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Chapter 24: Q24P. (page 1287)

Question. Propose a mechanism for the coupling of acetic acid and aniline using DCC as a coupling agent.

Short Answer

Expert verified

Representation of DCC as c-Hx

Mechanism for the coupling of acetic acid aniline

Step by step solution

01

Step1: Introduction to DCC

DCC or N,N’-Dicyclohexylcarbodiimide is used in the coupling reactions of amino acids during peptide synthesis. It has a low melting point and can be handled easily. Often, in reactions, we use the abbreviation ‘c-Hx’ to stand for cyclohexyl in DCC.

Representation of DCC as c-Hx

02

Mechanism for the reaction

The nitrogen of aniline firstly abstracts hydrogen from acetic acid-forming acetate ion and anilinium ion. Then, the oxygen of the acetate ion acts as a nucleophile and attacks thecarbon center of DCC, which is an electrophile. DCC thereby activatesthe carboxyl group,converting it into an excellent leaving group. Further, a reaction with another molecule of aniline occurs inthis activated carboxyl group, and lastly, the leaving group leaves, which is resonance-stabilized, and the final product is formed.

Mechanism for the coupling of acetic acid aniline

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Most popular questions from this chapter

Complete hydrolysis of an unknown basic decapeptide gives Gly, Ala, Leu, Ile, Phe, Tyr, Glu, Arg, Lys, and Ser. Terminal residue analysis shows that the N terminus is Ala, and the C terminus is Ile. Incubation of the decapeptide with chymotrypsin gives two tripeptides, A and B, and a tetrapeptide, C. Amino acid analysis shows that peptide A contains Gly, Glu, Tyr, and; peptide B contains Ala, Phe, and Lys; and peptide C contains Leu, Ile, Ser, and Arg.Terminal residue analysis gives the following results.

Incubation of the decapeptide with trypsin gives a dipeptide D, a pentapeptide E, and a tripeptide F. Terminal residue analysis of F shows that the N terminus is Ser and the C terminus is Ile. Propose a structure for the decapeptide and for fragments A through F.

A molecular weight determination has shown that an unknown peptide is a pentapeptide, and an amino acid analysis shows that it contains the following residues: one Gly, two Ala, one Met, one Phe. Treatment of the original pentapeptide with carboxypeptidase gives alanine as the first free amino acid released. Sequential treatment of the pentapeptide with phenyl isothiocyanate followed by mild hydrolysis gives the following derivatives:

Propose a structure for the unknown pentapeptide.

Suggest a method for the synthesis of the natural L enantiomer of alanine from the readily available L enantiomer of lactic acid.

Histidine is an important catalytic residue found at the active sites of many enzymes. In many cases, histidine appears to remove protons or to transfer protons from one location to another.

(a) Show which nitrogen atom of the histidine heterocycle is basic and which is not.

(b) Use resonance forms to show why the protonated form of histidine is a particularly stable cation.

(c) Show the structure that results when histidine accepts a proton on the basic nitrogen of the heterocycle and then is deprotonated on the other heterocyclic nitrogen. Explain how histidine might function as a pipeline to transfer protons between sites within an enzyme and its substrate.

Draw the resonance forms of a protonated guanidino group and explain why arginine has such a strongly basic isoelectric point.

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