A-substitution reaction

The results in table 2.6 show that the rates of reaction of compounds such as phenol and i-napthol are equal to the encounter rate. This observation is noteworthy because it shows that despite their potentially very high reactivity these compounds do not draw into reaction other electrophiles, and the nitronium ion remains solely effective. These particular instances illustrate an important general principle if by increasing the reactivity of the aromatic reactant in a substitution reaction, a plateau in rate constant for the reaction is achieved which can be identified as the rate constant for encounter of the reacting species, and if further structural modifications of the aromatic in the direction of further increasing its potential reactivity ultimately raise the rate constant above this plateau, then the incursion of a new electrophile must be admitted. 

In a simple pyrometaHurgical reduction, the reduciag agent, R, combines with the nonmetal, X, ia the metallic compound, MX, according to a substitution reaction of the foUowiag type ... 

Chlorobutyl rubber is prepared by chlorination of butyl rubber (chlorine content is about 1 wt%). This is a substitution reaction produced at the allylic position, so little carbon-carbon double unsaturation is lost. Therefore, chlorobutyl rubber has enhanced reactivity of the carbon-carbon double bonds and supplies additional reactive sites for cross-linking. Furthermore, enhanced adhesion is obtained to polar substrates and it can be blended with other, more unsaturated elastomers. 

Reaction types are classified by specifying the class and the reagent type thus a nucleophilic substitution (Sn) is a substitution reaction by a nucleophilic reagent, as in Eq. (1-1). 

Mechanistically the reaction can be divided into two steps. Initially the alkyl halide 1 reacts with sodium to give an organometallic species 3, that can be isolated in many cases. In a second step the carbanionic R of the organometallic compound 3 acts as nucleophile in a substitution reaction with alkyl halide 1 to replace the halide ... 

Closely related to the carboxylic acids and nitriles discussed in the previous chapter are the carboxylic acid derivatives, compounds in which an acyl group is bonded to an electronegative atom or substituent that can net as a leaving group in a substitution reaction. Many kinds of acid derivatives are known, but we ll be concerned primarily with four of the more common ones acid halides, acid anhydrides, esters, and amides. Esters and amides are common in both laboratory and biological chemistry, while acid halides and acid anhydrides are used only in the laboratory. Thioesters and acyl phosphates are encountered primarily in biological chemistry. Note the structural similarity between acid anhydrides and acy) phosphates. 

We said in A Preview ofCnrbonyl Compounds that much of the chemistry of carbonyl compounds can be explained by just four fundamental reaction types nucleophilic additions, nucleophilic acyl substitutions, o substitutions, and carbonyl condensations. Having studied the first two of these reactions in the past three chapters, let s now look in more detail at the third major carbonyl-group process—the a-substitution reaction. 

Active Figure 22.3 MECHANISM General mechanism of a carbonyl a-substitution reaction. The initially formed cation loses H+ to regenerate a carbonyl compound. Sign in at www.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

The halogenation is a typical a-substitution reaction that proceeds by acid-catalyzecl formation o.f an enol intermediate, as shown in Figure 22.4. 

The a-substitution reaction of a carbonyl compound through either an enoi or enolate ion intermediate is one of the four fundamental reaction types in carbonyl-gron p chem istrv. 

The anion reacts with bronine in an a-substitution reaction to give an N-bromoamide. 

Arenediazonium salts are extremely useful because the diazonio group (N2) can be replaced by a nucleophile in a substitution reaction. 

Carboxylic acid derivative (Chapter 21 introduction) A compound in which an acyl group is bonded to an electronegative atom or substituent Y that can act as a leaving group in a substitution reaction, RCOY. 

Leaving group (Section 11.2) The group that is replaced in a substitution reaction. 

A substitution reaction is one in which an atom or group of atoms in a molecule is replaced by a different atom or group. Substitution reactions are very common in organic chemistry. Examples include the following ... 

Ir(OH)g is formed by a substitution reaction and is isolable as red crystals. Similar complexes have been isolated for the heavier group (II) metals. 

