Alkyl Halides & Aryl Halides Test - 4

A 1° alkyl halide has only one alkyl group, so it is relatively unstable. It is unlikely to form a 1° carbocation in an S_N1 reaction. Instead, it will take the lower-energy S_N2 path as 1° alkyl is sterically unhindered. More the steric hinderance the substrate becomes less susceptible to S_N2 attack.

Alkyl halides are considered to be very reactive compounds towards nucleophile because they have an electrophilic carbon & a good leaving group as we go down the periodic table, halides that are larger in size will also be able to distribute their charge over a larger volume, making them less reactive (less basic). This is why fluoride is a much poorer leaving group than any of the other halides. So, alkyl halides are good for nucleophilic substitution reactions.

As we can see below, carbon attached to the halogen atom is further attached to one carbon atom.
Example: CH₃-X → Methyl halide
CH₃-CH₂-X → Ethyl halide
CH₃-CH₂-CH₂-X → n-Propyl halide.

Hydration of alkene is electrophilic addition of H₂O to alkenes which forms alcohol not alkyl halides.

The best method of preparation of alkyl halides is a reaction of alcohol with SOCl₂/ Pyridine because by-products formed in the reaction are SO₂ and HCl which are in gaseous form and escape into the atmosphere leaving behind pure alkyl chlorides.

CH₃CH₂−OH + SOCl₂ → CH₃CH₂−Cl + SO₂↑ + HCl↑

Chlorobenzene is less reactive than benzyl chloride. In chlorobenzene the lone pairs present on Cl atom get involved in resonance with π electrons of benzene due to which C−Cl bond acquires double bond character Hence, reactivity decreases. C₂H₅−Cl is more reactive than C₆H₅−CH₂−Cl.

In this reaction aniline is to be treated with nitrous acid to form benzene diazonium chloride, which is further reacted with HBF₄ to form benzene diazonium fluorobarate. This is when heated, undergoes decomposition to give fluorobenzene. This reaction is called as Balz-Shiemann reaction.

The substitution of an aromatic amino group is possible via preparation of its diazonium salt and subsequent displacement with a nucleophile (Cl^–, I^–, CN^–, RS^–, HO^–) is known as Sandmeyer’s reaction.

An industrial process for making chlorobenzene (and phenol) by a gas-phase reaction between benzene vapour, hydrogen chloride, and oxygen (air) at 230°C. The catalyst is copper(II) chloride.

2C₆H₆ + 2HCl + O₂ → 2H₂O + 2C₆H₅Cl

Benzene will react with three molecules of chlorine in the presence of sunlight to give benzene hexachloride.