O chem question 11

[luv8724]

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Is it usually the e- donating group that is a good leaving group?

like how grignard reaction on ester?

 

[UndergradGuy7]

Usually we think of it in terms of basicity. A strong base would not be likely to leave. A weak base would be likely to leave. Think a strong base wants to react complete, so it can't stay alone or leave. If it did leave it would react right away and re-join. Weak one can separate and stay separate.

Example...
In benzene we learned NH2 was the best donating group. -OR is also a good donating group...

In the carboxylic acid derivatives... NH2 is the less reactive compared to esters...
So your theory does not work?

 

[luv8724]

Then how do I determine which group is a better leaving group in carboxylic acid derivatives?

 

[orgohacks]

Then how do I determine which group is a better leaving group in carboxylic acid derivatives?

pKa. This is a really clear trend.

The best leaving groups will be the weakest bases. If you look on your pKa table the weaker the base, the better the leaving group. This is why, say, Cl is a great leaving group (conjugate base of strong acid HCl) and H2N- is a terrible leaving group (conjugate base of weak acid NH3).
Only tricky thing to watch out for is carboxylic acids - under basic conditions, they will deprotonate to give conjugate base, and thus the leaving group would have to be O (2-).

 

[rnbaxter]

Then how do I determine which group is a better leaving group in carboxylic acid derivatives?

Memorization.
The best leaving group for acid derivatives is Cl in the acid chloride, because it is a withdrawing group (from the carbonyl Carbon).

The other derivatives of carboxylic acids contain donating groups each one is more and more donating and less and less reactive because it does not withdraw charges from the carbonyl carbon.

Therefore in terms of decreasing reactivity (acid chlorides, anhydrides, esters, amides, carboxylates)

Peace