CHAPTER 19: CARBONYL COMPOUNDS III
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1 CHAPTER 19: CARBONYL COMPOUNDS III A hydrogen bonded to a carbon adjacent to a carbonyl carbon is sufficiently acidic to be removed by a strong base. The carbon adjacent to a carbonyl carbon is called an α-carbon. A hydrogen bonded to an α- carbon is therefore called an α-hydrogen The Acidity of an α-hydrogen A compound that contains a relatively acidic hydrogen bonded to an sp 3 carbon is called a carbon acid. The α-hydrogen of a ketone or an aldehyde is more acidic than the α-hydrogen of an ester. An α-hydrogen is more acidic because the base formed when a proton is removed from an α- carbon is relatively stable. The electrons left behind when a proton is removed from the α-carbon of an ester are not as readily delocalized onto the carbonyl oxygen as they would be in an aldehyde or a ketone. This is because the oxygen of the OR group of the ester also has a lone pair that can be delocalized onto the carbonyl oxygen. Thus, the 2 pairs of electrons compete for delocalization onto the same oxygen. If the α-carbon is between 2 carbonyl groups, the acidity of an α-hydrogen is even greater. Β-keto ester an ester with a second carbonyl group at the β-position. Β-diketone a ketone with a second carbonyl group at the β-position.
2 The acidity of α-hydrogens bonded to carbons flanked by 2 carbonyl groups increases because the electrons left behind when the protons is removed can be delocalized onto 2 oxygen atoms Keto Enol Tautomers Tautomers are isomers that are in rapid equilibrium. Keto-enol tautomers differ in the location of a double bon and a hydrogen. For most ketones, the enol tautomer is much less stable than the keto tautomer Keto Enol Interconversion The interconversion of the keto and enol tautomers is called keto-enol interconversion, keto-enol tautomerization, or enolization. The interconversion can be catalyzed by either a base or an acid.
3 19.4 How Enolate Ions and Enols React The resonance contributors of the enolate ion show that it has 2 electron-rich sites: the α- carbon and the oxygen. The resonance contributor with negatively charged oxygen makes the greater contribution to the hybrid. The enolate ion is an example of an ambident nucleophile. An ambident nucleophile is a nucleophile with 2 nucleophilic sites ( two teeth ). Which nucleophilic site (C or O) reacts with an electrophile depends on the electrophile and on the reaction conditions. Protonation occurs preferentially on oxygen (the kinetic product), because of the greater concentration of negative charge on the more electronegative oxygen atom. When the electrophile is something other than a proton, carbon is more likely to be the nucleophile because carbon is a better nucleophile than oxygen. The mechanism for the formation of an enolate ion and its subsequent reaction with an electrophile is shown below.
4 The overall reaction is an α-substitution reaction; one electrophile (E + ) is substituted for another (H + ) at the α-carbon. The resonance contributors of the enol show that it too has 2 electron-rich sites: the α-carbon and the oxygen. The mechanism for the formation of an enol and its subsequent reaction with an electrophile is shown below. In summary, a carbonyl compound with an α-hydrogen can undergo a substitution reaction at the α-carbon. The substitution reaction can be catalyzed by a base (forming an enolate ion) or by an acid (forming an enol) Halogenation of the α-carbon of Aldehydes and Ketones When Br 2, Cl 2, or I 2 is added to a solution of an aldehyde or a ketone, a halogen replaces one or more of the α-hydrogens of the carbonyl compound. Acid-Catalyzed Halogenation In the acid-catalyzed reaction, the halogen replaces one of the α-hydrogens:
5 Base-Promoted Halogenation When excess Br 2, Cl 2, or I 2 is added to a basic solution of an aldehyde or a ketone, the halogen replaces all the α-hydrogens.
