What is the mechanism for nucleophilic addition reactions at carbonyls?

The carbonyl bond is polar as the oxygen atom is more electronegative than the carbon atom. This results in a partial positive charge on the carbon and a partial negative charge on the oxygen. A nucleophile (electron pair donor) is able to attack into the elctron-deficient carbon of the C=O bond. This pushes a pair of electrons from the C=O double bond on to the oxygen atom, leaving the oxygen with a negative charge. The negatively charged oxygen of the intermediate is then able to pick up an electron-deficient hydrogen from the reaction solvent, leading to the alcohol product. An example of this type of reaction is the reduction of the ketone propanone with hydride (WILL GO THROUGH MECHANISM ON WHITEBOARD). NaBH₄ is a source of hydride ions, H^- . The hydride attacks the carbon of the carbonyl and electrons are pushed on to the oxygen, leaving it with a negative charge. This intermediate reacts with the solvent to form a secondary alcohol as the product. When drawing mechanisms, remember to draw clear curly arrows and include relevant lone pairs and dipoles.