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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鈥�-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 鈥榗-Hx鈥� 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

Peptides often have functional groups other than free amino groups at the N terminus and other than carboxyl groups at the C terminus.

(a) A tetrapeptide is hydrolyzed by heating with 6 M, and the hydrolysate is found to contain Ala, Phe, Val, and Glu. When the hydrolysate is neutralized, the odor of ammonia is detected. Explain where this ammonia might have been incorporated in the original peptide.

(b) The tripeptide thyrotropic hormone releasing factor(TRF) has the full name pyroglutamylhistidylprolinamide. The structure appears here. Explain the functional groups at the N terminus and at the C terminus.

(c)On acidic hydrolysis, an unknown pentapeptide gives glycine, alanine, valine, leucine and isoleucine. No odor of ammonia is detected when the hydrolysate is neutralized. Reaction with phenyl isothiocyanate followed by mild hydrolysis gives nophenylthiohydantoin derivative. Incubation with carboxypeptidase has no effect. Explain these findings.

The Sanger method for N-terminus determination is a less common alternative to the Edman degradation. In the Sanger method, the peptide is treated with the Sanger reagent, 2,4-dinitrofluorobenzene, and then hydrolyzed by reaction with 6 M aqueous HCl. The N-terminal amino acid is recovered as its 2,4-dinitrophenyl derivative and identified.

(a)Propose a mechanism for the reaction of the N terminus of the peptide with 2,4-dinitrofluorobenzene.

(b) Explain why the Edman degradation is usually preferred over the Sanger method.

Most naturally occurring amino acids have chiral centers (the asymmetric 伪 carbon atoms) that are named (S) by the Cahn-Ingold-Prelog convention (Section 5-3). The common naturally occurring form of cysteine has a chiral center that is named (R), however.

(a) What is the relationship between (R)-Cysteine and (S)-alanine? Do they have the opposite three-dimensional configuration (as the names might suggest) or the same configuration?

(b) (S)-Alanine is an L-amino acid (Figure 24-2). Is (R)-cysteine a D-amino acid or an L-amino acid?

Although tryptophan contains a heterocyclic amine, it is considered a neutral amino acid. Explain why the indole nitrogen of trytophan is more weakly basic than one of the imidazole nitrogens of histidine.

Show the third and fourth steps in the sequencing of oxytocin. Use figure 24-14 as a guide.
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