6 These 2 steps are repeated until all the α-hydrogens are replaced by the halogen. The Haloform Reaction In the presence of excess base and excess halogen, a methyl ketone is converted into a carboxylate ion. The conversion of a methyl ketone to a carboxylate ion is called a Haloform reaction because one of the products is Haloform either CHCl 3 (chloroform), CHBr 3 (bromoform), or CHI 3 (iodoform) Halogenation of the α-carbon of Carboxylic Acids: The Hell-Volhard-Zelinski Reaction Hell-Volhard-Zelinski reaction (HVZ reaction) heating a carboxylic acid with Br 2 + P in order to convert it into an α-bromocarboxylic acid.
7 19.7 α-halogenated Carbonyl Compounds are Useful in Synthesis Removing a proton from an α-carbon makes the α-carbon a nucleophile. When the α-position is halogenated, the α-carbon becomes electrophilic it reacts with nucleophiles. Substituting a bromine for a hydrogen bonded to an α-carbon makes the α-carbon an electrophile Using LDA to Form an Enolate Ion When LDA (lithium diisopropylamide) is used to removed the α-hydrogen, essentially all the carbonyl compound is converted to the enolate ion because LDA is a much stronger base than the base being formed. Therefore, LDA is the base of choice for those reactions that require the carbonyl compound to be completely converted to an enolate ion before it reacts with an electrophile.
8 19.9 Alkylating the α-carbon of Carbonyl Compounds Putting an alkyl group on the α-carbon of a carbonyl compound is an important reaction because it gives us another way to form a carbon-carbon bond. Alkylation is carried out by first removing a proton from the α-carbon with a strong base such as LDA and then adding the appropriate alkyl halide. Because the alkylation is an S N 2 reaction, it works best with methyl halides and primary alkyl halides. Enolate ions can be alkylated on the α-carbon Alkylation and Acylation of the α-carbon Using an Enamine Intermediate Enamines react with electrophiles in the same way that enolate ions do.
9 Because the alkylation step is an S N 2 reaction, only primary alkyl halides or methyl halides should be used. One advantage to using an enamine intermediate to alkylate an aldehyde or a ketone is that only the monoalkylated product is formed. In addition to using enamine intermediates to alkylate aldehydes and ketones, they can also be used to acylate these compounds Alkylation of the β-carbon: The Michael Reaction Michael reaction the addition of an α-carbanion to the β-carbon of an α, β-unsaturated carbonyl compound. The enolate ions that work best in Michael reactions are those formed from carbon acids that are flanked by 2 electron-withdrawing groups: enolate ions of β-diketones, β-diesters, β-keto esters, and β-keto nitriles. Because these enolate ions are relatively weak bases, conjugate addition occurs that is, addition at the β-carbon of α, β-unsaturated aldehydes and ketones. These enolate ions also add to the β-carbon of α, β-unsaturated esters and amides because of the low reactivity of the carbonyl group. Notice that Michael reactions form 1, 5-dicarbonyl compounds; that is, if a carbonyl carbon of the enolate ion is given the 1-position, the carbonyl carbon of the other reactant is at the 5-position.
10 Enamines can be used in place of enolate ions in Michael reactions. When an enamine is used as a nucleophile in a Michael reaction, the reaction is called a Stork enamine reaction.
11 19.12 An Aldol Addition Forms β-hydroxyaldehydes or β-hydroxyketones An aldol addition is a reaction in which both of these activities are observed: one molecule of a carbonyl compound after a proton is removed from an α-carbon reacts as a nucleophile and adds to the electrophilic carbonyl carbon of a second molecule of the carbonyl compound. Thus, and aldol addition is a reaction between 2 molecules of an aldehyde or 2 molecules of a ketone.
