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 Now, it has been replaced by some other compounds because of its objectionable smell.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.
Why are aryl halides less reactive towards nucleophilic substitution reactions than alkyl halides? How can we enhance the reactivity of aryl halides?
Aryl halides are less reactive towards nucleophilic substitution reaction due to the following reasons
(i) In haloarenes, the lone pair of electron on halogen are in resonance with benzene ring. So, $\mathrm{C}-\mathrm{Cl}$ bond acquires partial double bond character which strengthen $\mathrm{C}-\mathrm{Cl}$ bond. Therefore, they are less reactive towords nucleophilic substitution reaction.
(ii) In haloarenes, the carbon atom attached to halogen is $s p^2$ hybridised. The $s p^2$ hybridised carbon is more electronegative than $s p^3$ hybridised carbon. This $s p^2$-hybridised carbon in haloarenes can hold the electron pair of $C-X$ bond more tightly and make this $\mathrm{C}-\mathrm{Cl}$ bond shorter than $\mathrm{C}-\mathrm{Cl}$ bond of haloalkanes.
Since, it is difficult to break a shorter bond than a longer bond, therefore, haloarenes are less reactive than haloarenes.
In haloarenes, the phenyl cation will not be stabilised by resonance therefore $\mathrm{S}_{\mathrm{N}} 1$ mechanism ruled out.
(iv) Because of the repulsion between the nucleophile and electron rich arenes, aryl halides are less reactive than alkyl halides.
The reactivity of aryl halides can be increased by the presence of an electron withdrawing group $\left(-\left(\mathrm{NO}_2\right)\right.$ at ortho and para positions. However, no effect on reactivity of haloarenes is observed by the presence of electron withdrawing group at meta-position. Mechanism of the reaction is as depicted with ${ }^{-} \mathrm{OH}$ ion.
From the above resonance, it is very clear that electron density is rich at ortho and para positions. So, presence of EWG will facilitate nucleophilic at ortho and para postitions not on meta position.