Pictures of the Day 2-06-08

 
Four addition reactions to aldehydes and ketones. In each case, the nucleophilic site of the molecule (electron rich, red or orange color) interacts with the carbonyl carbon atom that has a partial positive charge (blue color). Each of these reactions follows Mechanism A and they all make carbon-carbon bonds (Hooray!!! Trumpet fanfare!!!!). The only difference is that in the case of the reaction with HCN, the chemist does not have to open the flask and add acid for the second step to occur, in the other three cases the chemist does. Also note that in the case of CN- and the Grignard reaction it is not so obvious which nucleophilic site reacts with the carbonyl. Each has more than one partial negative charge. The site that reacts is not always the one with the most electron density, it is the one that can form the most stable bond. Reactions need a motive to react, namely stable products (lower in energy than starting materials) as well as the opportunity (nucleophile reacts with electrophile). Thus, reactions are like crimes and you need to look for both motive and opportunity. In the case of the Grignard and CN- reaction, the more stable bonds by far are the carbon-carbon bonds, so these are what form in the reactions.
Above is shown the simplest Wittig reagent, namely the ylide made from a methyl halide and triphenyl phosphine. Note how in the electrostatic potential surface the negative charge is on the carbon atom, explaining why it is such a good nucleophile and reacts with carbonyl groups
Shown here is the four-membered ring intermediate that occurs when the Wittig reagent derived from a methyl halide reacts with acetone. The unique feature of the Wittig reaction is the mechanism in whichthe famous four-membered ring intermediate, called an oxaphophetane, is formed. This is completely different than the other reactions we have seen,(it does not follow Mechanisms A-D). When you think Wittig, I want you to think four-membered ring intermediate.