Sjsjl mechanism

Snce it h been denmnstrated, however ttiat in Sjsjl mechanism, proceedii with partic tion of the ion-pairs, some inversion of configuration is possible (earlier reserved exclusively for 2 processes involving oxonium ions) the stereochemistry of the products lost its diagnostic power for reaction mechanism. Thus, it would be premature to draw conclusions about the structures of growing species on the basis of data given by Blanchard Nevertheless, the h%h probability of racemic diads formation (P, = 0.62) may indicate participation of carbenium ions. 

In almost all cases, tertiary alkyl halides react rapidly with nucleophiles by the Sjsjl mechanism. The nature of the nucleophile is not important it does not affect the rate and carbocations are reactive enough to combine with even quite weak nucleophiles. 

The Sjsjl mechanism for nucleophilic aromatic substitution— diazonium compounds... 

The mechanism is a rather unusual type of ester cleavage reaction. You met, in Chapter 17 and again in Chapter 25, the cleavage of t-butyl esters in acid solution via an Sjsfl mechanism. In the reaction we are now considering, the same bond breaks (O-alkyl)—but not, of coiuse, via an Sjsjl mechanism because the alkyl group is Me. Instead the reaction is an 5 2 substitution of carboxylate by Cl. ... 

The Sjsjl mechanism begins with ionization to form a carbocation, attack of a nucleophile, and in the case of ROH nucleophiles, removal of a proton by a base to form a neutral product. 

The carbanion is in an sp orbital in the plane of the ring. Indeed, this intermediate is very similar to the aryl cation intermediate in the Sjsjl mechanism from diazonium salts. That had no electrons in the sp orbital the carbanion has two. 

The experiments also provide stereochemical evidence that a carbocation is an intermediate in both reactions. Both starting materials are cis-decalins but the product is a fra s-decalin. The carbocation intermediate has no stereochemistry and can react with the nitrile from either face. Axial attack is preferred and it gives the stable fram-decalin. The formation of the carbocation is shown only by the Beckmann fragmentation formation from the alcohol by the Sjsjl mechanism is obvious. 

Steric bulk means that f-butyl esters are resistant to nucleophilic attack at the carbonyl group, and that includes hydrolysis under basic conditions (nucleophilic attack by HO"). But they do hydrolyse relatively easily in acid, because the mechanism of hydrolysis of f-butyl esters in acid is quite different. It does not involve nucleophilic attack at the carbonyl group and is a favourable Sjsjl reaction at the f-butyl group (Chapter 17). 

Even in the transition state, the angles are increasing towards 120° and all interactions with the leaving group are diminishing as it moves away. There is steric acceleration in the Sjsjl reaction rather than steric hindrance. This, as well as the stability of t-alkyl cations, is why f-alkyl compounds react by the Sj[Pg.425]

Once again, we do not know for sure whether this displacement by cyanide goes by the Sjsjl, S f2, or S>j2 mechanism, as the reagents equilibrate under the reaction conditions. However, the chlorides do not equilibrate and so, if we want a clear cut result on a single well-defined starting material, the chlorides are the compounds to use. 

The Sjsjl reaction involves a two-step mechanism. A slow ionization gives a carbocation that reacts quickly with a (usually weak) nucleophile. Reactivity 3° > 2° > 1... 

We have now seen that substitution reactions can occur through either a concerted mechanism (Sisj2) or a stepwise mechanism (Sj l) (Figure 7.19). When drawing the mechanism of an Si.j2 or Sjsjl process, additional mechanistic steps will sometimes be required. In this section, we will focus on the additional steps that can accompany an S l process. Recall from Chapter 6 that ionic mechanisms are constructed using only four different types of arrow-pushing patterns. This will now be important, as all fom patterns can play a role in Sjvjl processes. 

The first core step of an Sjsjl process is loss of a leaving group to generate a carbocation. Recall from Chapter 6 that carbocations are susceptible to rearrangement via either a hydride shift or a methyl shift. Here is an example of an S l mechanism with a carbocation rearrangement ... 

Four factors have an impact on whether a particular reaction will occur via an S],j2 or an S],jl mechanism (1) the substrate, (2) the leaving group, (3) the nucleophile, and (4) the solvent (Figure 7.25). We must learn to look at aU four factors, one by one, and to determine whether the factors favor Sjsjl or Si,j2. 

---