Chem.243ch18 - Chemistry 243 Chapter 18-Aldehydes and...

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Chemistry 243 Chapter 18—Aldehydes and Ketones revisited!! Recall the main regions of reactivity in aldehydes and ketones? Let us review: a. Where do nucleophiles attack? b. Where do electrophiles attack? C O Let us recall take a look at the anatomy of a carbonyl with hydrogens. What is special about the -hydrogens α as compared to the others?: C H 3 CC H 2 C H 2 CH 3 O Well done! The main reaction to be investigated here is the acidity of the alpha hydrogens . Recall that acidity is increased by proximity to an electron-withdrawing group, or two! Can you arrange the compounds below in increasing acidities of their -hydrogens α ? O propanal propane O O 2,4-pentanedione
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In addition to the fact mentioned above, we have the wonderful answer to everything in organic chemistry: resonance! The conjugate base is stabilized by the inductive effect of the positively polarized carbonyl and……delocalization through resonance! Let us try some examples: H O H O O The anions that result are known as: We have to remember that each resonance form can be brought out in reactions. That is why the enolate ion is called : ambident. The next term we need to relearn is: tautomerism . Through this process, isomers formed are called enol and keto tautomers. Which predominates?
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Chem.243ch18 - Chemistry 243 Chapter 18-Aldehydes and...

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