diazonium group if potassium iodide is added to the solution containing diazonium salt.
Fluorination occurs when the aryl diazonium salt is heated with fluoroboric acid. This reaction is known as the “Schiemann reaction“.
(b) By Gattermann Reaction:
In this reaction, chloro and bromo arenes are prepared by heating aryl diazonium salt with copper powder in presence of halogen acid( HCl or HBr).
By Hundsdiecker Reaction:
In this reaction, bromoarenes are prepared by heating silver salt of an aromatic acid with bromine in CCl4following decarboxylation.
Melting and boiling points of haloarenes are the same as those of haloalkanes with the same number of carbon atoms. The melting point gradually increases as the size of the halogen atom decreases ( if the aryl group is the same). There is also an increase in melting and boiling points with the increase in the number of carbon atoms (if the halogen atom is the same). They are insoluble in water. They do not form hydrogen bonds with water. However, they are quite soluble in organic solvents of low polarity such as petroleum, ether, benzene, chloroform carbon tetrachloride, etc. As far as the density is concerned al haloarenes are heavier than water.
Reactions of halorenes are classified as :
(1) Nucleophilic substitution reactions
(2) Electrophilic substitution reactions
(3) Miscellaneous reactions
(1) Nucleophilic Substitution reactions:
Haloarenes are less reactive towards nucleophilic substitution reactions than haloalkanes. Despite having a C-X bond haloarenes are less reactive(whereas haloalkanes are highly reactive) towards nucleophilic substitution reactions. The low reactivity of haloarenes is attributed to the following reasons :
(a) Resonance Effect:
Haloarenes can be represented as a resonance hybrid of the following structures
As a result of resonance C−X bond acquires some double bond character. Consequently, the C−X bond in haloarenes is much stronger than that in haloalkanes. Besides this energy of activation for the displacement of a halogen atom from haloarenes nucleophilically is much greater than that of alkyl halides.
The carbon atoms of haloarenes and haloalkanes are sp2 and sp3 hybridized respectively Therefore, the C-X bond should be shorter and stronger in haloarenes than those in haloalkanes.
(c) Polarity of carbon halogen bond:
The sp2 – carbon atom in haloarenes is more electronegative than that of the Sp3 – carbon atom of haloalkanes. Due to sp2 hybridization and the more electronegative nature of carbon, the C−X bond in haloarene is less polar than that of the C−X bond in haloalkane.
Haloarenes undergo nucleophilic substitution reactions under extreme conditions as halo substituents withdraw electrons. If electron-withdrawing groups are present ortho or para to the leaving group nucleophilic substitutions can occur without using extreme conditions.
Mechanism I :
Nucleophilic aromatic substitution takes place by a two-step reaction known as an SN Ar reaction substitution nucleophilic aromatic. In the first step, the nucleophile attacks the carbon bearing the leaving group perpendicular to the aromatic ring. Thus, nucleophilic attack forms a resonance-stabilized carbanion intermediate. In the second step of the reaction, the leaving group departs a pair of electrons and regains the aromaticity of the ring.
Since the first step is addition and the second elimination this bimolecular mechanism is also known as the ‘addition-elimination mechanism‘.
Evidence: The involvement of the intermediate and therefore, the presence of two steps can be supported by the energy profile which is shown in figure 6.01.
Elimination-Addition or Benzyne mechanism:
Treatment of chlorobenzene with sodamide in liquid ammonia at −33∘C gives aniline.
Chlorobenzene labeled with 14C at the position bearing chlorine gives an equimolar mixture of unrearranged and rearranged products. Point out the mediation of a symmetrical species, benzyne which is formed from chlorobenzene by an E2 type elimination and reacts with ammonia to give aniline. Thus is the addition of H+ and NH2– results in the final product- aniline. These different media give aniline. Thus, an as elimination-addition mechanism.
Other examples of elimination-addition reactions are :
(d) Reaction with cuprous cyanide:
When heated with cuprous cyanide at 200∘C in presence of pyridine or DMF, the bromine atom of the bromoarene is replaced by the CN group.
Note: Benzonitrile is a useful compound for the synthesis of many other compounds such as benzylamine, benzoic acid, benzamide, etc.
(2) Electrophilic Substitution reactions:
Haloarenes undergo the Hallmark electrophilic substitution reaction of the benzene ring such as halogenation, nitration, sulphonation, and Friedel-Crafts reactions. The halogens deactivate the ring towards electrophilic attack but direct ortho and para positions. Therefore, if further substitution occurs the electrophile attacks at ortho and para positions with respect to the halogen atom already attached. However, because of a steric hindrance at the ortho positions, para-product usually predominates over the ortho product.
(3) Miscellaneous Reactions :
(i) Reaction with sodium: Haloarenes when treated with sodium in presence of an ethereal solution of at alkyl halide, yield alkyl benzene. This reaction is called the “Wurtz-Fittig reaction“.
When haloarenes alone are treated with sodium, diaryls are formed. This reaction is called the “Fitting reaction“.
(ii) Reaction with copper: When an iodoarene is heated with copper powder in a sealed tube, a diaryl is formed. This reaction is called the “Ullmann reaction“. e.g.
(iii) Reaction with magnesium: When etheral solution of bromoarenes and iodoarenes are treated with magnesium turning, aryl magnesium bromides and aryl magnesium iodides are formed. On the other d chloroarenes react with magnesium in presence of tetrahydrofuran(THF) to form aryl magnesium ides. These products are called “Grignard reagents“
(iv) Reaction with lithium: Bromo and iodoarenes react with lithium metal in dry ether to form their responding organolithium compounds.
(v) Reduction: Haloarenes on reduction with nickel-aluminum in presence of alkalies, yield spending arenes.
The halogens ( F, Cl, Br, and I) deactivate the ring towards electrophilic attack but direct ortho and para why?
Reason: We have seen in the previous chapter that deactivating groups are meta-directors toward the electrophilic but in the case of halogens that is not so. Halogen deactivates the benzene ring yet o,p-directors. The only way to is if there are two opposing effects- electron donation by conjugation and electron withdrawal by induction. The has three lone pairs, one of which may conjugate with the ring just like phenol or aniline.
Since the ring is deactivated by induction, there is an opposing effect-electron donation by ionization which certainly dominates and thereby activates the ring. Consequently, there is an ortho-directing effect.