12 Because an aldol addition reaction occurs between 2 molecules of the same carbonyl compound, the product has twice as many carbons as the reacting aldehyde or ketone Dehydration of Aldol Addition Products Forms α, β-unsaturated Aldehydes and Ketones If the product of an aldol addition undergoes dehydration, the overall reaction is called an aldol condensation. A condensation reaction is a reaction that combines 2 molecules by forming a new C C bond while removing a small molecule (usually water or an alcohol). An aldol addition product loses water to form an aldol condensation product. An aldol condensation forms an α, β-unsaturated aldehyde or an α, β-unsaturated ketone. E1cB (elimination unimolecular conjugate base) reaction a two step elimination reaction that forms a carbanion intermediate. E1cB reactions occur only if the carbanion can be stabilized by electron delocalization The Crossed Aldol Addition
13 Crossed aldol addition or mixed aldol addition an aldol addition in which 2 different carbonyl compounds are used. If 2 different carbonyl compounds are used in an aldol addition, 4 products can be formed because each enolate ion can react with either of the 2 carbonyl compounds A Claisen Condensation Forms a β-keto Ester Claisen Condensation When 2 molecules of an ester undergo a condensation reaction, the reaction is called a Claisen condensation. The product of a Claisen condensation is a β-keto ester. In a Claisen condensation, as in an aldol addition, one molecule of carbonyl compound is the nucleophile and a second molecule is the electrophile. The new C C bind connects the α- carbon of one molecule and the carbon that was formerly the carbonyl carbon of the other molecule.
14 After nucleophilic addition, the Claisen condensation and the aldol addition differ. In the Claisen condensation, the negatively charged oxygen re-forms the carbon-oxygen π bond and eliminates the OR group. In the aldol addition, the negatively charged oxygen obtains a proton from the solvent. A Crossed Claisen Condensation A crossed Claisen condensation is a condensation reaction between 2 different esters. Like a crossed aldol addition, a crossed Claisen condensation is a useful reaction only if it is carried out under conditions that foster the formation of primarily one product. Primary one product will be formed from crossed Claisen condensation if one of the esters has no α-hydrogens (and therefore cannot form an enolate ion) and the ester with α- hydrogens is added slowly to a solution of the ester without α-hydrogens and the alkoxide ion Intramolecular Condensation and Addition Reactions A compound with 2 ester groups would undergo an Intramolecular Claisen condensation, and a compound with 2 aldehyde or ketone groups would undergo an Intramolecular aldol addition.
15 Intramolecular Claisen Condensations Dieckmann condensation an Intramolecular Claisen condensation. Intramolecular Aldol Additions Because a 1, 4-diketone has 2 different sets of α-hydrogens, 2 different Intramolecular addition products can potentially form, one with a 5-membered ring, the other with a 3- membered ring. The greater stability of 5-and-6-membered rings caused them to be formed preferentially. In fact, the 5-membered ring product is the only product formed from the Intramolecular aldol addition of a 1, 4-diketone. The Intramolecular aldol addition of a 1, 6-diketone can potentially lead to either a 7- or a 5- membered ring product. Again, the more stable product the one with the 5-membered ring is the only product of the reaction.
16 1, 5-Diketones and 1, 7-diketones undergo Intramolecular aldol additions to form 6- membered ring products The Robinson Annulation Robinson Annulation a Michael reaction followed by an intramolecular aldol condensation. An annulation reaction is a ring-forming reaction. Robinson annulation results in a product that has a 2-cyclohexenone ring.
17 19.19 Carboxylic Acids with a Carbonyl Group at the 3-Position can be Decarboxylated Decarboxylation loss of CO 2 molecule 3-Oxocarboxylic acids decarboxylate when heated. Carboxylic acids with a carbonyl group at the 3-position lose CO 2 when they are heated The Malonic Ester Synthesis: A Way to Synthesize a Carboxylic Acid Malonic ester synthesis the synthesis of a carboxylic acid, using diethyl malonate as the starting material.
18 19.21 The Acetoacetic Ester Synthesis: A Way to Synthesize a Methyl Ketone The only difference between the Acetoacetic ester synthesis and the Malonic ester synthesis is the use of Acetoacetic ester rather than Malonic ester as the starting material. An acetoacetic ester synthesis forms a methyl ketone with 2 more carbons than the alkyl halide. The steps in the acetoacetic ester synthesis are the same as those in the Malonic ester synthesis.
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