This is an example of a substitution reaction, a reaction in which one 1 ewis base takes the place of another. Here the CN ions drive out H20 molecules from the coordination sphere of the [Fe(H20)6]2+ complex and take their place. Replacement is less complete when certain other ions, such as Cl, are added to an iron(II) solution ... 

The palladium(O) complex undergoes first an oxydative addition of the aryl halide. Then a substitution reaction of the halide anion by the amine occurs at the metal. The resulting amino-complex would lose the imine with simultaneous formation of an hydropalladium. A reductive elimination from this 18-electrons complex would give the aromatic hydrocarbon and regenerate at the same time the initial catalyst. 

Salts of aliphatic or aromatic carboxylic acids can be converted to the corresponding nitriles by heating with BrCN or CICN. Despite appearances, this is not a substitution reaction. When R COO was used, the label appeared in the nitrile, not in the C02, and optical activity in R was retained. The acyl isocyanate... 

In both Sn2 and SnI reactions, a nucleophile is attacking an electrophile, giving a substitution reaction. That explains the Sn part of the name. But what do the 1 and 2 stand for To see this, we need to look at the mechanisms. Let s start with Sn2 ... 

In the previous chapter, we saw that a substitution reaction can occur when a compound possesses a leaving group. In this chapter, we will explore another type of reaction, called elimination, which can also occur for compounds with leaving groups. In fact, substitution and elimination reactions frequently compete with each other, giving a mixture of products. At the end of this chapter, we will learn how to predict the products of these competing reactions. For now, let s consider the different outcomes for substitution and elimination reactions ... 

In a substitution reaction, the leaving group is replaced with a nucleophile. In an elimination reaction, a beta ((3) proton is removed together with the leaving group, forming a double bond. In the previous chapter, we saw two mechanisms for substitution reactions (SnI and Sn2). In a similar way, we will now explore two mechanisms for elimination reactions, called El and E2. Let s begin with the E2 mechanism. 

Now let s consider the effect of the substrate on the rate of an E2 process. Recall from the previous chapter that Sn2 reactions generally do not occur with tertiary substrates, because of steric considerations. But E2 reactions are different than Sn2 reactions, and in fact, tertiary substrates often undergo E2 reactions quite rapidly. To explain why tertiary substrates will undergo E2 but not Sn2 reactions, we must recognize that the key difference between substitution and elimination is the role played by the reagent. In a substitution reaction, the reagent functions as a nucleophile and attacks an electrophilic position. In an elimination reaction, the reagent functions as a base and removes a proton, which is easily achieved even with a tertiary substrate. In fact, tertiary substrates react even more rapidly than primary substrates. 

If we want to perform a substitution reaction with an alcohol, we have the same issue that we had when we explored elimination reactions a few moments ago—the OH group is not a good leaving group. So, we must convert the OH into a better leaving group. There are several ways to do that for substitution reactions. We will look at four different ways ... 

Via an Sn2 process. With primary or secondary alcohols, we can still protonate the OH group and get a substitution reaction (it will just be Sn2 instead of SnI) ... 

A third way to perform a substitution reaction is to convert the OH group into a tosylate, and then do an Sn2 reaction ... 

There is one other way to convert an OH group into a better leaving group for the purpose of doing a substitution reaction. We can use thionyl chloride (SOCI2) to convert an alcohol into an alkyl chloride ... 

If vinylogous imidazole-A-carboxylates (route A) are treated with nucleophiles such as alkoxides or amines, the corresponding vinylogous carbonic esters or amides are obtained. While reaction of the vinylogous imidazole-TV-carboxylate with a thiol (route A) yields the addition product only, that of the corresponding imidazolium compound (route B) leads to the carbonic thioester in a substitution reaction [3]... 

The encapsulating hexadentate ligand (148) forms [Ni(148)]2+ with a distorted octahedral geometry. This can be oxidized to a Ni111 species in which a substitution reaction by sulfate in the NiN6 chromophore is observed.514... 

---