The reactivity of carboxylic acids and derivatives toward nucleophiles is similar to the reactivity of aldehydes and ketones. The carbonyl carbon possesses a partial positive charge due to the polarity of the carbon-oxygen bond. There is also the contribution of a resonance form which introduces a partial positive charge to the carbon, making it susceptible to attack by nucleophiles. Furthermore, protonation of the carbonyl portion of the carboxyl group under conditions of acid catalysis (which also can occur with aldehydes and ketones), increases the susceptibility of the group toward attack by the nucleophile. Acyl transfer reactions as a class begin by formation of a tetrahedral intermediate which generally resolves by an elimination step that restores the carbonyl group. Addition-elimination has occurs with the original group bound to the carbonyl portion having been substituted by another. The acyl transfer mechanism of addition-elimination among the carboxylic acid derivatives is one of the most important in chemistry.