Pictures of the Day 2-04-08

 
Shown above is the electrostatic potential surface of a carbonyl, in this case formaldehyde (left) and acetaldehyde (right). This picture is most of what you need to remember about carbonyls in terms of nucleophilic attack. The electrostatic force will direct nucleophiles to the carbonyl carbon atom, while electrophiles (Lewis acids and protons) will be directed to the oxygen atom. The pi bond will break upon nucleophilic attack to create the ubiquitous, always present, never to be forgotten, can't live without it, gotta have it, gotta love it, TETRAHEDRAL INTERMEDIATE. The main differences between the carbonyl mechanisms involve the timing of protonation and/or whether there is a leaving group (Cl-, -OCH3) attached to the carbonyl. These latter statements will make a lot of sense to you very soon!
The above four mechanisms describe all of the different reactions you will see for carbonyl species such as aldehydes, ketones and carboxylic acid derivatives. They are really just various combinations of the four mechanism elements A) Add a proton, B) Remove a proton, C) Attack with a nucleophile, and D) Departure of a leaving group. They all have a tetrahedral intermediate, and they all involve a nucleophile attacking the carbon atom of a carbonyl. The difference comes down to whether there is a leaving group involved, then the timing of protonation/deprotonation.