Q.(a)

Birch reduction reduces an aromatic ring to a 1,4-diene using Na/liquid NH3/alcohol; environmental pollution can be curbed through cleaner fuels, wastewater treatment, and afforestation/waste management.

1. Birch reduction: Birch reduction is a method for the partial reduction of an aromatic ring to a non-conjugated (1,4-) diene, using an alkali metal (typically sodium, though lithium or potassium also work) dissolved in liquid ammonia, together with an alcohol (such as ethanol) as a proton source. Benzene, when treated with Na / liquid NH3 in the presence of ethanol, is reduced to 1,4-cyclohexadiene: C6H6 (benzene) --(Na, liquid NH3, EtOH)--> 1,4-cyclohexadiene (C6H8) Mechanistically, the alkali metal donates an electron to the aromatic ring to form a radical anion; the alcohol then protonates this intermediate, and a second electron/proton addition follows, ultimately breaking the aromaticity and leaving two of the original three double bonds, specifically in the 1,4- (non-conjugated) arrangement rather than the conjugated 1,3-diene. This selective, partial reduction (leaving some unsaturation, unlike full catalytic hydrogenation, which would give cyclohexane) is the distinguishing feature of Birch reduction.
2. Three strategies to control environmental pollution:

1. Use cleaner fuels and emission controls for vehicles: switching to unleaded petrol, CNG, or other low-emission fuels, and fitting catalytic converters, reduces the release of carbon monoxide, unburnt hydrocarbons, and oxides of nitrogen/sulphur into the air, cutting air pollution. …