The product of nucleophilic addition to a carbonyl group is not always a stable compound
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص197-198
2025-05-08
597
Addition of a Grignard reagent to an aldehyde or ketone gives a stable alkoxide, which can be protonated with acid to produce an alcohol (you met this reaction in Chapter 9). The same is not true for addition of an alcohol to a carbonyl group in the presence of base—in Chapter 6 we drew a reversible, equilibrium arrow for this transformation and said that the product, a hemiacetal, is formed to a significant extent only if it is cyclic. The reason for this instability is that RO− is easily expelled from the molecule. We call groups that can be expelled from molecules, usually taking with them a negative charge, leaving groups. We’ll look at leaving groups in more detail later in this chapter and again in Chapter 15.
●Leaving groups Leaving groups are anions such as Cl−, RO−, and RCO2 − that can be expelled from molecules taking their negative charge with them.
So, if the nucleophile is also a leaving group, there is a chance that it will be lost again and that the carbonyl group will reform—in other words, the reaction will be reversible. The energy released in forming the C=O bond (bond strength 720 kJ mol−1) makes up for the loss of two C–O single bonds (about 350 kJ mol−1 each), one of the reasons for the instability of the hemiacetal product in this case. The same thing can happen if the starting carbonyl compound contains a potential leaving group. The unstable negatively charged intermediate in the red box below is formed when a Grignard reagent is added to an ester.
Again, it collapses with loss of RO− as a leaving group. This time, though, we have not gone back to starting materials: instead, we have made a new compound (a ketone) by a substitu tion reaction—the OR group of the starting material has been substituted by the Me group of the product. In fact, the ketone product can react with the Grignard reagent a second time to give a tertiary alcohol. Later in this chapter we’ll discuss why the reaction doesn’t stop at the ketone.
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