Reactivity of Carboxylic Acid Derivatives

Reactivity of Carboxylic Acid Derivatives - Reactivity of...

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Reactivity of Carboxylic Acid Derivatives There are at least five important carboxylic acid derivatives that we will be focusing on and these are shown in the figure below. R Cl O Acid Chlorides R O O O R' Anhydrides R S O R' Thioesters R O O R' Esters R NH 2 O Amides R N H O R' R N O R' R" High Reactivity Low Reactivity The reactions we will focus on are the substitution reactions that take one carboxylic acid derivative to another one. In general, this will involve reacting the carbonyl derivative with an appropriate nucleophile. The reactivity of these derivatives varies greatly and they are ordered above from the most reactive (blazingly fast reactions) to the least reactive (slow as my sprinting speed). The Uphill/Downhill Rule In general, any of the compounds below a derivative on the reactivity scale can be made from that derivative, but anything above cannot. For example, thioesters can be made into esters or amides, but not anhydrides or acid chlorides. It is easy to go downhill but much tougher to go up. 1 Remember, Nature loves to lose energy and does so by making more stable compounds. Downhill can be equated to exothermic (heat releasing) reactions. In 1 This uphill-downhill rule generally holds but there are exceptions. For example, it is possible to take an amide and convert it to an ester by treating it with an alcohol and an acid catalyst. N H O + OH H O O + H 3 N It is best to view this as an exception rather than the rule. By in large, we will stick to the uphill-bad/downhill-good rule.
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truth, all of these are equilibrium reactions and the real issue is where the equilibrium lies. R S O R' O H Thioesters H R O O + R'S H O O R O + R'S H O O This concept of equilibrium is important for us to fully understand the relative reactivity of these compounds. In many cases we will “drive” the reaction using techniques that push or pull the equilibrium in the desired direction. Driving a reaction in the forward direction: push it and/or pull it O O + H 2 O OH O + OH Distill off High Concentrations Pull Push Interestingly, any of these derivatives can be easily hydrolyzed to the parent carboxylic acid. In turn, the carboxylic acid can be made into an acid chloride to get us right back to the top of the chart. The reagents used to convert carboxylic acids to acid chlorides are either thionyl chloride or oxalyl chloride.
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R Cl O Acid Chlorides R O O O R' Anhydrides R S O R' Thioesters R
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Reactivity of Carboxylic Acid Derivatives - Reactivity of...

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