Why is amide a poor leaving group?

[pacer]

Why is amide a poor leaving group (is less reactive compared to other carboxylic acid derivatives)?

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[pacer]

Can we assign proton NMR to carbonyl carbons based on hydrogens on adjacent carbons?

 

[Teleologist]

Why is amide a poor leaving group (is less reactive compared to other carboxylic acid derivatives)?

Wouldn't a poor leaving group be more reactive?

 

[justadream]

@pacer

Do you mean why is NH2- a poor leaving group (as in, when NH2- leaves following nucleophilic acyl substitution)?

I don't think amides usually act as leaving groups (@Teleologist probably knows more about this). Here's a table on wiki (http://en.wikipedia.org/wiki/Leaving_group) that shows that amides are pretty poor leaving groups.

If you really mean NH2- as a leaving group, then you can rationalize this by noting that NH2- is somewhat basic (not super weak). Thus, they are somewhat reactive and make poor leaving groups.

Ideally, you want your leaving group to be as unreactive as possible. For example Cl- is unreactive (it it electron withdrawing and can stabilize the negative charge) which makes acid chlorides reactive (since it has a good Cl leaving group).

 

[type12]

Why is amide a poor leaving group (is less reactive compared to other carboxylic acid derivatives)?

Because he works in finance and was laid off, and still recovering from the depression.

 

[Teleologist]

If you really mean NH2- as a leaving group, then you can rationalize this by noting that NH2- is somewhat basic (not super weak). Thus, they are somewhat reactive and make poor leaving groups.

Amides are poor leaving group because when they leave they break the bond heterolytically as to create a cation and H2N(-) (nitrogen with two lone pairs). In water solution, amide anions are strong bases (not just somewhat basic but very basic). This makes sense; ammonia is a weak base and an even crappier acid; the conjugate base of a crappy acid is a strong base.

 

[justadream]

@Teleologist

When people say an "amide" leaving group, does that mean like a CONH2 as a leaving group or does that mean like NH2-?

You seem to imply that NH2- is an amide?

Originally, I thought the OP was asking why CONH2 would make a poor leaving group.

 

[Teleologist]

@Teleologist

When people say an "amide" leaving group, does that mean like a CONH2 as a leaving group or does that mean like NH2-?

You seem to imply that NH2- is an amide?

Originally, I thought the OP was asking why CONH2 would make a poor leaving group.

Oops, missed that. I see now that the OP was referring specifically to carboxylic acid derivatives. But yes, the amide leaving group can refer to NH2(-). http://en.wikipedia.org/wiki/Amide

 

[justadream]

@Teleologist

This may be a stupid question but:

1) Why is NH2- called an amide when it lacks a carbonyl group? It seems like it should be an amine like NH2 (at least to me lol)

2) If someone talks about an "amide" in general, do you need to rely on context to determine whether it is talking about a NH2- or a CONH2?

 

[Teleologist]

@Teleologist

This may be a stupid question but:

1) Why is NH2- called an amide when it lacks a carbonyl group? It seems like it should be an amine like NH2 (at least to me lol)

Ions end in "-ide." Consider the sulfur anion: S(2-). Also known as the sulfide anion. Consider the oxygen anion. Also known as the oxide anion, O(2-).

2) If someone talks about an "amide" in general, do you need to rely on context to determine whether it is talking about a NH2- or a CONH2?

Context. In any case I'm still confused by the OP: is a carboxylic amide a good or a poor leaving group?

 

[justadream]

@Teleologist

Right, that makes sense.

Is CoNH2 an amide because it technically has negative charge somewhere? If not, why is that also an amide?

 

[The Brown Knight]

the carbonyl carbon-nitrogen bond in amide is resonance stabilized in comparison to an oxygen containing substituent. the nitrogen due to its lower electronegativity donates electrons to the electrophilic carbon and doesn't mind the positive formal charge as much as an oxygen or halide would. this resonance stabilization gives the carbon-nitrogen bond a between 1-2 bond order and hence a stronger bond than between the carbonyl carbon and a halide/alkoxy group.
on a side note, in an acid-catalyzed hydrolysis of an amide, the proton would actually bind to the oxygen rather than the nitrogen because the latter has a resonance-based positive formal charge; you'd need a strong base to displace the amine group from the amide.