The product in such cases is always a dimer of the reactant carbonyl compound. Second, the substitutions are limited to hydrogen atoms. The rates of halogenation and isotope exchange are essentially the same (assuming similar catalsts and concentrations), and are identical to the rate of racemization for those reactants having chiral alpha-carbon units. It is necessary, therefore, to achieve complete conversion of aldehyde or ketone reactants to their enolate conjugate bases by treatment with a very strong base (pKa > 25) in a non-hydroxylic solvent before any alkyl halides are added to the reaction system. Methyl ketones undergo a unique oxidative cleavage called The Haloform Reaction. Nu:$ Nu ¥We used the example of hydration (formation of gem-diols) to illustrate nucleophilic addition to carbonyl compounds; however, other nucleophiles can If the alpha-carbon is a chiral center, as in the second example, the products of halogenation and isotopic exchange are racemic. Thus, the non-bonding π orbital lobes' extensions substantiate the fact that the addition of acids and other electrophiles to alkenes, hydroxyl anion is a very poor leaving group. Some examples are shown below, and in most cases beta-elimination of water occurs under the conditions used. The first three questions concerns their nomenclature. Not all carbonyl compounds exhibit these characteristics, the third ketone being an example.

Three examples of the base-catalyzed aldol reaction are shown in the following diagram, and equivalent acid-catalyzed reactions also occur. The exception, reaction #3, is conducted under mild conditions with an excess of the reactive aldehyde formaldehyde serving in the role of electrophilic acceptor. The illustration of the non-bonding π orbital is indicative of As a result, Ring closures to four, five, six and seven-membered are also possible by intramolecular enolate alkylation, as illustrated by the following example. and the nucleophilic qualities of the enolate anion are mostly represented through the The monoalkylated product shown in the equation still has an acidic hydrogen on the central carbon, and another alkyl group may be attached there by repeating this sequence. From previous knowledge and experience, we surmise that the common intermediate is an enol tautomer of the carbonyl reactant. While the pKa values for alkyl C-H bonds is typically on the order of 40-50, pKa values for these alpha hydrogens is more on the order of 19-20. Acidity of a-Hydrogens. Here the two hydrogen atoms activated by both carbonyl groups are over 1010 times more acidic than the methyl hydrogens on the ends of the carbon chain. The most surprising aspect of beta-elimination, however, is that it can be base-catalyzed. Nevertheless, the conditions required for the beta-elimination are found to be milder than those used for simple alcohols. Nevertheless, these very strong bases are useful in making soluble amide bases. If an alkyl halide undergoes an SN2 reaction at the carbon atom of an enolate anion the product is an alkylated aldehyde or ketone. For more information contact us at or check out our status page at the otherwise highly energized carbanion. Aldol condensations between different carbonyl reactants are called crossed or mixed reactions, and under certain conditions such crossed aldol condensations can be effective. Prof. Steven Farmer (Sonoma State University), Clarke Earley (Department of Chemistry, Kent State University Stark Campus). The last example (reaction #4) is an interesting case of intramolecular alkylation of an enolate anion. Why then should the base-catalyzed elimination of water occur in aldol products? Reactions in which a larger molecule is formed from smaller components, with the elimination of a very small by-product such as water are termed Condensations. The dichloro alkylating agent used in reaction #1 nicely illustrates the high reactivity of allylic halides and the unreactive nature of vinylic halides in SN2 reactions. The equilibrium reaction between the ketone or aldehyde and the enol form is commonly referred to as "keto-enol tautomerism". In the following table, the acidity of the H for various enolate systems and other closely related systems are given. 06.03 Acidity and Basicity of Carbonyl Compounds - Duration: 12:10. The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. The tenth illustrates three ways questions about chemical reactions may be formulated. If we substitute molecules for hampsters, their numbers will be extraordinarily large (recall the size of Avogadro's number), but the equilibrium between keto tautomers (hampsters in the cage) and enol tautomers (hampsters in the annex) is so rapid that complete turnover of all the molecules in a sample may occur in fractions of a second rather than minutes or hours.

Only one of the two aldol products can undergo a beta-elimination of water, so the eventual isolated product comes from that reaction sequence.

Reactants having two or more reactive sites are called ambident, so this term is properly applied to enolate anions. If all the reactions occurred at the same rate, equal quantities of the four products would be obtained. It has been found that this competition is sensitive to a number of factors, including negative charge density, solvation, cation coordination and product stability. α carbon is a soft base, while the carbonyl oxygen is a hard base. A clever way of overcoming this disadvantage has been found. at the carbonyl oxygen clearly supports the fact that the electron (density) is bound The following abbreviated formulas illustrate the possible products in such a case, red letters representing the acceptor component and blue the donor. The eleventh and twelveth questions require you to draw the structural formulas for the products of many aldehyde and ketone reactions. Ketones such as cyclohexanone are much more acidic than their parent hydrocarbons (by at least 25 powers of ten); nevertheless they are still very weak acids (pKa = 17 to 21) compared with water. ¥The sp2 hybridization of the carbonyl compound means that attack of the nucleophile on the carbonyl carbon may occur from either face. hydrogen that is bound to the carbon adjacent to the carbonyl carbon. Together with some related acidities, this is listed in the following table.

#3 above), and the equilibrium product concentration is small. This resembles closely the first step in the addition of acids and other electrophiles to alkenes. soft Lewis acids (electrophiles), such as alkyl halides and the carbonyl carbon of Because alkylations are usually irreversible, their products should reflect the inherent (kinetic) reactivity of the different nucleophilic sites. Indeed, they are sufficiently acidic (pKa = 9) to allow complete conversion to the enolate anion in aqueous or alcoholic solutions. Examples of such doubly (and higher) activated carbon acids are given elsewhere. The five-carbon chain of the dichloroketone can adopt many conformations, two of which are approximated in the preceding diagram. The fundamental transformation in this reaction is a dimerization of an aldehyde (or ketone) to a beta-hydroxy aldehyde (or ketone) by alpha C–H addition of one reactant molecule to the carbonyl group of a second reactant molecule. more strongly and closely to the oxygen atom than is the case with the α carbon. At low to moderate halogen concentrations, the rate of halogen substitution is proportional (i.e. The dehydration step of an aldol condensation is also reversible in the presence of acid and base catalysts.


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