{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

Chapter 23 Ester Enolates and the Claisen Condensation

Chapter 23 Ester Enolates and the Claisen Condensation -...

Info icon This preview shows pages 1–4. Sign up to view the full content.

View Full Document Right Arrow Icon
Chemistry 140C Winter 2010 (K. Albizati) 1 Chapter 23 Ester Enolates and the Claisen Condensation β -dicarbonyl compounds are versatile compounds which have significant industrial importance. They are also an important class of biochemical intermediates. β -Ketoester Synthesis via the Claisen Condensation General Form Example
Image of page 1

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Chemistry 140C Winter 2010 (K. Albizati) 2 Mechanism This step is not favorable due to the great difference in pK a between ethanol (about 17) and ethyl acetate (about 25). Very little of the ester enolate is present at equilibrium , but it is enough for the reaction to proceed. This step is a simple nucleophilic addition of an enolate to a carbonyl. This step is an elimination step. Although alkoxide is a relatively strong base and is not a good leaving group in an S N 2 (displacement) sense, it is an OK leaving group in an elimination reaction under strongly basic conditions. ANIMATION MECHANISM Up to here, all of the reactions have been reversible; that is, the equilibrium constants are not all that favorable. However the final step in the mechanism is a strongly favored acid-base reaction and serves to drive the whole sequence to this thermodynamic endpoint: O OEt O OEt + H H O OEt O + H EtOH pK a = 11 pK a = 17
Image of page 2
Chemistry 140C Winter 2010 (K. Albizati) 3 At this point, the reaction is worked up with aqueous acid and the β -ketoester enolate is protonated and can be isolated and purified: In general, protons that are between 2 carbonyl groups have enhanced acidity because of resonance delocalization in the anion: And the hybrid can be drawn as : with the negative charge spread out over 5 atoms. See Table 23-1 for a list of pK a values of β -dicarbonyls and related compounds. This is not for memorization but you should have an idea of the general neighborhood of the pKa of a β -dicarbonyl and what is more or less acidic than these. The Claisen condensation fails for esters that have only one acidic hydrogen next to the ester carbonyl. Why do you suppose this is the case?
Image of page 3

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Image of page 4
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern