Nucleophilic substitution acid-base reaction

Substitution of CO by phosphines 145 The Dotz reaction 149 Rearrangement reactions with loss of CO 151 Photochemical reactions 153 Reactions at the carbene carbon 158 General features 158 Amine nucleophiles 159 Phospine and phosphite nucleophiles 167 Alcohols and alkoxide ion nucleophiles 171 Thiol and thiolate ion nucleophiles 179 Intramolecular nucleophilic reactions 191 Hydroxide ion and water as nucleophiles 194 Insertion reactions initiated by nucleophilic attack Acid-base reactions at the a-carbon 207 General features and methods 207 Kinetic and thermodynamic acidities 209 Effect of structure on pKa values 210 Intrinsic rate constants for proton transfer 219 Thermodynamic acidities in organic solvents 223 Hydrolysis of ionizable carbene complexes 228 Acknowledgments 232 References 233... 

Step 3 This step is a fast acid base reaction that follows the nucleophilic substitution Water acts as a base to remove a proton from the alkyloxonium ion to give the observed product of the reaction tert butyl alcohol... 

Amines like ammonia are weak bases They are however the strongest uncharged bases found m significant quantities under physiological conditions Amines are usually the bases involved m biological acid-base reactions they are often the nucleophiles m biological nucleophilic substitutions... 

In this context, an avalanche of studies were devoted to acid-base reactions in their broadest sense (i.e., the Lewis picture), also involving complexation reactions, to the typical organic reactions of addition, substitution, and elimination types, involving nucleophilic and electrophilic reagents including the case of radicalar reactions and excited states (for a review see Ref. [11]) in which our group has... 

Allyl esters, carbonates, and carbamates readily undergo C-O bond cleavage upon reaction with palladium(O) to yield allyl palladium(II) complexes. These complexes are electrophilic and can react with nucleophiles to form products of allylic nucleophilic substitution. Linkers based on this reaction have been designed, which are cleavable by treatment with catalytic amounts of palladium complexes [165,166], For the immobilization of carboxylic acids, support-bound allyl alcohols have proven suitable (Figure 3.12, Table 3.7). 

The nucleophilic substitution of carboxylic acids is complicated due to the fact that an acidic proton is present. Since most nucleophiles can act as bases, the reaction of a carboxylic acid with a nucleophile results in an acid-base reaction rather than nucleophilic substitution. 

Since 05 is a conjugate base of weak acid, HO2 (p ATa=4.9), and also an anion radical at physiological pH, it can perform both ionic and radical reactions. As an anion 05 has reactivies such as nucleophilic substitution and addition reactions, and as a radical, it performs hydrogen-withdrawal reactions, one-electron reduction or oxidation reactions and the disproportionation reaction. In aqueous medium, the radical reactions are... 

Show the products of these reactions. (Remember that acid—base reactions are usually much faster than nucleophilic substitution reactions.)... 

Because the.se substitution reactions involve electron-rich nucleophiles, they are called nucleophilic substitution reactions. Examples are shown in Equations [l]-[3]. Nucleophilic substitutions are Lewis acid—base reactions. The nucleophile donates its electron pair, the alkyl halide (Lewis acid) accepts it, and the C-X bond is heterolytically cleaved. Curved arrow notation can be used to show the movement of electron pairs, as shown in Equation [3]. 

Although nucleophilicity and basicity are interrelated, they are fundamentally different. Basicity is a measure of how readily an atom donates its electron pair to a proton it is characterized by an equilibrium constant Kg in an acid-base reaction, making it a thermodynamic property. Nucleophilicity is a measure of how readily an atom donates its electron pair to other atoms it is characterized by the rate constant, k, of a nucleophilic substitution reaction, making it a kinetic property. 

Carboxylic adds are strong organic acids. Because acid-base reactions proceed rapidly, any nucleophile that is also a strong base will react with a carboxylic acid by removing a proton first, before any nucleophilic substitution reaction can take place. 

An acid-base reaction (Reaction [1]) occurs with OH, NH3, and amines, all common nucleophiles used in nucleophilic acyl substitution reactions. Nonetheless, carboxyhc acids can be converted to a variety of other acyl derivatives using special reagents, with acid catalysis, or sometimes, by using rather forcing reaction conditions. These reactions are summarized in Figure 22.2 and detailed in Sections 22.10A—22.10D. 

The direct conversion of a carboxylic acid to an amide with NH3 or an amine is very difficult, even though a more reactive acyl compound is being transformed into a less reactive one. The problem is that carboxylic acids are strong organic acids and NH3 and amines are bases, so they undergo an acid-base reaction to form an ammonium sait before any nucleophilic substitution occurs. 

An acid—base reaction forms a nucleophilic anion that can react with an unhindered alkyl halide— that is, CH3X or RCH2X—in an 5 2 reaction to form a substitution product. This alkylated imide is then hydrolyzed with aqueous base to give a 1° amine and a dicarboxylate. This reaction is similar to the hydrolysis of amides to afford carboxylate anions and amines, as discussed in Section 22.13. The overall result of this two-step sequence is nucleophilic substitution of X by NH2, so the Gabriel synthesis can be used to prepare 1° amines only. 

Alcohols react with hydrogen halides by nucleophilic substitution. The OH group is replaced by a halogen water is the byproduct. In the reaction mechanism, the first step involves formation of an oxonium ion by the Lewis acid-base reaction of the hydrogen ion of the hydrogen halide and alcohol oxygen. The rest of the reaction occurs by one of the nucleophilic substitution mechanisms depending on structure of the alcohol. In the Sn2 reaction, the next step involves displacement of the water molecule by halide ion to form the final products. In the Sn1... 

Acid - base reactions Substitution reactions Elimination reactions Addition reactions Nucleophilic acyl addition Oxidation reactions Reduction reactions... 

It was noted that nucleophiles are less nucleophilic if they that can delocalize electrons by resonance. An example is the phenoxide anion, where electron density is delocalized away from oxygen by the adjacent phenyl ring, meaning that oxygen cannot donate electrons to carbon as effectively and is less nucleophilic. It should also be noted that nucleophilic reactions are usually under kinetic control, whereas acid-base reactions are under thermodynamic control. Under kinetic control, the most nucleophilic species will react faster and dominate the substitution reaction. 

This type of reaction begins when a ir bond of an alkene donates an electron pair to an acid (H+)—an acid-base reaction where the alkene is a weak base. The Jt bond is broken as the new Br—H bond is formed, and the remaining carbon of the former double bond becomes a carbocation. The reaction of cyclohexene with acid to form secondary cation 294 illustrates this process. The cationic center then reacts with the nucleophilic gegenion (Br" from HBr) to produce bromocyclohexane. The latter portion of this sequence is analogous to the second step (coupling) of an Sjsfl reaction. The initial reaction usually involves formation of a solvent separated carbocation intermediate, but this depends on the solvent. A tight ion pair intermediate can react in the substitution step to give the same product. The net result of this cationic reaction is addition of H and Br across the jt bond. 

Substitution reactions, which occur in all areas of chemistry, are those in which an atom or group of atoms is substituted for another. A Lewis base is an electron pair donor, and a Lewis acid is an electron pair acceptor. Some common Lewis bases are H2O, NH3, OH , F , etc., while some common Lewis acids are AICI3, BCI3, carbocations (RsC ), etc. In a Lewis acid-base reaction, a coordinate bond is formed between the acid and base with the base donating the pair of electrons. Lewis bases are known as nucleophiles and Lewis acids are known as electrophiles. In fact, when A is a... 

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