Chimica organica

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CH C C C C–C −EtO 3 22O O O molecule of ester, forming aL’eliminazione di , il gruppo uscente, forma un β-chetoestere [3]CH• CH CHOEt OEt[1] 33 2 In Step [1], the base removeOEt OEt •OEt +– `CH CH CHH C C C This joins the carbobond.EtOHGli stadi [1] e [3] sono reversibili• Steps [2]–[3] Nucleophilic addition and loss of the leaving group2 2 2α – α resonancarbon to form a– + EtO the carbonyl carbon of a seOEt OEt OEtEtO nucleophilic attack loss of the leaving group enolate.– In Step [2], the nucleophilic e•resonance-stabilized enolateO O OO Elimination of the leaving gro•O O[2] [3] electrophilic carbonyl carbon+ β-keto– a ester in Step [3]. StCH C CHCH C C C CC 3 22 molecule of ester, forming aCHCH CHOEt OEtSteps [2]–[3] Nucleophilic addition and loss of the leaving group reversible equilibria.33 2OEt OEtOEt `This joins the carbbond.–+ EtO– In Step [2], the nucleophilic

protonation

La formazione di tale enolato stabilizzato per risonanza sposta l'equilibrio nella reazione di• resonance-stabilized The formation of– (Step [4]).

β-ketoa ester in Step [3]. S+ H OClaisen EtO 2enolate β-ketoBecause the ester forO O •O O O O stabilized enolate drives threversible equilibria.

La protonazione di questo enolato con acidi forti riforma il composto neutro, completando la• + +EtOH H O has especially acidic protonthe Claisen reaction.

C C3–C C C Creazione CH CH OEtCHCH C COEt OEt carbonyl groups, a proton is[4] [5] 3 23 3Steps [4]–[5] Deprotonation and protonation Protonation of this enolate w•β-keto ester the basic reaction conditionsHH H β-ketore-forms the neutral eβ-ketoBecause the ester foO Oresonance-stabilized •O O O O The formation of– (Step [4]).+ H OEtO the reaction.2+enolate H O has especially acidic protonstabilized enolate drives thC C3–C C C C+ EtOH CH CH OEtCHCH C COEt

OEt carbonyl groups, a proton i[4] [5] the Claisen reaction.3 23 3 β-keto ester the basic reaction conditionHH H Protonation of this enolate w•Because the generation of a resonance-stabilized enolate from the produresonance-stabilized The formation o– (Step [4]).+ H OEtO β-ketore-forms the neutral e2enolatethe Claisen reaction (Step [4] of Mechanism 24.4), only esters with twostabilized enolate drives tthe reaction.+ EtOHαthe carbon undergo this reaction; that is, esters must have the general sthe Claisen reaction.RCH CO R'. Protonation of this enolate•2 2 β-ketore-forms the neutralBecause the generation of a resonance-stabilized enolate from the produKeep in mind: The characteristic reaction of esters is nucleophilic• the reaction.the Claisen reaction (Step [4] of Mechanism 24.4), only esters with twoClaisen reaction is a nucleophilic substitution in which an enolate iαthe carbon undergo this reaction; that is, esters must have the

general sRCH CO R'.Figure 24.6 compares the general reaction for nucleophilic substitution2 2Because the generation of a resonance-stabilized enolate from the prodPagina 13 di 22 Derivati degli acidi carbossiliciClaisen reaction. Sample Problem 24.3 reinforces the basic features of thethe Claisen reaction (Step [4] of Mechanism 24.4), only esters with twoKeep in mind: The characteristic reaction of esters is nucleophilic• αthe carbon undergo this reaction; that is, esters must have the generalClaisen reaction is a nucleophilic substitution in which an enolateProblem 22.27 CHOCO(CH ) CHWhat is the composition of the soap prepared by hydrolysis of this triacylglycerol?2 15 3CH OCO(CH ) CH CH(CH ) CHOCO(CH ) CHCH2 2 7 2 7 32 2 15 3 Scienze BiologichecisCHOCO(CH ) CH2 15 3Reazioni delle ammidi CH OCO(CH ) CH CH(CH ) CH2 2 7 2 7 322.13 Reactions of Amides cisLe ammidi possiedono il peggior gruppo uscente di tutti i derivati degli acidi carbossilici e di• conseguenza

sotto forma di sale• + R' NHCamine is formed. =R' H or alkyl 2–R Od’ammonio In acid, the amine by-product is protonated as an ammonium ion, whereas in base, a neutralOIn ambiente acido si ottengono ammine libere• O+Examples H Oamine is formed. 3C + 4+C NHCH NH CH OH3 2 3O O+Examples H O3OC +O 4+C NHCH NH CH OH3 2 3C CH O +2 –NHCH O CH NH3 3 2–O OH OC CH O +2 –NHCH O CH NH3 3 2The relative lack of reactivity of the amide bond is notable in proteins, which are polymers– OHof amino acids connected by amide linkages (Section 22.6B). Proteins are stable in aqueousThe relative lack of reactivity of the amide bond is notable in proteins, which are polymersof amino acids connected by amide linkages (Section 22.6B). Proteins are stable in aqueousLa relativa perdita di reattività delle ammidi è importante nelle proteine, che sono polimeri di• amminoacidi legati attraverso legami ammidiciLe proteine sono stabili in

soluzione acquosa e in assenza di acidi o basi, così possono esplicare le loro varie funzioni nell'ambiente cellulare acquoso

Il meccanismo di idrolisi delle ammidi è il tipico meccanismo a due stadi per la reazione di sostituzione nucleofila acilica, con l'aggiunta di uno stadio in cui è coinvolto un trasferimento protonico

L'acido carbossilico che si ottiene inizialmente reagisce ulteriormente nelle condizioni basiche per formare un anione carbossilato stabilizzato per risonanza, reazione che sposta l'equilibrio verso il completamento della reazione

The hydrolysis of the amide bond requires a variety of specific enzymes.

acid is exactly the same as the mechanism o− NHPerché avvenga l’idrolisi delle ammidi l’intermedio tetraedrico deve perdere , una base• 2lysis in acid (Section 22.11A) except that the leaving group is different.− OHmolto forte e quindi un gruppo uscente molto debole se paragonato aThe mechanism of amide hydrolysis in base has the usual two steps of the generof the nucleophile lossfor nucleophilic acyl substitution—addition followed byIdrolisi delle ammidi in ambiente basico group—plus an additional proton transfer. The initially formed carboxylic acid− OHI primi due stadi danno luogo all’addizione del nucleofilo , seguita dall’eliminazione del• under basic conditions to form the resonance-stabilized carboxylate anion, and t− NHgruppo uscente reaction to completion. Mechanism 22.10 is written for a 1° amide.2Poiché l’acido carbossilico è un acido organico forte ed il gruppo uscente