Question

Write short notes on the following: (i) Freons (ii) Sandmeyer reaction (iii) Friedel Craft acylation of haloarene. (2+1½+1½=5)
OR
What happens when (write chemical equation only): (i) Propene reacts with HI (ii) Ethyl bromide reacts with magnesium in the presence of dry ether (iii) Chlorobenzene is heated with sodium in dry ether (iv) The mixture of chlorobenzene and methyl chloride is heated with sodium in the presence of dry ether (v) Isopropyl alcohol reacts with conc. HCl and ZnCl₂. (1+1+1+1+1=5)

Hint:

Part 1: Freon-12 by Swarts reaction, Sandmeyer uses diazonium + cuprous halide, and the ring halogen is ortho/para directing in acylation. Part 2: apply Markovnikov (i), Grignard formation (ii), Fittig (iii) and Wurtz-Fittig (iv) coupling, and Lucas reagent (v).

Answer 1

Option 1 — Short notes

(i) Freons: Freons are the trade name for chlorofluorocarbons (CFCs), the fully halogenated derivatives of methane and ethane in which every hydrogen is replaced by chlorine and fluorine atoms. The most common member is Freon-12, dichlorodifluoromethane \( CCl_2F_2 \). It is manufactured from carbon tetrachloride by the Swarts reaction, in which chlorine is exchanged for fluorine using antimony halides as catalyst: \[ CCl_4 + 2HF \xrightarrow{SbF_5} CCl_2F_2 + 2HCl \] Freons are colourless, odourless, non-toxic, non-corrosive and chemically inert gases. Because of these properties they are used as refrigerants in refrigerators and air conditioners and as aerosol propellants. In the stratosphere ultraviolet light splits them to give chlorine free radicals which destroy the ozone layer, so their use is now restricted.

(ii) Sandmeyer reaction: When a primary aromatic amine (aniline) is treated with sodium nitrite and dilute HCl at 273–278 K it forms a benzene diazonium salt. When the diazonium chloride solution is treated with cuprous halide (cuprous chloride in HCl, or cuprous bromide in HBr), the diazonium group \( -N_2^+ \) is replaced by the halogen and nitrogen gas is evolved. This is the Sandmeyer reaction: \[ C_6H_5N_2^+Cl^- \xrightarrow{CuCl / HCl} C_6H_5Cl + N_2 \] \[ C_6H_5N_2^+Cl^- \xrightarrow{CuBr / HBr} C_6H_5Br + N_2 \] With cuprous cyanide (CuCN) the cyano group is introduced to give benzonitrile. It is a convenient way to place a halogen exactly where the amino group was on the ring.

(iii) Friedel-Craft acylation of haloarene: A haloarene such as chlorobenzene reacts with an acyl chloride (for example acetyl chloride, \( CH_3COCl \)) in the presence of the Lewis acid catalyst anhydrous aluminium chloride \( AlCl_3 \). The \( AlCl_3 \) generates an acylium ion \( CH_3CO^+ \) that attacks the ring. Although the halogen is deactivating, it is ortho and para directing, so the acyl group enters mainly at the para position (with some ortho): \[ C_6H_5Cl + CH_3COCl \xrightarrow{\text{anhyd. } AlCl_3} p\text{-}ClC_6H_4COCH_3 + HCl \] The main product is para-chloroacetophenone.