An abbreviated mechanistic description for DCC-mediated amide bond formation is

Question

An abbreviated mechanistic description for DCC-mediated amide bond formation is illustrated as Path A. An alternative pathway for this process has been proposed as well, illustrated in Path B. In this alternative pathway, a second molecule of acid 1 out-competes the amine 4 for the pivotal activated acyl derivative 3, leading to a transient symmetrical anhydride 7 (with discharge of 6). In Path B, anhydride 7 then is the active electrophile that reacts with amine 4 to give the observed amide product 5 with regeneration of one molecule of acid 1.

Select one:

a. Chiral amide 11a only.

b. Chiral amide 11b only.

c. An equal mix of 11a and 11b (= racemic mix).

d. An unequal mix of 11a and 11b.

Explanation / Answer

if the reaction follows only path A then we will obtain an unequal mixture of 11a and 11b. Path A is the normal mechanism (well documented) for the given reaction of carboxylic acids with amine with DCC coupling to form amides.

Due to hinderance the attack from back side will be more as compared to front side attack, so we will not obtain racemic mixture but an unequal mixture.

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