Assertion (A) It is difficult to replace chlorine by -OH in chlorobenzene in comparsion to that in chloroethane.
Reason (R) chlorine carbon ( $\mathbf{C}-\mathbf{C l}$ ) bond in chlorobenzene has a partial double bond character due to resonance
Assertion (A) Hydrolysis of (-)-2-bromooctane proceeds with inversion of configuration.
Reason (R) This reaction proceeds through the formation of a carbocation.
Assertion (A) Nitration of chlorobenzene leads to the formation of m-nitrochlorobenzene.
Reason (R) - $\mathrm{NO}_2$ group is a m-directing group.
Some alkyl halides undergo substitution whereas some undergo elimination reaction on treatment with bases. Discuss the structural features of alkyl halides with the help of examples which are responsible for this difference.
Primary alkyl halides follow $S_N 2$ mechanism in which a nucleophile attacks at $180^{\circ}$ to the halogen atom. A transition state is formed in which carbon is bonded to two nucleophiles and finally halogen atom is pushed out. In $\mathrm{S}_{\mathrm{N}}{ }^2$ mechanism, substitution of nucleophile takes place as follows
Thus, in $S_N 2$ mechanism, substitution takes place. Tertiary alkyl halides follow $S_N 1$ mechanism. In this case, tert alkyl halides form $3^{\circ}$ carbocations. Now, if the reagent used is a weak base then substitution occur while if it is a strong base than instead of substitution elimination occur.
Here, the reagent used is aq. KOH . It is a weak base so, substitution takes place.
As alc. KOH is a strong base, so elimination competes over substitution and alkene is formed.
Some halogen containing compounds are useful in daily life. Some compounds of this class are responsible for exposure of flora and fauna to more and more of UV light which causes destruction to a great extent. Name the class of these halocompounds. In your opinion, what should be done to minimise harmful effects of these compounds.
Some halogen containing compounds are useful in daily life are as follows
Dichloromethane It is used as a solvent as a paint remover, as a propellant in aerosols, and as a process solvent in the manufacture of drugs. It is also used as a metal cleaning and finishing solvent.
Trichloromethane It is employed as a solvent for fats, alkaloids, iodine and other substances.
Triodomethane It is used as an antiseptic. Now, it has been replaced by some other compounds because of its objectionable smell.
But some compounds of this class are responsible for exposure of flora and fauna to more and more of UV light which causes destruction to great extent.
These are as follows
(i) Tetrachloromethane When carbon tetrachloride is released into the air, it rises to the atmosphere and depletes the ozone layer. Depletion of the ozone layer is believed to increase human exposure to UV rays leading to increased skin cancer, eye diseases and disorders, and possible disruption of the immune system. These UV rays cause damage to plants, and reduction of plankton populations in the ocean's photic zone.
(ii) Freons Freon-113 is likely to remain in the air long enough to reach the upper atomsphere. Here, it provides chlorine atoms which damage the ozone layer. Because of this depletion UV rays enters in our atmosphere and become responsible for damage to great extent.
(iii) $p-p^{\prime}$ Dichlorodiphenyl trichloroethane (DDT)
DDT is not completely biodegradable. Instead, it gets deposited in fatty tissues. If ingestion continues for a long time, DDT builds up within the animal and effect the reproductive system.
To minimise the harmful impacts of these compounds i.e., freons, hydrofluorocarbons, fluorocarbons and hydrocarbons can be straight used to make refrigerants and air-conditioning equipments. They are stable in the stratosphere and secure for flora and fauna.