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An Overview of the Topic There Are Several Important Name Reactions in Organic Chemistry, Called Such Because They Either Bear

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An Overview of the topic There are several important name reactions in organic chemistry, called such because they either bear the names of the persons who described them or else are called by a specific name in texts and journals. Sometimes the name offers a clue about the reactants and products, but not always. 1. Aldol Reaction or Aldol Addition The aldol addition reaction is the combination of an alkene or ketone and the carbonyl of another aldehyde or ketone to form a β-hydroxy aldehyde or ketone. Aldol is a combination of the terms 'aldehyde' and 'alcohol.' 2. Aldol Condensation Reaction The aldol condensation removes the hydroxyl group formed by the aldol addition reaction in the form of water in the presence of a acid or base. The aldol condensation forms α,β-unsaturated carbonyl compounds. 3. Baeyer-Villiger Oxidation - Named Organic Reactions The Baeyer-Villiger oxidation reaction converts a ketone into an ester. This reaction requires the presence of a peracid such as mCPBA or peroxyacetic acid. Hydrogen peroxide can be used in conjunction with a Lewis base to form a lactone ester. 4. Baker-Venkataraman Rearrangement The Baker-Venkataraman rearrangement reaction converts an ortho-acylated phenol ester into a 1,3-diketone. 5. Beckmann Rearrangement Reaction The Beckmann rearrangement reaction converts oximes into amides. Cyclic oximes will produce lactam molecules. 6. Benzilic Acid Rearrangement The benzilic acid Rearrangement reaction rearranges a 1,2-diketone into an α- hydroxycarboxylic acid in the presence of a strong base. Cyclic diketones will contract the ring by the benzilic acid rearrangement. 7. Benzoin Condensation Reaction The benzoin condensation reaction condenses a pair of aromatic aldehydes into an α- hydroxyketone. 8. Friedel-Crafts Reaction A Friedel-Crafts reaction involves the alkylation of benzene. When a haloalkane is reacted with benzene using a Lewis acid (commonly an aluminum halide) as a catalyst, it will attach the alkane to the benzene ring and produce excess hydrogen halide. It is also called Friedel-Crafts alkylation of benzene. Proper description of the named reactions ALDOL CONDENSATION An aldol condensation is a condensation reaction in organic chemistry in which an enol or an enolate ion reacts with a carbonyl compound to form a β- hydroxyaldehyde or β-hydroxyketone, followed by dehydration to give a conjugated enone. Aldol condensations are important in organic synthesis, because they provide a good way to form carbon–carbon bonds. For example, the Robinson annulation reaction sequence features an aldol condensation; the Wieland-Miescher ketone product is an important starting material for many organic syntheses. Aldol condensations are also commonly discussed in university level organic chemistry classes as a good bond-forming reaction that demonstrates important reaction mechanisms. In its usual form, it involves the nucleophilic addition of a ketone enolate to an aldehyde to form a β-hydroxy ketone, or "aldol" (aldehyde + alcohol), a structural unit found in many naturally occurring molecules and pharmaceuticals. The name aldol condensation is also commonly used, especially in biochemistry, to refer to just the first (addition) stage of the process—the aldol reaction itself—as catalyzed by aldolases. However, the aldol reaction is not formally a condensation reaction because it does not involve the loss of a small molecule. The reaction between an aldehyde/ketone and an aromatic carbonyl compound lacking an alpha-hydrogen (cross aldol condensation) is called the Claisen-Schmidt condensation. This reaction is named after two of its pioneering investigators Rainer Ludwig Claisen and J. G. Schmidt, who independently published on this topic in 1880 and 1881. An example is the synthesis of dibenzylideneacetone. Quantitative yields in Claisen-Schmidt reactions have been reported in the absence of solvent using sodium hydroxide as the base and plus benzaldehydes. Mechanism The first part of this reaction is an aldol reaction, the second part a dehydration—an elimination reaction (Involves removal of a water molecule or an alcohol molecule). Dehydration may be accompanied by decarboxylation when an activated carboxyl group is present. The aldol addition product can be dehydrated via two mechanisms; a strong base like potassium t-butoxide, potassium hydroxide or sodium hydride in an enolate mechanism, or in an acid-catalyzed enol mechanism. Depending on the nature of the desired product, the aldol condensation may be carried out under two broad types of conditions: kinetic control or thermodynamic control. Condensation Types It is important to distinguish the aldol condensation from other addition reactions of carbonyl compounds. When the base is an amine and the active hydrogen compound is sufficiently activated the reaction is called a Knoevenagel condensation. In a Perkin reaction the aldehyde is aromatic and the enolate generated from an anhydride. A Claisen condensation involves two ester compounds. A Dieckmann condensation involves two ester groups in the same molecule and yields a cyclic molecule A Henry reaction involves an aldehyde and an aliphatic nitro compound. A Robinson annulation involves a α,β-unsaturated ketone and a carbonyl group, which first engage in a Michael reaction prior to the aldol condensation. In the Guerbet reaction, an aldehyde, formed in situ from an alcohol, self- condenses to the dimerized alcohol. In the Japp–Maitland condensation water is removed not by an elimination reaction but by a nucleophilic displacement DIECKMANN CONDENSATION The Dieckmann condensation is the intramolecular chemical reaction of diesters with base to give β-ketoesters. It is named after the German chemist Walter Dieckmann (1869–1925). The equivalent intermolecular reaction is the Claisen condensation. Reaction Mechanism Deprotonation of an ester at the α-position generates an enolate ion which then undergoes a 5-exo-trig nucleophilic attack to give a cyclic enol. Protonation with a Bronsted-Lowry acid (H3O+ for example) re-forms the β-keto ester. Owing to the steric stability of five- and six-membered ring structures, these will preferentially be formed. So 1,6 diesters will form five-membered cyclic β-keto esters, while 1,7 diesters will form six-membered β-keto esters. Animation of the reaction mechanism CLAISEN REARRANGEMENT The Claisen rearrangement (not to be confused with the Claisen condensation) is a powerful carbon–carbon bond-forming chemical reaction discovered by Rainer Ludwig Claisenin 1912. The heating of an allyl vinyl ether will initiate a [3,3]-sigmatropic rearrangement to give a γ,δ-unsaturated carbonyl. The Claisen rearrangement is the first recorded example of a [3,3]-sigmatropic rearrangement. The Claisen rearrangement is an exothermic, concerted (bond cleavage and recombination) pericyclic reaction. Woodward–Hoffmann rules show a suprafacial, stereospecific reaction pathway. The kinetics are of the first order and the whole transformation proceeds through a highly ordered cyclic transition state and is intramolecular. Crossover experiments eliminate the possibility of the rearrangement occurring via an intermolecular reaction mechanism and are consistent with an intramolecular process. There are substantial solvent effects observed in the Claisen rearrangement, where polar solvents tend to accelerate the reaction to a greater extent. Hydrogen-bonding solvents gave the highest rate constants. For example, ethanol/water solvent mixtures give rate constants 10-fold higher than sulfolane. Trivalent organoaluminium reagents, such as trimethylaluminium, have been shown to accelerate this reaction. The first reported Claisen rearrangement is the [3,3]-sigmatropic rearrangement of an allylphenyl ether to intermediate 1, which quickly tautomerizes to an ortho-substituted phenol. Meta-substitution affects the regioselectivity of this rearrangement. For example, electron withdrawing groups (e.g. bromide) at the meta-position direct the rearrangement to the ortho-position (71% ortho-product), while electron donating groups (e.g. methoxy), direct rearrangement to the para-position (69% para-product). Additionally, presence of ortho-substituents exclusively leads to para-substituted rearrangement products (tandem Claisen and Cope rearrangement). If an aldehyde or carboxylic acid occupies the ortho or para positions, the allyl side- chain displaces the group, releasing it as carbon monoxide or carbon dioxide, respectively. Bellus–Claisen rearrangement The Bellus–Claisen rearrangement is the reaction of allylic ethers, amines, and thioethers with ketenes to give γ,δ-unsaturated esters, amides, and thioesters. This transformation was serendipitously observed by Bellus in 1979 through their synthesis of a key intermediate of an insecticide, pyrethroid. Halogen substituted ketenes (R1, R2) are often used in this reaction for their high electrophilicity. Numerous reductive methods for the removal of the resulting α-haloesters, amides and thioesters have been developed. The Bellus-Claisen offers synthetic chemists a unique opportunity for ring expansion strategies. Eschenmoser–Claisen rearrangement The Eschenmoser–Claisen rearrangement proceeds by heating allylic alcohols in the presence of N,N-dimethylacetamide dimethyl acetal to form γ,δ-unsaturated amide. This was developed by Albert Eschenmoser in 1964. Eschenmoser-Claisen rearrangement was used as a key step in the total synthesis of morphine. Ireland–Claisen rearrangement The Ireland–Claisen rearrangement
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  • Aldol Condensation

