Two Mechanisms for Nucleophilic Substitution

This bond is broken... as. ..this bond is formed. 

If the C-X bond is broken as the C-Nu bond is formed, the mechanism has one step. As we learned in Section 6.9, the rate of such a bimolecular reaction depends on the concentration of both reactants that is, the rate equation is second order. 

The preceding discussion has generated two possible mechanisms for nucleophilic substitution a one-step mechanism in which bond breaking and bond making are simultaneous, and a two-step mechanism in which bond breaking comes before bond making. In Section 7.10 we look at data for two specific nucleophilic substitution reactions and see if those data fit either of these proposed mechanisms. 

Rate equations for two different reactions give us insight into the possible mechanism for nucleophilic substitution. 

The numbers 1 and 2 in the names Sn1 and Sn2 refer to the kinetic order of the reactions. For exampie, Sn2 means that the kinetics are second order. The number 2 does not refer to the number of steps in the mechanism. 

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In the last chapter we looked at two mechanisms for nucleophilic substitution SN1 and Sn2. We saw that the Sn2 reaction involved an inversion at the carbon centre. Recall that the incoming nucleophile had to attack the a orbital of the C-X bond. This meant that it had to approach the leaving group directly from behind, leading to inversion of configuration. 

There are two mechanisms for elimination—E2 and El—just as there are two mechanisms for nucleophilic substitution—Sn2 and SnI. 

In the last chapter we looked at two mechanisms for nucleophilic substitution and 5 2. We saw... 

Nomenclature Physical properties Interesting alkyl halides The polar carbon-halogen bond General features of nucleophilic substitution The leaving group The nucleophile Possible mechanisms for nucleophilic substitution Two mechanisms for nucleophilic substitution The S 2 mechanism Application Useful Snj2 reactions... 

There are two mechanisms for nucleophilic substitutions, plus one that is rather special and less common. These are designated, and divided, by their kinetics. The first of these is designated as Sj.jl this stands for substitution, nucleophilic, unimolecular. The rate of the reaction is proportional only to the concentration of the substrate only the substrate is involved in the RDS. The reaction involves two steps, the first of which is rate determining. The example shown in Figure 9.2 will illustrate. In the first, rate-determining, step of the reaction, the carbon-bromine bond is broken heterolytically, to give a bromide ion and a carbocation. The carbocation is then attacked by the lone pair of electrons on a water molecule in a fast step, and finally a proton is shed from the oxygen to yield an alcohol. 

Substitution nucleophilic unimolecular(SNl) mechanism (Sec tions 4 9 and 8 8) Mechanism for nucleophilic substitution charactenzed by a two step process The first step is rate determining and is the ionization of an alkyl halide to a carbocation and a halide ion... 

The ionization mechanism for nucleophilic substitution proceeds by rate-determining heterolytic dissociation of the reactant to a tricoordinate carbocation (also sometimes referred to as a carbonium ion or carbenium ion f and the leaving group. This dissociation is followed by rapid combination of the highly electrophilic carbocation with a Lewis base (nucleophile) present in the medium. A two-dimensional potential energy diagram representing this process for a neutral reactant and anionic nucleophile is shown in Fig. 

This mechanism is exactly analogous to the allylic rearrangement mechanism for nucleophilic substitution (p. 421). The UV spectra of allylbenzene and 1-propenylbenzene in solutions containing NH2 are identical, which shows that the same carbanion is present in both cases, as required by this mechanism. The acid BH protonates the position that will give the more stable product, though the ratio of the two possible products can vary with the identity of BH". It has been shown that base-catalyzed double-bond shifts are partially intramolecular, at least in some cases. The intramolecularity has been ascribed to a conducted tour mechanism (p. 766) in which the base leads the proton from one carbanionic site to the other ... 

Alkyl halides (RX) are good substrates for substitution reactions. The nucleophile (Nu ) displaces the leaving group (X ) from the carbon atom by using its electron parr or lone pair to form a new a bond to the carbon atom. Two different mechanisms for nucleophilic substitution are SnI and 8 2 mechanisms. In fact, the preference between S l and 8 2 mechanisms depends on the structure of the alkyl halide, the reactivity and structure of the nucleophile, the concentration of the nucleophile and the solvent in which reaction is carried out. 

How can these two different results be explained Although these two reactions have the same nucleophile and leaving group, there must he two different mechanisms because there are two different rate equations. These equations are specific examples of two well known mechanisms for nucleophilic substitution at an sp hybridized carbon ... 

There are two mechanisms for nucleophilic aromatic substitution. Both occur in two important steps. In one mechanism, an addition is followed by an elimination. In the other mechanism, an elimination is followed by an addition. 

The paper discussed possible mechanisms for nucleophilic substitution at a cyclopropane carbon with particular attention to retentive replacements. From the NMR spectra of the syn- and anti-bromides and acetates shown above, it was concluded that smooth stereospecific substitution of bromide by acetate had taken place under typical Sis 2-type conditions. For the anti-pair, comprising two solids (one member of the syn pair is an oil), this conclusion was confirmed by X-ray structure determination of both educt and product. 

The two mechanisms for Friedel-Crafts alkylation are not dissimilar to the two mechanisms for nucleophilic aliphatic substitution. In an S,j1 mechanism, a carbocation is generated from an alkyl halide before the nucleophile attacks, but in an S 2 reaction the halide departs simultaneously with the nucleophile attacking the R group. In the Friedel-Crafts reaction, benzene behaves as the nucleophile. 

The two main mechanisms for nucleophilic substitution of alkyl halides are SN1 and SN2. These represent the extreme mechanisms of nucleophilic substitution, and some reactions involve mechanisms which lie somewhere in between the two. 

Fig. 4.7. Two-dimensional reaction energy diagram showing concerted, ion pair intermediate, and stepwise mechanisms for nucleophilic substitution.

There are two possible mechanisms for nucleophilic substitution. In a reaction the molecule first forms a carbonium ion for example ... 

On the basis of a wealth of experimental observations developed over a 70-year period, chemists have proposed two limiting mechanisms for nucleophilic substitutions. A fundamental difference between them is the timing of bond breaking between carbon and the leaving group and of bond forming between carbon and the nucleophile. 

There are two limiting mechanisms of /3-elimination reactions. A fundamental difference between them is the timing of the bond-breaking and bond-forming steps. Recall that we made this same statement about the two limiting mechanisms for nucleophilic substitution reactions in Section 7.4. 

The net result of this process is substitution of the —OR group of the alcohol for the —OH group of the acid. Hence the reaction is referred to as nucleophilic acyl substitution. But the reaction is not a direct substitution. Instead, it occurs in two steps (1) nucleophilic addition, followed by (2) elimination. We will see in the next and subsequent sections of this chapter that this is a general mechanism for nucleophilic substitutions at the carbonyl carbon atoms of carboxylic acid derivatives. 

There are two limiting mechanisms for nucleophilic substitution, namely Sj 2 and Sj l. 

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