Haloalkanes and Haloarenes 3

nucleophiles are electron rich species. Therefore, they attack at that part of the substrate molecule which is electron deficient. The reaction in which a nucleophile replaces already existing nucleophile in a molecule is called nucleophilic substitution reaction. Since the substitution reaction is initiated by a nucleophile, it is called nucleophilic substitution reaction.

Haloalkanes are substrate in these reactions. In this type of reaction, a nucleophile reacts with haloalkane (the substrate) having a partial positive charge on the carbon atom bonded to halogen. A nucleophile, the attacking reagent, attacks the partial positively charged carbon and weakens the C-X bond. A substitution reaction takes place and halogen atom, called leaving group departs as halide ion.

Groups like cyanides and nitrites possess two nucleophilic centres and are called ambident nucleophiles. i) Actually cyanide group is a hybrid of two contributing structures and therefore can act as a nucleophile in two different ways [ºC≡N: « :C≡Nº], i.e., linking through carbon atom resulting in alkyl cyanides and through nitrogen atom leading to isocyanides. ii) Similarly nitrite ion also represents an ambident nucleophile with two different points of linkage [–O— N:=O]. The linkage through oxygen results in alkyl nitrites while through nitrogen atom, it leads to nitroalkanes.

KCN is predominantly ionic and provides cyanide ions in solution. Although both carbon and nitrogen atoms are in a position to donate electron pairs, the attack takes place mainly through carbon atom and not through nitrogen atom since C—C bond is more stable than C—N bond. However, AgCN is mainly covalent in nature and nitrogen is free to donate electron pair forming isocyanide as the main product.

SN2 reactions follow second order kinetics,ie.,the rate of reaction depends upon the concentration of both the reactants(Haloalkanes and nucleophiles). The reaction between methyl chloride and hydroxide ion to yield methanol and chloride ion follows second order kinetics. Rate= k[CH3Cl][OH-]. This rate law implies that both the alkyl halide and the nucleophile are taking part simultaneously in the rate determining step of the reaction.

the incoming nucleophile interacts with alkyl halide causing the carbon-halide bond to break and a new bond is formed between carbon and attacking nucleophile. Here it is C-O bond formed between C and -OH. These two processes take place simultaneously in a single step and no intermediate is formed. The SN2 mechanism is a one-step process in which a nucleophile attacks the substrate, and a leaving group, L, departs simultaneously. Because the reaction occurs in one step, it is concerted. The substrate and the nucleophile are both present in the transition state for this step

SN2 reactions occur through a transition state in which both the reactants are partially bonded to each other. In the transition state, the carbon atom is simultaneously bonded to the incoming nucleophile and the outgoing leaving group. In other words, in the transition state, carbon atom is bonded to five atoms and thus the transition state is unstable and hence cannot be isolated. It ultimately decomposes to form the product and the leaving group.

i)As the reaction progresses and the bond between the incoming nucleophile and the carbon atom starts forming, the bond between carbon atom and leaving group weakens. As this happens, the three carbon-hydrogen bonds of the substrate start moving away from the attacking nucleophile. ii)In transition state all the three C-H bonds are in the same plane and the attacking and leaving nucleophiles are partially attached to the carbon. iii) As the attacking nucleophile approaches closer to the carbon, C-H bonds still keep on moving in the same direction till the attacking nucleophile attaches to carbon and leaving group leaves the carbon. As a result configuration is inverted, This process is called as inversion of configuration.=also called as walden inversion

In SN2 reactions, the attack of the nucleophile occurs from the back side and the leaving group leaves from the front side. SN2 reactions are always accompanied by inversion of configuration.

Since this reaction requires the approach of the nucleophile to the carbon bearing the leaving group, the presence of bulky substituents on or near the carbon atom have a dramatic inhibiting effect. Of the simple alkyl halides, methyl halides react most rapidly in SN 2 reactions because there are only three small hydrogen atoms. Tertiary halides are the least reactive because bulky groups hinder the approaching nucleophiles. Thus the order of reactivity followed is: Primary halide > Secondary halide > Tertiary halide.

SN1 reactions are generally carried out in polar solvents such as water, alcohol, acetic acid. It follows first order kinetics, ie.,the rate of reaction depends upon the concentration of alkyl halide only and is independent of the concentration of nucleophile.

. In the first step, the alkyl halide undrgo ionization to produce carbocation and a halide ion. This step is slow and reversible and hence is the rate-determinig step of the reaction. Step I is the slowest and reversible. It involves the C–Br bond breaking for which the energy is obtained through solvation of halide ion with the proton of protic solvent. Since the rate of reaction depends upon the slowest step, the rate of reaction depends only on the concentration of alkyl halide and not on the concentration of hydroxide ion.

In the second step, the carbocation being a reactive chemical species, is immediately attacked by the nucleophile to give the product. This step is fast and hence does not affect the rate of the reaction.

Further, greater the stability of carbocation, greater will be its ease of formation from alkyl halide and faster will be the rate of reaction. In case of alkyl halides, 30 alkyl halides undergo SN1 reaction very fast because of the high stability of 30 carbocations. 30 carbocation > 20 carbocation > 10 carbocation > CH3+, therefore, reactivity of alkyl halides towards SN1 reactions decreases in the same order, i.e., 30 alkyl halides > 20 alkyl halides > 10 alkyl halides > methyl halides. Further, due to greater stabilization of allyl and benzyl carbocation intermediates by resonance, even primary allylic and primary benzylic halides show higher reactivity in SN1 reactions than other simple primary halides.

For a given alkyl group, the reactivity of the halide, R-X, follows the same order in both the mechanisms R-I> R-Br>R-Cl>R-F. This can be explained in terms of the bond dissociation enthalpies of the C-X bond. C-I has the minimum bond dissociation enthalpy while C-F has the maximum. Therefore, C-I bond is the easiest to break and hence iodoalkanes are the most reactive.