    Aldol Condensation

    Chemistry 212 Laboratory Dibenzalacetone via Crossed Aldol Condensation Prelab: Calculate the amounts of all chemicals needed in measurable amounts (i.e. grams or milliliters rather than moles.) Introduction: Aldol condensations are important in organic synthesis, providing a good way to form carbon–carbon bonds. The "aldol" (aldehyde + alcohol) product is a structural unit found in many naturally occurring molecules and pharmaceuticals, and is therefore important. In an Aldol condensation an enolate ion reacts with a carbonyl compound to form a β- hydroxyaldehyde or β-hydroxyketone, followed by dehydration to give a conjugated enone. The general equation is shown in Figure 1. O O O R" B: H R R'" R "R R'" loss of H2O H R' R' Figure 1. The equation for the Aldol Condensation. The reaction involves the nucleophilic addition of an enolate to an aldehyde to form a β-hydroxy carbonyl. The β-hydroxy carbonyl is readily dehydrated under mild conditions. The aldol reaction occurs under both acidic and basic conditions as seen in Figure 2. ENOL pathway (reacts in H O protonated OH form) O O catalytic H+ O O H H R' H2O lost R' R R R' R R H aldol addition product aldol condensation product ENOLATE pathway O O M O M O base O H R' R R' R R enolate H Figure 2. The Aldol reaction and subsequent dehydration under acidic and basic conditions. The reaction we will be doing this week involves the reaction between benzaldehyde and acetone to do a double Aldol Condensation. The overall equation is shown in Figure 3.