If your institution subscribes to this resource, and you don't have a MyAccess Profile, please contact your library's reference desk for information on how to gain access to this resource from off-campus. In this chapter and the ones that follow, we will discuss certain reactions of drug molecules and enzymes — biological macromolecules that break and form chemical bonds.
In this chapter we discuss substitution, addition, and elimination reactions. The main focus is on substitution reactions, which are prevalent in physiological and metabolic processes, in the action of some drugs, and in the chemical synthesis of nearly all drugs. The topic of addition reactions is introduced here and expanded upon in the following chapter on carbonyl chemistry.
Substitution reactions involve the reaction of nucleophiles with electrophiles. Some examples of nucleophiles and electrophiles are shown in Figure 6. Nucleophiles generally are anionic or neutral with a lone pair of electrons to donate. Electrophiles are positively charged or have a polarized bond with partial positive character. Electrophiles capable of undergoing substitution reactions have a leaving group , a species that can accept and stabilize the pair of electrons that make up the bond being broken. Examples of some good nucleophiles, electrophiles, and leaving groups.
Addition and Elimination Reactions of Aliphatic Compounds, Volume 9 - 1st Edition
In the sections that follow, we will discuss in more detail the factors that make for a good nucleophile, electrophile, or leaving group. We will also review the various reaction mechanisms by which substitution, addition, and elimination reactions occur. By the end of the chapter you should have developed a sound understanding of the factors that govern these reactions and be able to predict reaction products when provided with the reactants and reaction conditions. You should also be able to write reasonable mechanisms for your reactions, making the proper use of curly arrows to show the movement of electrons as chemical bonds are formed and broken.
A more general description of acids and bases is that first proposed by the chemist Gilbert N. Lewis, who described a covalent bond as the sharing of an electron pair between two atoms. Thus, a Lewis acid is a species that can accept an electron pair and a Lewis base is a species that can donate an electron pair in the formation of a covalent bond. Skip to content. Search for books, journals or webpages All Pages Books Journals. View on ScienceDirect. Editors: C.
Bamford R. Compton C. Imprint: Elsevier Science. Published Date: 1st January Page Count: Unlike the nucleophilic substitution reactions, this reduction probably proceeds by a radical mechanism. These aryl diazonium substitution reactions significantly expand the tactics available for the synthesis of polysubstituted benzene derivatives.
Consider the following options:. A nitro substituent deactivates an aromatic ring and directs electrophilic substitution to meta locations.
The following examples illustrate some combined applications of these options to specific cases. You should try to conceive a plausible reaction sequence for each. Once you have done so, you may check suggested answers by clicking on the question mark for each. A resonance description of diazonium ions shows that the positive charge is delocalized over the two nitrogen atoms. It is not possible for nucleophiles to bond to the inner nitrogen, but bonding or coupling of negative nucleophiles to the terminal nitrogen gives neutral azo compounds.
As shown in the following equation, this coupling to the terminal nitrogen should be relatively fast and reversible. In practice it is found that the E-isomer predominates at equilibrium.
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- Elimination reaction.
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Unless these azo products are trapped or stabilized in some manner, reversal to the diazonium ion and slow nucleophilic substitution at carbon with irreversible nitrogen loss will be the ultimate course of reaction, as described in the previous section. For example, if phenyldiazonium bisufate is added rapidly to a cold solution of sodium hydroxide a relatively stable solution of sodium phenyldiazoate the conjugate base of the initially formed diazoic acid is obtained.
Lowering the pH of this solution regenerates phenyldiazoic acid pK a ca. Aryl diazonium salts may be reduced to the corresponding hydrazines by mild reducing agents such as sodium bisulfite, stannous chloride or zinc dust. The bisulfite reduction may proceed by an initial sulfur-nitrogen coupling, as shown in the following equation. The most important application of diazo coupling reactions is electrophilic aromatic substitution of activated benzene derivatives by diazonium electrophiles.
The products of such reactions are highly colored aromatic azo compounds that find use as synthetic dyestuffs, commonly referred to as azo dyes. Some examples of azo coupling reactions are shown below. Otherwise ortho-coupling will occur. You should try to conceive a plausible product structure for each of the following couplings.
Addition and Elimination Reactions of Aliphatic Compounds, Volume 9
Once you have done so, you may check your answers by clicking on the question mark for each. Amine functions seldom serve as leaving groups in nucleophilic substitution or base-catalyzed elimination reactions. Indeed, they are even less effective in this role than are hydroxyl and alkoxyl groups. In the case of alcohols and ethers, a useful technique for enhancing the reactivity of the oxygen function was to modify the leaving group OH — or OR — to improve its stability as an anion or equivalent. This stability is conveniently estimated from the strength of the corresponding conjugate acids.
For example, heating an amine with HBr or HI does not normally convert it to the corresponding alkyl halide, as in the case of alcohols and ethers.
In this context we note that the acidity of the putative ammonium leaving group is at least ten powers of ten less than that of an analogous oxonium species. The loss of nitrogen from diazonium intermediates is a notable exception in this comparison, due to the extreme stability of this leaving group the conjugate acid of N 2 would be an extraordinarily strong acid.
Most applications involving this class of compounds are eliminations, but a few examples of S N 2 substitution have been reported.
Substitution and elimination reactions
Example 1 below shows a typical Hofmann elimination. Obviously, for an elimination to occur one of the alkyl substituents on nitrogen must have one or more beta-hydrogens, as noted earlier in examining elimination reactions of alkyl halides. The chief product from the elimination is the alkene having the more highly substituted double bond, reflecting not only the numerical advantage of those beta-hydrogens, but also the greater stability of the double bond.
Exhaustive methylation is shown again in example 4. The tendency of Hofmann eliminations to give the less-substituted double bond isomer is commonly referred to as the Hofmann Rule , and contrasts strikingly with the Zaitsev Rule formulated for dehydrohalogenations and dehydrations. In cases where other activating groups, such as phenyl or carbonyl, are present, the Hofmann Rule may not apply.