which type of atomic displacement reaction show by tertiary secondary and primary halides explain the mechanism
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Answer:
24.4 Nucleophilic Substitution
The typical SN2 and SN1 mechanisms of alkyl halides do not occur for aryl halides. Nucleophilic substitution does occur, but by two different mechanisms termed addition–elimination and elimination–addition reactions.
The addition–elimination reaction results from attack of a nucleophile at the carbon atom bearing a leaving group, forming a tetrahedral intermediate. We recall that an intermediate is not a transition state. Although the intermediate in this addition–elimination reaction may resemble the transition state structure in an SN2 mechanism, however an intermediate has a lifetime that in some cases may allow for its isolation. The intermediate is not formed by attack of the nucleophile from the back of the carbon–halogen bond, but rather from “front” side.
The addition–elimination reaction, also known as nucleophilic aromatic substitution, occurs only if electron withdrawing groups are bonded to the ring to stabilize the negative charge of the cyclohexadienyl anion. The strongly electron withdrawing nitro group is most effective, but it must be ortho or para to the carbon–halogen bond for resonance stabilization to occur.
The second step in the addition–elimination reaction is the ejection of the halide ion as the leaving group. This step is not rate determining. Thus, the normal order of leaving group tendencies of the halide ions is not observed. Rather, the effect of the halogen atom in stabilizing the cyclohexadienyl anion is observed. As a result of inductive electron withdrawal, the fluorine atom is the most effective in stabilizing the anion and hence the transition state leading to that anion is lowest in energy for the fluoro compound.
The elimination–ddition mechanism of aryl halides requires an amide ion, a very strong base, to abstract a proton from the position ortho to the halogen atom. An elimination reaction in occurs in which the halide ion is the leaving group. A very reactive intermediate called benzyne results. The two carbon atoms in the triple bond of benzyne are equivalent. Attack of the amide ion at either of carbon atom gives an aryl anion which is subsequently protonated by transfer of a proton from ammonia. This step regenerates an amide ion.
When substituents such as a methyl group are also present, the two carbon atoms of the benzyne intermediate are no longer structurally equivalent, and mixtures of anilines result. There is little regioselectivity, so mixtures of isomers usually form. Some regioselectivity may result if only the ring substituent inductively stabilizes charge. Because the charge is located in an sp2-hybridized orbital, resonance stabilization of charge cannot occur.