Why is Cl a better leaving group than an anhydride?

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In summary, chloride is a better leaving group than an anhydride due to its electron withdrawing ability and stability as an anion. However, when considering the molecule as a whole and the interactions between the leaving group and the reaction center, the anhydride is actually a better leaving group. This is because the carboxyl group in the anhydride has resonance, making it a more stable and efficient leaving group.
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nobahar
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Hello!
I just read that chloride is a better leaving group than an anhydride (when adjacent to a carbonyl, hence the anhydride). (i.e. R-COCl, an Acyl chloride and R-COOCO-R', an anhydride).
I do not understand why this is so. A good leaving group is characterized by being electron withdrawing and a stable anion. Oxygen is more electronegative and a carboxyl group has resonance, which chloride doesn't.
I'd then conclude that the anhydride is a better leaving group; but apparently not!
Any help appreciated.
 
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Instead of just thinking about the leaving group as a discrete entity, think about the molecule as a whole, and about what interactions can be taking place between the LG and the reaction center.
 

1. Why is Cl a better leaving group than an anhydride?

Chlorine (Cl) is a better leaving group than an anhydride because it is more electronegative than oxygen, making it easier to leave a molecule and form a stable anion. In addition, chlorine is smaller in size than oxygen, allowing for a more efficient departure from the molecule.

2. What factors influence the leaving ability of a group?

The leaving ability of a group is influenced by its stability as an anion and its ability to disperse negative charge. Groups with high stability as anions, such as halogens, are better leaving groups. In addition, groups that are able to disperse negative charge, such as highly conjugated groups, also make good leaving groups.

3. How does the acidity of the leaving group affect its leaving ability?

The acidity of the leaving group does not directly affect its leaving ability. However, more acidic leaving groups tend to be more stable as anions, which can indirectly enhance their leaving ability.

4. Can an anhydride ever be a better leaving group than Cl?

Yes, under certain conditions, an anhydride can be a better leaving group than Cl. This is because the stability of an anhydride as an anion can be enhanced by resonance, making it a more favorable leaving group in certain reactions.

5. How does the reaction mechanism affect the leaving ability of a group?

The reaction mechanism can greatly influence the leaving ability of a group. For example, in reactions that involve nucleophilic substitution, a good leaving group is necessary for the reaction to occur. In contrast, in reactions that involve elimination, a poor leaving group may actually be preferred in order to form a more stable product.

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