HALOALKANES:
Production:
-FRSR
-Electrophilic Addition on Alkene
*issme Halide jaake uss Carbon se lagta hai jispe zyada alkyl groups houn - major prod.
In the addition of halogens to alkenes, one of the halogen atoms acts as an electrophile and the other as a nucleophile.
-Substitution of an alcohol:
In the substitution of alcohols an alcohol group is replaced by a halogen to form a halogenoalkane
This substitution can be done by reacting alcohol with:
1- HX or KBr with H2SO4 H3PO4 to make Haloalkane
2- PCl3 + heat
3- PCl5 at r.t.p
4- SOCl4
Haloalkanes = Primary (attached to one alkyl group) least stable
Secondary (attached to two alkyl groups)
Tertiary (attached to three alkyl groups) most stable
Reactions of Haloalkanes:
1-Haloalkanes react with NaOH (aqueous) to form alcohol.
- The halogen is replaced by the OH-
- The aqueous hydroxide (OH- ion) behaves as a nucleophile by donating a pair of electrons to the carbon atom bonded to the halogen.
2- Haloalkanes react with ethanolic KCN and are heated under reflux to form Nitrile.
CN acts as a nucleophile.
A 'C' is added to the original chain.
3- Haloalkanes react with ethanolic excess ammonia to form Amine.
Ethylamine formed can further act as a nucleophile and attack haloalkanes to form diethylamine.
4- Haloalkane react w/ aqueous nitrate.
Halogenoalkanes can be broken down under reflux by water to form alcohols (HYDROLYSIS)
This reaction is classified as a nucleophilic substitution reaction with water molecules in aqueous silver nitrate solution acting as nucleophiles, replacing the halogen in the halogenoalkane
We react haloalkanes with aqueous silver nitrate in ethanol to check which halide is present.
Elimination Reaction of Halogens:
In haloalkane's elimination reaction, they lose the hydrogen halide.
Haloalkanes are heated with ethanolic NaOH, causing C-X bond to break heterolytically, forming an X- ion and leaving an alkene as an organic product.
If NaOH(ethanol) is used, an elimination reaction takes place to form an alkene from a halogenoalkane
If NaOH(aq) is used, a nucleophilic substitution reaction takes place to form an alcohol from a halogenoalkane
SN1 & SN2
SN2: Primary Haloalkane:
The SN2 mechanism is a one-step reaction. The nucleophile donates a pair of electrons to the δ+ carbon atom to form a new bond. At the same time, the C-X bond is breaking and the halogen (X) takes both electrons in the bond (heterolytic fission). The halogen leaves the halogenoalkane as an X- ion.
SN1: Tertiary Haloalkane:
2 step process:
C-Br bond breaks forming a carbocation. Carbocation is then attacked by OH.
Since substitution reactions involve breaking the carbon-halogen bond the bond energies can be used to explain their different reactivities.
