Alpha substitution reaction carbonyl condensation reactions

Both in the laboratory and in living organisms, the reactions of carbonyl compounds take place by one of four general mechanisms nucleophilic addition, nucleophilic acyl substitution, alpha substitution, and carbonyl condensation. These... 

Most reactions of carbonyl groups occur by one of four general mechanisms nucleophilic addition, nucleophilic acyl substitution, alpha substitution, am carbonyl condensation. These mechanisms have many variations, just a alkene electrophilic addition reactions and 8 2 reactions do, but the varia tions are much easier to learn when the fundamental features of the mechanisms are understood. Let s see what the four mechanisms are and what kinds of chemistry carbonyl groups undergo. 

Up to now, we have studied two of the main types of carbonyl reactions nucleophilic addition and nucleophilic acyl substitution. In these reactions, the carbonyl group serves as an electrophile by accepting electrons from an attacking nucleophile. In this chapter, we consider two more types of reactions substitution at the carbon atom next to the carbonyl group (called alpha substitution) and carbonyl condensations. 

There is no simple answer to this question, but the exact experimental conditions usually have much to do with the result. Alpha-substitution reactions require a full equivalent of strong base and are normally carried out so that the carbonyl compound is rapidly and completely converted into its enolate ion at a low temperature. An electrophile is then added rapidly to ensure that the reactive enolate ion is quenched quickly. In a ketone alkylation reaction, for instance, we might use 1 equivalent of lithium diisopropylamide (LDA) in lelrahydrofuran solution at -78 °C. Rapid and complete generation of the ketone enolate ion would occur, and no unreacled ketone would be left so that no condensation reaction could take place. We would then immediately add an alkyl halide to complete the alkylation reaction. 

Carbonyl Condensation Reactions versis Alpha-Substitution Reactions 941... 

Carbonyl Condensation Reactions versus Alpha-Substitution Reactions... 

Chapter 22 Condensations and Alpha Substitutions of Carbonyl Compounds 22-68 Predict the products of the following reactions. 

Alpha substitutions and condensations of carbonyl compounds are some of the most common methods for forming carbon-carbon bonds. These types of reactions are common in biochemical pathways, particularly in the biosynthesis and metabolism of carbohydrates and fats. A wide variety of compounds can participate as nucleophiles or electrophiles (or both) in these reactions, and many useful products can be made. We begin our study of these reactions by considering the structure and formation of enols and enolate ions. 

Under basic conditions, the aldol condensation occurs by a nucleophilic addition of the enolate ion (a strong nucleophile) to a carbonyl group. Protonation gives the aldol product. Note that the carbonyl group serves as the electrophile that is attacked by the nucleophilic enolate ion. From the electrophile s viewpoint, the reaction is a nucleophilic addition across the carbonyl double bond. From the viewpoint of the enolate ion, the reaction is an alpha substitution The other carbonyl compound replaces an alpha hydrogen. 

CHAPTER 17 CARBONYL ALPHA-SUBSTITUTION AND CONDENSATION REACTIONS... 

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