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1-18-2024

Organometallic Reagents: Partial negative charge on CARBON!
The important feature of organometallic reagents like organolithium and Grignard reagents is that the carbon atom attached to the Li or Mg atom has a partial negative charge. The partial negative charge can be seen as the red color on the carbon atom attached to the Li or Mg atoms on the above organolithium and Grignard reagents, respectively. The partial negative charge means that the carbon atom acts as a nucleophile with electrophiles such as protons in –OH groups or, more importantly, Grignard reagents react with electrophilic carbon atoms such as the epoxide as shown below. Note that relief of the ring strain within the three-membered epoxide ring adds a driving force (motive) to the latter reaction. 
Above is the entire reaction between and Grignard reagent and an epoxide. The nucleophilic carbon of the Grignard reagent attacks the electrophilic carbon of the epoxide. Note that the less-hindered epoxide carbon is attacked as expected for any SN2-like (C-C bond-making and C-O bond-breaking occur simultaneously) process. The magnesium salt of an alkoxide is obtained. The reaction is completed with the chemist opens the flask and adds mild aqueous acid to protonate and create the product alcohol. A key feature of this process is that a carbon-carbon bond is created!!!! Note how the stereochemistry at the lone chiral center is retained throughout the reaction, leading to production of a single stereoisomer (of the S configuration) product.
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 1) Make a newbond between a nucleophile and electrophile, 2) Break a bond so that realtively stable molecules or ions are formed, 3) Add a proton, and 4) Take a proton away.

All of the carbonyl mechanisms have a tetrahedral intermediate, and they all involve a nucleophile attacking the carbon atom of a carbonyl with simultaneous breaking of the C=O pi bond. The differences comes down to whether there is a leaving group involved and the timing of protonation/deprotonation.