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In Clayden et al., Organic Chemistry, p. 332, the authors compare the reaction energy profile for ester formation from a) acyl chloride and b) anhydride:

Clayden et al., Organic Chemistry, OUP 2001

They then write:

The energy levels of the starting materials, the transition state, and the intermediate are all lower in the anhydride reaction than in the chloride reaction.

Are they really comparing absolute energies between different species?

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These authors are comparing relative energies. Look at what they set as their reference... the products. They could have easily made "starting materials" as their 0.0 reference point and stated that the products of the acyl chloride reaction is lower in energy relative to the anhydride reaction mechanism.

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