CH18_Lecture

CH18_Lecture - Chapter 18: Enols and Enolates Summary In...

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Unformatted text preview: Chapter 18: Enols and Enolates Summary In this chapter we look at another type of reactivity of carbonyl compounds where they function as nucleophiles. Before starting this, you should make sure you are familiar with the contrasting reactions where the aldehydes and ketones are attacked by nucleophiles due to the electrophilic character of the carbonyl C and undergo nucleophilic addition. Treatment of aldehydes and ketones with a suitable base can lead to the formation of a nucleophilic species called an enolate that reacts with electrophiles. Treatment of esters with a suitable base can also lead to the formation of a nucleophilic species called an ester enolate that will react with an electrophile. These C nucleophiles are useful for making new carbon-carbon bonds. Acidity of a-Hydrogens Compared to simple hydrocarbons, the a-protons adjacent to carbonyl groups are much more acidic and can be removed by common bases (e.g. HO-, RO- etc.). For example, compare the acidity of propanone and propane : pKa = 19 pKa 50 As we have advocated before, look at the stabilization of the conjugate base. Notice the proximity of the adjacent p system, and hence the possibility for RESONANCE stabilization by delocalization of the negative charge to the more electronegative oxygen atom. A very similar case is the comparison of the acidity of carboxylic acids (pKa = 5) and alcohols (pKa = 16). The acid is more acidic since the negative charge can be delocalized to a second electronegative oxygen atom. This delocalization makes the carboxylate more stable (more favorable). This delocalization is not possible in the alcohol. In fact, the -OH proton is also an a- proton. pKa = 5 pKa = 16 The same pattern occurs with the a-hydrogens in aldehydes and ketones... a resonance structure can be drawn with the negative charge relocated on the electronegative oxygen atom..... The more effective the resonance stabilization of the negative charge, the more stable the conjugate base is and therefore the more acidic the parent system. Typical pKa values for a ketone and an aldehyde are shown: pKa = 19 pKa = 17 We can rationalize the trend by comparing the two structures, the difference simply being the alkyl- group versus the hydrogen. Since alkyl groups are weakly electron donating, they tend to destabilize anions (you should recall that they stabilize carbocations). This is because they will be "pushing" electrons towards a negative system which is unfavorable electrostatically. Hence the anions of ketone where there are extra alkyl groups are less stable than those of aldehyde, and so, ketones are less acidic. In some cases there could be H atoms that are adjacent to 2 carbonyl groups. This means that there is more resonance stabilization of the anion since the charge can be delocalized to 2 electronegative oxygen atoms. As a result, we have an even more acidic pKa. These type of compounds are sometimes called "active methylene". compounds are sometimes called "active methylene"....
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This note was uploaded on 04/09/2008 for the course CHEM 313 taught by Professor Narske during the Spring '08 term at Augustana.

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CH18_Lecture - Chapter 18: Enols and Enolates Summary In...

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