Haloalkanes And Haloarenes Notes Class 12
PREPARATION OF ALKYL HALIDES
Numerous ways to make alkyl halides.
- Free Radical Halogenation
(a) Alkane +Cl2 or Br2, heat or light replaces C-H with C-X but gives mixtures
(i) Hard to control (ii) Via free radical mechanism
(b) It is usually not a good idea to plan a synthesis that uses this method – multiple products.
Radical Halogenation: Selectivity
If there is more than one type of hydrogen in an alkane, reactions favor replacing the hydrogen at the most highly substituted carbons.
1. Based on quantitative analysis of reaction products, relative reactivity is estimated for
Cl2: (5:3.5:1 for 3o: 2o: 1o)
2. Order parallels stability of radicals
3. Reaction distinction is more selective with bromine than
chlorine (1700:80:1 for 3o: 2o: 1ο)
- Allylic Bromination: (Allylic means adjacent to a C=C double bond)
The bromination of cyclohexene produces a high yield of 3-bromocyclohexene.
An allylic hydrogen has been substituted for a bromine. The bromine atom abstracts an allylic hydrogen because the allylic radical is resonance stabilized. The radical then reacts with a bromine molecule to continue the chain.
- From Alcohols
(a) Preparation of alkyl chloride:
Note: Tertiary alcohols react with HCl (g) even in the absence of anhydrous ZnCl2.
(b) Preparation of alkyl bromides:
(c) Preparations of alkyl iodides:
- Borodine Hunsdiecker Reaction:
5. Finkelstein Reaction:
The reverse reactions are not possible because NaCl and NaBr are insoluble in CH3OH or acetone.
Aryl halides are compounds where halogen is directly attached to an aromatic ring. They have the general formula ArX, where Ar is phenyl substituted phenyl or a group derived from some other aromatic system e.g.
An aryl halide is not just any halogen compound containing an aromatic ring [C6H5 – CH2 – Cl] is not an aryl halide for the halogen is not attached to the benzene ring. The properties of aryl halides are entirely diﬀerent from that of alkylhalides.
METHODS OF PREPARATION OF ARYL HALIDES
(a) From Diazonium Salts
The Gatternann reaction is a modification of the Sandmeyer reaction. In Sandmeyer reaction, cuprous halides are used which are unstable and difficult to handle, however in Gattermann reaction copper power and a hydrogen halide are used.
(b) By Direct Halogenation of Aromatic Hydrocarbon
This reaction is reversible due to the formation of HI which is a strong reducing agent. To get iodobenzene, HI must be removed from the reaction mixture. To achieve this some oxidising agent like HIO3, HNO3 or HgO is used.
PHYSICAL PROPERTIES OF ARYL HALIDES
(a) Aryl halides are colourless liquids and colourless solids with a characteristic odour.
(b) The boiling point of aryl halide follows the order ArI > ArBr > ArCl > ArF
(c) The melting point of p-isomer is more than o- and m-isomer.
Structure and Reactivity of Aryl Halide and Vinyl Halides:
Chlorobenzene is a resonance hybrid of 5 resonating structures.
Contribution by II, III and IV give a double bond character to the carbon-chlorine bond. Hence C-Cl bond in chlorobenzene is strong. As a result, aryl halides are less reactive compared to the corresponding alkyl halide towards nucleophilic substitution reaction. Similar is the case with vinyl halides.
(a) Therefore attempts to convert aryl halides into phenols, ethers, amines with the usual nucleophilic reagents and conditions are unsuccessful. e.g R-Cl + aq.NaOH → ROH + NaCl
(b) The carbon-halogen bonds of aryl halides and vinyl halides are usually short.
(c) Dipole moments of aryl and vinyl halides are usually small
(d) In chlorobenzene, the chlorine atom is attached to a sp2 hybridized carbon atom whereas, in alkyl chloride, the chlorine atom is attached to a sp3 hybridized carbon atom.
The sp2 hybridized carbon atom is more electronegative than the sp3 hybridized carbon atom, thereby the release of electrons to chlorine atoms is less in chlorobenzene and more in alkyl chloride.
(e) The resonating structure of chlorobenzene indicate that the benzene ring carries a –ve charge at o- and p-positions w.r.t. chlorine atom. Thus the benzene ring definitely takes part in electrophilic substitution reactions.
CHEMICAL REACTIONS OF ARYL HALIDES
The reaction of Aryl halides can be grouped as:
- Nucleophilic substitution reactions
- Electrophilic substitution reactions
- Miscellaneous reactions
Nucleophilic substitution reactions
(a) Dow’s Process: The presence of a nitro group at ortho or para to chlorine increases its reactivity. Further, as the number of such NO2 groups increases the reactivity is increased.
Like NO2, certain other groups have been found to increase the reactivity of chlorobenzene if present at ortho or para to the chlorine atom. These groups are,
Here again the presence of NO2 groups at ortho or para position w.r.t. Cl group increases the reactivity.
Mechanism: The nucleophilic aromatic substitution reaction can be well explained by a bimolecular mechanism.
The intermediate carbonium ion is stabilized due to resonance. The stability of such carbonium ion can be further increased by –R or –M groups at ortho or para positions.
(b) Elimination – Addition Reaction: Reaction with sodamide
2. Electrophilic Substitution Reaction:
Halogens are unusual in their eﬀect on electrophilic substitution reactions: They are electron withdrawing yet ortho and para-directing.
To understand the inﬂuence of halogens, let us consider the intermediate formed when an electrophile attacks the halobenzene at ortho, meta and para positions.
In A, B and C if one considers the inductive eﬀect i.e. (-I eﬀect) of X then A and B would be unstable because the (+) charge comes on the carbon atom carrying the halogen atom X. The structure C will be most stable and the (+) charge does not come on the carbon atoms carrying the halogen atom X.
We should, therefore, expect that halogen atoms attached to the benzene ring would be meta. While directing for electrophilic substitution reactions, the existence of halonium ions has shown that halogen can share a pair of electrons and can accommodate a positive charge. When this idea is applied to the present problem the carbocations formed when an electrophile attacks at ortho or para position i.e. (A) and (B) would be stabilized as below. Whereas the carbocation formed when the electrophile attacks the meta position on halobenzene i.e. C would be destabilized.
The inductive eﬀect causes electrons withdrawing deactivation- the resonance eﬀect tends to oppose the inductive eﬀect for the attack at ortho and para position, and hence makes the deactivation less for ortho and para than for meta. This shows reactivity is controlled by the inductive eﬀect, and orientation is controlled by resonance eﬀect.
Chlorobenzene does not undergo Ullmann’s reaction but if a deactivating group is attached to chlorobenzene then the substituted chlorobenzene can take part in Ullmann’s reaction.