"The dipolar Lewis structure indicates the polarization of the C=O bond. Even though it is a minor contributor (because of the lack of an octet on carbon), it explains the deshielding of the carbonyl carbon. In carboxylic acids, however, the corresponding dipolar form contributes less to the resonance hybrid: The hydroxy oxygen can donate an electron pair to give a third arrangement in which the carbon and both oxygen atoms have octets. The degree of positive charge on the carbonyl carbon and therefore its deshielding are greatly reduced. The changes in electron density distribution around the carbonyl function when going from acetone to acetic acid are shown in their respective electrostatic potential maps."