Acylation A-Level Chemistry PDF

A LEVEL CHEMISTRY 3.3.9 CARBOXYLIC ACIDS & DERIVATIVES ACYLATION In this section we’ll learn about the reactions of two derivatives of carboxylic acids. Acyl Chlorides Acid...

A LEVEL CHEMISTRY 3.3.9 CARBOXYLIC ACIDS & DERIVATIVES ACYLATION In this section we’ll learn about the reactions of two derivatives of carboxylic acids. Acyl Chlorides Acid Anhydrides O O R C R C O Cl R C O Essentially a carboxylic acid Essentially two carboxylic where the -OH is replaced acids bonded together. by a Cl. _____anoic anhydride _____anoyl chloride e.g. ethanoic anhydride e.g. ethanoyl chloride (where both R groups are 2 carbons in length) O O H 3C C H 3C C Cl O H 3C C O Both are highly reactive derivatives of carboxylic acids. Both undergo nucleophilic addition-elimination reactions with: Water Alcohols Ammonia 1o Amines However, you only need to know the mechanisms for the reactions of acyl chlorides. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.9 CARBOXYLIC ACIDS & DERIVATIVES ACYL CHLORIDES You need to know the following reactions of acyl chlorides. All of which have the same nucleophilic addition-elimination mechanism. The only difference between them is the nucleophile that the acyl chloride reacts with. All of these reactions occur at room temperature. Acyl Chloride + Water (hydrolysis) δ- O– O O δ+ R C R C Cl R C + HCl(g) Cl +O OH O H H H H This is a violent and highly exothermic reaction. Forms a carboxylic acid + HCl(g). Observations: Fizzing + Steamy fumes of HCl(g) R-COCl + H2O R-COOH + HCl Acyl Chloride + Alcohol δ- O– O O δ+ R C R C Cl R C O R Cl +O + HCl(aq) O R H R H This is a highly exothermic reaction. Forms an ester + HCl(g). Unlike the method of using a carboxylic acid to form an ester, it is not reversible and it does not require a catalyst. However, highly toxic HCl(g) is produced as a by- product instead of water (not good!) Observations: Fizzing + Steamy fumes of HCl(g) R-COCl + R-OH R-COO-R + HCl AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.9 CARBOXYLIC ACIDS & DERIVATIVES Acyl Chloride + Ammonia δ- O– O H O δ+ R C R C Cl R C N H Cl H N+ H + NH4Cl(s) H N H H H This is a highly exothermic reaction. Forms a primary amide + NH4Cl(s) Observations: The formation of NH4Cl(s) is observed as white smoke. Where did the NH4Cl come from!? Any HCl that would normally be produced by this reaction is neutralised by the NH3! Overall Reaction: R-COCl + 2NH3 R-CONH2 + NH4Cl Acyl Chloride + 1o Amine δ- O– O H O δ+ R C R C Cl R C N R Cl H N+ H + NH4Cl(s) H N H R R This is a highly exothermic reaction. Forms a secondary amide + NH4Cl(s) Observations: The formation of NH4Cl(s) is observed as white smoke. As in the previous reaction, NH4Cl is formed for the same reasons. Overall Reaction: R-COCl + 2R-NH2 R-CONH-R + NH4Cl AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.9 CARBOXYLIC ACIDS & DERIVATIVES ACID ANHYDRIDES You need to know the following reactions of acid anhydrides. All of which are similar to the reactions of acyl chlorides. They are all nucleophilic addition-elimination, but you do NOT have to know the mechanism. All of these reactions occur at room temperature. Acid Anhydride + Water (hydrolysis) O R C O O + H 2O 2R C R C OH O Forms 2 carboxylic acid molecules Observations: None (two colourless liquids give a colourless liquid) Acid Anhydride + Alcohol O O R C O O + R OH R C O R + R C R C OH O Forms an ester and a carboxylic acid. Observations: None (two colourless liquids give a colourless liquid) AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.9 CARBOXYLIC ACIDS & DERIVATIVES Acid Anhydride + Ammonia O O H R C O O + 2 NH3 R C N H + R C R C O – NH4 + O Forms a 1o amide and an ammonium salt (e.g. ammonium ethanoate) Observations: None (two colourless liquids give a colourless liquid) Acid Anhydride + 1o Amine O O H R C O O + R NH2 R C N R + R C R C O – + H 3N R O Forms a 2o amide and an amide salt (e.g. ethylammonium ethanoate) Observations: None (two colourless liquids give a colourless liquid) HINTS | TIPS | HACKS Don’t overcomplicate this section. How To Recall Nucleophilic Notice how the mechanism for each of the 4 Addition-Elimination reactions of acyl chlorides is the same Notice how the products of the 4 reactions of acyl chlorides and acid anhydrides are the same. Only the by-products are different. Take note of the observations of the reactions of acyl chlorides. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.9 CARBOXYLIC ACIDS & DERIVATIVES PRODUCING ASPIRIN Aspirin is produced by reacting 2-hydroxybenzoic acid with ethanoic anhydride in a nucleophilic addition-elimination reaction. COOH O COOH O OH H 3C C O C CH3 + O O H 3C C + H 3C C O OH 2-hydroxybenzoic ethanoic aspirin ethanoic acid acid anhydride This reaction is no different to the one of an acid anhydride with an alcohol. The products are the same in that we make an ester and a carboxylic acid. It’s just that the alcohol (-OH) group is bonded to a benzene ring. COOH O Ester group formed O C CH3 This reaction is no different to the one of an acid anhydride with an alcohol. The products are the same in that we make an ester and a carboxylic acid. It’s just that the alcohol (-OH) group is bonded to a benzene ring. This reaction could also be achieved by reacting the 2-hydroxybenzoic acid with ethanoyl chloride. However, in industry, ethanoic anhydride is the preferred reactant because it: a) is less likely to be hydrolysed by water b) is cheaper c) is less corrosive d) produces a less toxic by-product (the acyl chloride reaction would produce HCl(g)) AQA www.chemistrycoach.co.uk © scidekick ltd 2024