Haloalkanes are highly reactive compounds and undergo a number of reactions such as nucleophilic substitution reaction, elimination reaction, reactions with metals etc.
The halogen atom in haloalkanes is more electronegative than the carbon atom attached to it. As a result, haloalkanes are polar i.e., the carbon atom acquires a partial positive charge and the halogen atom acquires a partial negative charge.
Because of the presence of this partial positive charge on the carbon atom, the carbon atom is prone to getting attacked by nucleophilic reagents (electron rich species). When a strong nucleophile attacks this carbon atom, a new bond between the carbon atom and the incoming nucleophile is formed and the halide ion gets removed.
Nucleophilic substitution reaction can take place in two ways :
In SN1 reaction, the alkyl halide dissociates first to give carbocation and halide ion in a slow process.
The carbocation being unstable is highly reactive and immediately reacts with the nucleophile. This happens very fast.
The second step in SN1 reaction is too fast to affect the rate of reaction. Thus, the rate of reaction only depends on step 1 (slow step). Since only one reactant participates in slow step, the order is 1 (unimolar) and the rate is K[R−X].
In SN2 reaction, the nucleophile attacks the carbon atom from back side of the leaving group (in this case : halide ion). The intermediate state is formed, which is highly unstable and decomposes into products.
Since the rate of reaction depends upon both alkyl halide and nucleophile, it is a second order reaction (bimolar).
The bond dissociation energies of C-X bonds follow the order : C−F > C−Cl > C−Br > C−I. Since the higher the bond dissociation energy, the more difficult it is to break the bond. The reactivity follows the order : RI > RBr > RCl > RF.
SN1 reaction : SN1 reaction depends on the stability of carbocation. The more stable the carbocation, the better its chance to form. The stability of carbocation is : 1° < 2° < 3° (why?) . Therefore, the order of reactivity follows the same order.
SN2 reaction : In SN2 reaction, the attack of the nucleophile occurs from the back at the α-carbon (the carbon atom attached to halide), the presence of any alkyl group at the α-carbon tends to hinder the approach of nucleophile. As a result, the SN2 reactivity decreases with increase in number of alkyl groups at α-carbon.
Try to answer the following questions. Pay attention as they are very important questions.
In elimination reaction, some molecules leave the compound leading to the formation of a double or a triple bond.
Alkyl halides react with some metals to form compounds containing carbon-metal bond which are called organometallic compounds.
We will learn more about nucleophilic substitution reaction, elimination reaction and reaction of halogens with metals in separate sections.