dat_student said:
I looked at some of your references:
This is what one of them says:
http://www.hamline.edu/~rmajerle/Chem3460/Slides/April2406.ppt
"
Some ammonium salts of carboxylic acids
can be dehydrated to the amide
at high temperatures
This is generally a
poor method of amide synthesis"
If temperature is not high it is not even possible.... You need to heat it up to make it go through a second step. As your reference suggests this method doesn't work for all cases and is a very very very very poor method to make amides.
This is one of the usual ways to make amides in labs:
McMurry 4th edition page 1079 (McMurry is a professor at University of Pennsylvania)
the mechanism of amide formation by reaction of a carboxylic acid and an amine with DCC (dicyclohexylcarbodiimide)
Figure 27.8
1)
the carboxylic acid first adds to the carboiimide reagent to yield a reactive acylating agen (at this point we no longer have COOH, instead we have RCOO-(DCC) << This is done to prevent acid & base rxn)
2)
nucleophilic attack of the amide on the acylating agent gives a tetrahedral intermediate
3)
The intermediate loses dicyclohexylurea and yields the desired amide.
I don't consider that one step. If you think that's one step, fine it's one step. I don't think the question is asking for exceptional cases and exceptional set of parameters.
omg. are u for real? you are fun, yet dumb. no wonder you chose dental school. lol. good thing though my lab is closed down for 2 days and i got absolutely nothing to do today.
anyway,
everything is done in one step. let me just reminds you again and again that the number of experimental steps you take defines the rxn steps. if you add sugar and heat up until it dissolves, it is considered one step. if you add sugar stir until dissolves partially, and then you heat to melt more sugar, it is considered as a two step.
for the amide synthesis pathway i was talking about, you dump all the reagents and heat up all at one step. no additional separate steps are required. you dont heat separately after the acid-base rxn is done or not.
you just do all the step at once. dump and heat. leave it alone for a while.
first of all, like all the amide formation rxn with carboxylate derivatives, the reacting amine must not be tertiary. guess why? it needs to form a bond with carbonyl carbon and nitrogen-carbon bond is not as unstable as N-H bond. thus any tertiary amine will not form amide with any carbonyl compounds unless reaction condition is really harsh.
of course, the tertiary amine can act as base so it will form acid-base pair with the carboxylate. but it wont be able to make any new bonds with carbon. it could at certain temperature but it will be such extreme condition, the carbon chain will crack. you have heard about cracking, right? petroleum purification, modification, etc.
anyway, let me address some reasons why your copying and pasting of orgo. lab manual does no good in defending your case.
when you add DCC to carboxylic acid, you doing a different synthesis pathway from the one i was talking about earlier which involves salt.(do you even understand intro orgo?).
anionic oxygen of the carboxylate attacks the DCC, thus forming a better leaving group than oxygen. it is similar to acylation of carboxylic acid with SOCl2, PBr3, etc. another organic reagent you could use instead of DCC for the rxn you mentioned is HOBt.
so this particular pathway, you mentioned, requires the prior formation of carboxylate-DCC adduct before the addition of the amine. by the way, carbxylate-DCC adduct is functionally similar to anhydride.
you FIRST do the reaction that forms more reactive species. only AFTER the adduct rxn is completed(so you wait for this rxn to be completed before doing anything else to the rxn vessel) you add amine. now let me ask you why the hell amine was not added along with carboxylic acid and DCC ?
i will just give you the answer since it seems like you cannot think at all.
amine will react with carboxylic acid, so carboxylate-DCC adduct will form faster. but amine will be protonated, thus won't be able to function as good nucleophile. another reason is that amine can react with DCC thus reducing the effective concentratino of free DCC. therefore, two separate and distinct empirical steps are required for the amide synthesis pathway you mentioned.
back to the carboxylate-DCC pathway. as i stated above carboxylate-DCC is functionally similar to anhydride.
that is why it undergoes the typical addition-elimination rxn of the carboxylic-acid derivatives which involves the tetrahedral intermediate.
the formation of amide by heating carboxylic acid and amine mixture does NOT undergo same mechanistic pathway. it involves salt as intermediate. the rxn pathway you have mentioned actually prevents the ammonium carboxylate salt formation.
so when doe the salt forms in your rxn pathway?
as you can see clearly, why compare two rxns that are totally different?
Furthermore how can you draw a conclusion as you did?
to be clear, let me just mention again the fact that the urea/amide synthesis pathway i mentioned earlier using carbon dioxide and ammonia w/ heat involves no addition of DCC or any electrophiles that will ultimately serve as leaving group. also, addition of DCC to carboxylic acid prior to separate addition of amine prevents the formation of the ammonium carboxylate salt formation, which is central to the amide synthesis pathway i was talking about before.
so your supposedly counter-example does not refute or have any relevance to the fact that the synthesis i was talking about was one step.
it is one step.
also whether a particular rxn's yield(which defines whether a synthesis pathway is poor method or not) is poor or not does affect whether a rxn is single step or multiple step rxn. it just tells that there might alot of competing rxn.
then again, your stupidity was more than exposed when you got confused between single step rxn and simple rxn. where is the defense for that?
😱
lol.
