Thanks for your explanation but I'd like to know what you mean by "this resonance structure contributes more to the hybrid for an amide than it does for any of the other carboxylic acid derivatives". Contributing more to the hybrid for an amide, why? And why is it important in here.
As for "contributing to the hybrid" I mean that, when you have resonance structures that are not equivalent, the overall picture (hybrid) is a weighted average of the contributing canonical forms. Some canonical forms contribute more or less (lower weighting) than others based on separation of charge, full octets, electronegativity of atoms bearing charges, etc.
Between N & O, nitrogen is a better resonance donating group b/c N is less electronegative and holds on less tightly to it's lone pair of electrons. As such, in an amide the resonance structure where N donates a lone pair to the partial positive carbon is more significant (contributes more to the weighted average) than the analogous one in an ester.
Between Cl and the others, electronegativity doesn't explain all of it b/c O>Cl. There is also the difference in size between the 3p orbitals on Cl and 2p orbitals on carbon leading to less than ideal overlap.
Overall, the resonance stabilization increases (lowest to highest) in order of:
acid chloride < anhydride < ester < amide
Reactivity follows the opposite trend
Are these statements correct:
1-hydrolysis of acetyl chloride is because of the Cl ( a good leaving group).
2-acetamide is not readily hydrolyzed because NH2 is not a good leaving group.
I'd agree with both of those, but keep in mind the quality of the leaving group is most useful in determining which leaving group will be kicked out once a nucleophile attacks. How much resonance stabilization the starting material helps determine whether the weak nucleophile water will attack the carbonyl in the first place.
3-as far as resonance stabilization, there is more resonance in acetic anhydride than ethyl acetate. therefore acetic anhydride is a more stable compound than ethyl acetate. but why is it more readily hydrolyzed than ethyl acetate?
There is more resonance for the anhydride, but the middle oxygen of the anhydride has two carbonyl's that it can donate to...effectively halving it's donating ability to either one.