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Ebook Organic chemistry as a second language (3e): Part 2

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(BQ) Part 2 book "Organic chemistry as a second language" has contents: Ketones and aldehydes, carboxylic acid derivatives, enols and enolates, amines, nucleophilicity and basicity of amines. | CHAPTER 5 KETONES AND ALDEHYDES 5.1 PREPARATION OF KETONES AND ALDEHYDES Before we can explore the reactions of ketones and aldehydes, we must first make sure that we know how to make ketones and aldehydes. That information will be vital for solving synthesis problems. Ketones and aldehydes can be made in many ways, as you will see in your textbook. In this book, we will only see a few of these methods. These few reactions should be sufficient to help you solve many synthesis problems in which a ketone or aldehyde must be prepared. The most useful type of transformation is forming a C¨ O bond from an alcohol. Primary alcohols can be oxidized to form aldehydes: OH O R H R And secondary alcohols can be oxidized to form ketones: OH R O R R R Tertiary alcohols cannot be oxidized, because carbon cannot form five bonds: OH O Carbon NEVER has 5 bonds So, we need to be familiar with the reagents that will oxidize primary and secondary alcohols (to form aldehydes or ketones, respectively). Let’s start with secondary alcohols. A secondary alcohol can be converted into a ketone upon treatment with sodium dichromate and sulfuric acid: OH R O Na2Cr2O7 H2SO4, H2O R R R Alternatively, the Jones reagent can be used, which is formed from CrO3 in aqueous acetone: OH R O CrO3 R aqueous acetone heat R R 129 130 CHAPTER 5 KETONES AND ALDEHYDES Whether you use sodium dichromate or the Jones reagent, you are essentially performing an oxidation that involves a chromium oxidizing agent (the alcohol is being oxidized and the chromium reagent is being reduced). You should look through your lecture notes and textbook to see if you are responsible for the mechanisms of these oxidation reactions. Whatever the case, you should definitely have these reagents at your fingertips, because you will encounter many synthesis problems that require the conversion of an alcohol into a ketone or aldehyde. Chromium oxidations work well for secondary alcohols, but we run into a