Mechanism, tetrahedral

Instead of performing the one step bimolecular SN2 reaction, alkenes react via two closely related bimolecular pathways. The first of these is called the tetrahedral mechanism and proceeds via a negatively charged intermediate. This mechanism is sometimes called the addition/elimination reaction, which is given the label Adn/E. This alternative name is unfortunate, because the other pathway is called the addition/elimination mechanism and proceeds via a readily detectable neutral intermediate. This latter mechanism will be considered in the chapter on sequential addition/elimination reactions. In this book, in an attempt to reduce the confusion, we will call the mechanism that proceeds via an anionic intermediate the tetrahedral mechanism, and reserve the name addition/elimination mechanism for the mechanism that proceeds via a neutral species. 

Consider the general alkene, ZCH=CHX, and let it be attacked by a nucleophile, Y. Draw the mechanism for this attack, and suggest how the anionic intermediate may be stabilised. 

The adjacent carbon now bears a negative charge and this may be stabilised by either strongly -I groups, or better still -M groups such as carbonyl or cyano groups. 

The intermediate may either eliminate the Z group, which would result in overall substitution, or it could add an electrophile, which would result in overall addition to the alkene bond. These two reactions usually compete. 

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On the other hand, if the tetrahedral mechanism operates then the intermediate... 

Several studies have been made of the directionality of approach by the nucleophile. ° Menger ° has proposed for reactions in general, and specifically for those hat proceed by the tetrahedral mechanism, that there is no single definable preferred transition state, but rather a cone of trajectories. All approaches within this cone lead to reaction at comparable rates it is only when the approach comes outside of the cone that the rate falls. 

Some nucleophilic substitutions at a carbonyl carbon are catalyzed by nucleophiles.There occur, in effect, two tetrahedral mechanisms ... 

For a list of some of the more important reactions that operate by the tetrahedral mechanism, see Table 10.8. 

Nucleophilic substitution at a vinylic carbon is difficult (see p. 433), but many examples are known. The most common mechanisms are the tetrahedral mechanism and the closely related addition-elimination mechanism. Both of these mechanisms are impossible at a saturated substrate. The addition-elimination mechanism has... 

The tetrahedral mechanism, often also called addition-elimination (Ar N - E), takes place with much less facility than with carbonyl groups, since the negative charge of the intermediate must be borne by a carbon, which is less electronegative than oxygen, sulfur, or nitrogen ... 

Of course, we have seen (p. 430) that SnI reactions at vinylic substrates can be accelerated by a substituents that stabilize that cation, and that reactions by the tetrahedral mechanism can be accelerated by P substituents that stabilize the carbanion. Also, reactions at vinylic substrates can in certain cases proceed by addition-elimination or elimination-addition sequences (pp. 428, 430). 

In contrast to such systems, substrates of the type RCOX are usually much more reactive than the corresponding RCH2X. Of course, the mechanism here is almost always the tetrahedral one. Three reasons can be given for the enhanced reactivity of RCOX (1) The carbonyl carbon has a sizable partial positive charge that makes it very attractive to nucleophiles. (2) In an Sn2 reaction a cr bond must break in the rate-determining step, which requires more energy than the shift of a pair of n electrons, which is what happens in a tetrahedral mechanism. (3) A trigonal carbon offers less steric hindrance to a nucleophile than a tetrahedral carbon. 

Table 10.6 is an approximate listing of groups in order of SnI and Sn2 reactivity. Table 10.7 shows the main reactions that proceed by the Sn2 mechanism (if R = primary or, often, secondary alkyl) Table 10.8 shows the main reactions that proceed by the tetrahedral mechanism. 

TABLE 10.8 The More Important Synthetic Reactions of Chapter 10 that Take Place by the Tetrahedral Mechanism"... 

Not all the reactions in this chapter are actually nucleophilic substitutions. In some cases the mechanisms are not known with enough certainty even to decide whether a nucleophile, an electrophile, or a free radical is attacking. In other cases (such as 10-79), conversion of one compound to another can occur by two or even all three of these possibilities, depending on the reagent and the reaction conditions. However, one or more of the nucleophilic mechanisms previously discussed do hold for the overwhelming majority of the reactions in this chapter. For the alkylations, the Sn2 is by far the most common mechanism, as long as R is primary or secondary alkyl. For the acylations, the tetrahedral mechanism is the most common. 

Another way to esterify a carboxylic acid is to treat it with an alcohol in the presence of a dehydrating agent. One of these is DCC, which is converted in the process to dicyclohexylurea (DHU). The mechanism has much in common with the nucleophilic catalysis mechanism the acid is converted to a compound with a better leaving group. However, the conversion is not by a tetrahedral mechanism (as it is in nucleophilic catalysis), since the C—O bond remains intact during this step ... 

Ion 21 can either lose a proton or combine with chloride ion. If it loses a proton, the product is an unsaturated ketone the mechanism is similar to the tetrahedral mechanism of Chapter 10, but with the charges reversed. If it combines with chloride, the product is a 3-halo ketone, which can be isolated, so that the result is addition to the double bond (see 15-45). On the other hand, the p-halo ketone may, under the conditions of the reaction, lose HCl to give the unsaturated ketone, this time by an addition-elimination mechanism. In the case of unsymmetrical alkenes, the attacking ion prefers the position at which there are more hydrogens, following Markovnikov s rule (p. 984). Anhydrides and carboxylic acids (the latter with a proton acid such as anhydrous HF, H2SO4, or polyphosphoric acid as a catalyst) are sometimes used instead of acyl halides. With some substrates and catalysts double-bond migrations are occasionally encountered so that, for example, when 1 -methylcyclohexene was acylated with acetic anhydride and zinc chloride, the major product was 6-acetyl-1-methylcyclohexene. ... 

The first step is usually, but not always, rate determining. It can be seen that this mechanism greatly resembles the tetrahedral mechanism discussed in Chapter 10 and, in another way, the arenium ion mechanism of electrophilic aromatic substitution. In all three cases, the attacking species forms a bond with the... 

As with the tetrahedral mechanism at an acyl carbon, nucleophilic catalysis (p. 427) has been demonstrated with an aryl substrate, in certain cases. 

This is reminiscent of the nucleophilic tetrahedral mechanism at a vinylic carbon (p. 429). 

As is the case in the tetrahedral mechanism, it is also possible for the electrophilic species to attack first, in which case it goes to the heteroatom. This species is most... 

Reactivity factors in additions to carbon-hetero multiple bonds are similar to those for the tetrahedral mechanism of nucleophilic substitution. If A and/or B are electron-donating groups, rates are decreased. Electron-attracting substituents increase rates. This means that aldehydes are more reactive than ketones. Aryl groups are somewhat deactivating compared to alkyl, because of resonance that stabilizes the substrate molecule but is lost on going to the intermediate ... 

Nucleophilic Substitution at an Aliphatic Trigonal Carbon. The Tetrahedral Mechanism... 

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