(i) The polyhalogen compound for which Paul Muller was awarded the Nobel Prize is DDT. (Dichlorodiphenyltrichloroethane). The structure of DOT is given below:
(ii) The use of DOT is banned in some countries because:
(a) it is toxic to aquatic life.
(b) it is not degraded rapidly by soil microorganisms due to its complex structure.
(c) it is not metabolized rapidly by animals and deposited in the fatty tissues.
(d) Because of its stability, it accumulates in food chain leading to biomagnification. 
(iii) Environmental protection and sensitivity towards life.

(i) Halogen containing organic compounds are useful in industry as well as in day to day life. 
For example:
(a) Chloramphenicol: It is a chlorine containing antibiotic produced by soil microbes. It is very effective for the treatment of typhoid fever.
(b) Thyroxine: It is an iodine containing hormone secreted by the thyroid gland in our body. This hormone regulates the metabolic rate of the body and maintains basal metabolic rate. Deficiency of this hormone causes goitre. This disease can be prevented by taking iodised salt and sea foods.
(c) Chloroquine: It is a drug used for the treatment of malaria.
(ii) Awareness about the halogen containing organic compounds which are used for the treatment of diseases like typhoid and malaria and importance of iodine in our diet.

CFC (chlorofluoro carbons) are responsible for the destruction of ozone layer and UV light can reach earth which may cause skin cancer and affect flora and fauna.
These compounds should be banned and their use should be made to minimum extent.

(i) HOCI in presence of an acid generates the reactive electrophile, chloronium ion «r, which attacks benzene to give chlorobenzene (A).
(ii) Chlorobenzene (A) on treatment with NaNH2/liq. NH₃ undergaes-dehydrohalogenation via benzyne to afford aniline (B).
(iii) Aniline (B) on treatment with HBF4 forms the corresponding salt anilinium tetrafluoroborate (C) which on heating with NaNO₂ undergoes Balz-Schiemann reaction through the intermedium formation of benzenediazonium tetrafluoroborate to afford fluorobenzene (D).

We know that S_N2 reactions proceed with inversion of configuration at the stereocentre. If, however, two S_N2 reactions are carried out at the same stereocentre of a compound, retention of configuration will occur. Thus, (R)-2-octanethiol can be obtained from (R)-2-bromooctane by first reacting it with Nal in acetone and then with NaSH in ethanol

(i) HCN (PK_a 10) and NH3 (PK_a 11·25) are very weak: acids. Therefore, their conjugate bases, i.e., CN^- ion and NH₂ ^- are very strong bases. Since strong bases are bad leaving groups, their displacement in nucleophilic substitution reactions is never observed,
(ii) Iodide ion is a powerful nucleophile and hence reacts rapidly with RCI to form RI,
Further because 1- ion is a better leaving group than Cl- ion, therefore, RI is more rapidly hydrolysed than RCI to form ROH,
The I^- ion thus regenerated recycles in the above reaction thereby explaining its catalytic effect.

tert-Alkyl halides prefer to undergo dehydrohalogenation in presence of a strong base such as Na metal instead of undergoing Wurtz reaction as explained below:
\(\underset{tert-Butyl\ bromide}{(CH_3)_3C-Br}+2Na\longrightarrow\underset{tert-Butylsodium}{(CH_3)_3Na^+}+Na^+Br^-\)
Thus, only rand 2° alkyl halides undergo Wurtz reaction while 3° alkyl halides prefer to undergo dehydrohalogenation to form alkenes.

According to the question,
The relative ratios of these two isomeric chlorobutanes can be easily calculated by knowing: (i) the number and type of hydrogens (i.e. 1°, 2° or 3°) to be substituted and (ii) their relative rates of substitution (i.e. I : 3·8 : 5·0 for CI₂ at 298 K). Thus
\({1-Chlorobutane\over 2-Chlorobutane}={No.\ of\ 1^0H\over No.\ of\ 2^0H}\times{Reactivity\ o\ 1^0H\over Reaativity\ of\ 2^0H}={6\over 4}\times{1\over 3.8}={6\over 15.2}={28./.\over 72./.}\)

Free radical halogenation is not a suitable method for laboratory synthesis of alkyl halides because of the following two reasons :
(i) It gives a mixture of isomeric monohalogenated products whose boiling points are so close that they cannot be easily separated in the laboratory.
(ii) Polyhalogenation may also occur to some extent thereby making the mixture more complex and hence more difficult to separate.