A much better measure is the difference in IR stretching frequency of the C=O group ,  where we noted a competition between conjugation by lone-pair electron donation into the carbonyl from OCOR, OR, or NH₂ and inductive with drawal from the C=O group because of the electronegativity of the substituent. Conjugation donates electrons into the π* orbital of the π bond and so lengthens and weakens it. The C=O bond becomes more like a single bond and its stretching frequency moves towards the single bond region, that is, it goes down. The inductive effect removes electrons from the π orbital and so shortens and strengthens the π bond. It becomes more like a full double bond and moves up in frequency.

These effects are balanced in different ways according to the substituent. Chlorine is poor at lone-pair electron donation (its lone pair is in a large 3p orbital and overlaps badly with the 2p orbital on carbon) but strongly electron-withdrawing so acid chlorides absorb at high frequency, almost in the triple-bond region. Anhydrides have an oxygen atom between two carbonyl groups. Inductive withdrawal is still strong but conjugation is weak because the lone pairs are pulled both ways. Esters have a well-balanced combination with the inductive effect slightly stronger (oxygen donates from a compatible 2p orbital but is very electron egative and so withdraws electrons strongly as well). Finally, amides are dominated by con jugation as nitrogen is a much stronger electron donor than oxygen because it is less electronegative.

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