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Download 10.6 Carboxylic Acids Learning Outcomes (a) describe the formation
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10.6 Carboxylic Acids Learning Outcomes (a) describe the formation of carboxylic acids from alcohols, aldehydes and nitriles (b) describe the reactions of carboxylic acids in the formation of (i) salts (ii) esters Describe the formation of carboxylic acids from alcohols and aldehydes You need to use excess of the oxidizing agent and make sure the aldehyde formed half-way stays in the reaction mixture. Again we see a colour change in the chromate when the alcohol is oxidized to the aldehyde and/ or the aldehyde is oxidized to the acid. So both types of compounds can be used to begin with in concentrated acids and put in reflux apparatus with chromate (VII). Concentrated HCl is preferred. Making Carboxylic Acids by hydrolysing nitriles 1. Prepare nitriles Nucleophilic substitution Nucleophilic addition Two ways to do the hydrolysis: Acid hydrolysis Alkaline hydrolysis Acid hydrolysis The nitrile is heated under reflux with a dilute acid such as dilute hydrochloric acid. A carboxylic acid is formed. For example, starting from ethanenitrile you would get ethanoic acid. The ethanoic acid could be distilled off the mixture. Hydroxynitriles (cyanohydrins can also be used) RCH(CN)OH. The carboxylic acid is formed via the amide, but this is not isolated. RCH2CN H+ RCONH2 H+ RCOOH Alkaline hydrolysis The nitrile is heated under reflux with an alkali such as sodium hydroxide solution. This time you wouldn't, of course, get a carboxylic acid produced - any acid formed would react with the sodium hydroxide present to give a salt. You also wouldn't get ammonium ions because they would react with sodium hydroxide to produce ammonia. Acidification of this mixture would produce the carboxylic acid Simple reactions of Carboxylic acids Carboxylic acids are weak acids, as they ionize to a small degree in water; pH about 2-3. Carboxylate ions, e.g. sodium ethanoate stabilized by delocalisation of negative charge over two oxygen atoms making it more spread out and energetically more stable.