Leaving groups reactivity order

Indeed the order of leaving group reactivity m nucleophilic aromatic substitution is the... 

In the case of esters, carboxylate anions, amides, and acid chlorides, the tetrahedral adduct may undergo elimination. The elimination forms a ketone, permitting a second addition step to occur. The rate at which breakdown of the tetrahedral adduct occurs is a function of the reactivity of the heteroatom substituent as a leaving group. The order of stability of the... 

The objective in selecting the reaction conditions for a preparative nucleophilic substitution is to enhance the mutual reactivity of the leaving group and nucleophile so that the desired substitution occurs at a convenient rate and with minimal competition from other possible reactions. The generalized order of leaving-group reactivity RSOj" I- > BF > CF pertains for most Sw2 processes. (See Section 4.2.3 of Part A for more complete data.) Mesylates, tosylates, iodides, and bromides are all widely used in synthesis. Chlorides usually react rather slowly, except in especially reactive systems, such as allyl and benzyl. 

The rate at which dissociation of the tetrahedral adduct occurs depends on the reactivity of the heteroatom substituent as a leaving group. The order of stability of the tetrahedral... 

Indeed, the order of leaving-group reactivity in nucleophilic aromatic substitution is the opposite of that seen in aliphatic substitution. Fluoride is the most reactive leaving group in nucleophilic aromatic substitution, iodide the least reactive. 

The order of reactivity for R groups is aUyl = benzyl > tertiary > secondary > > > primary. For the halide leaving groups the order is I > Br > Q. 

Therefore, the mechanism must be concerted. However, in contrast to the concerted oxidative addition of H2, the addition of vinyl and phenyl C-X bonds occurs more rapidly with complexes than with d complexes. The reactions are still second order, and the normal order for leaving group reactivity is foimd. 

The order of alkyl halide reactivity in nucleophilic substitutions is the same as their order m eliminations Iodine has the weakest bond to carbon and iodide is the best leaving group Alkyl iodides are several times more reactive than alkyl bromides and from 50 to 100 times more reactive than alkyl chlorides Fluorine has the strongest bond to car bon and fluonde is the poorest leaving group Alkyl fluorides are rarely used as sub states m nucleophilic substitution because they are several thousand times less reactive than alkyl chlorides... 

Another factor which strongly affects the reactivity of these carboxylic acid derivatives is the leaving-group abihty of the substituents. The order is Cl > OAr > OR > NR2 > 0 so that not only does the ease of forming the tetrahedral intermediate decrease in the order Cl>0Ar>0R>NR2>0 , but the tendency for subsequent elimination to occur is also in the same order. Because the two factors work together, there are large differences in reactivity toward the nucleophiles. 

The role of the leaving group in determining the reaction rate is somewhat different from its role in 8 2 and 8 1 substitution at alkyl groups. In those cases, the bond strength is usually the dominant factor so that the order of reactivity of the halogens is... 

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