Chapter 26: Carboxylic Acids and Esters
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What functional group do carboxylic acids have, and what does this contain?
• Carboxyl groups • They contain both a carbonyl and hydroxyl group bonded to the same carbon atom, which is what makes them react differently
Are carboxylic acids soluble? Explain why.
• The C=O and O-H bonds are polar, which allows carboxylic acids to form hydrogen bonds with the water molecules and dissolve in water • However, once there are more than 4 carbon atoms in a carboxylic acid molecules, they are no longer soluble, as the non-polar carbon chain has a greater effect
What type of carboxylic acids are more soluble than the others, and why?
• Dicarboxylic acids • They have 2 polar carboxyl groups to form hydrogen bonds
What happens when carboxylic acids dissolve in water? What does this mean?
• They partially dissociate • They are weak acids
Give the equation for the dissociation of methanoic acid, including state symbols.
• HCOOH (aq) <-> H+ (aq) + HCOO- (aq)
Which reactions do carboxylic acids have in common with other acids?
• Redox reactions with metals • Neutralisation reactions with bases (alkalis, metal oxides and carbonates)
What observations can be made during the reaction of carboxylic acids with metals?
• The metal dissolves • Effervescence
What observations can be made during the reaction of carboxylic acids with carbonates?
• The carbonate dissolves if the carboxylic acid is in excess • Effervescence
How can carboxylic acids be told apart from other organic compounds? Why does this method work?
• They are reacted with carbonates • They are the only common organic compound that is acidic enough to react with carbonates
What are carboxylic acid derivatives?
• Compounds that are formed (derived) from carboxylic acids
List 4 derivatives of carboxylic acids, and the functional group of each.
• Esters, RCOOR (second R cannot be a hydrogen atom) • Acyl chlorides, RCOCl • Acid anhydrides, RCOOCOR/ (RCO)2O • Amides, RCONR2
What causes carboxylic acid derivatives to react in similar ways?
• They all have an acyl group (RC=O, called that and not a carbonyl group since it comes from a carboxylic acid) bonded to an electronegative atom
How are esters formed? What is this reaction called?
• Alcohol + carboxylic acid -> ester + water • Conditions: heat, sulfuric acid catalyst • Esterification
How are simple esters named?
• The alcohol’s name changes to end in -yl (e.g. methyl) • The name of the carboxylic acid changes to end in -oate (e.g. propanoate) • (E.g. methyl propanoate) • However, if the OH group is on the second carbon, for example, the ester would be called 2-propyl methanoate
How are esters with substituents named?
• The carbon atoms in the ester bond are both numbered 1 in their part of the chain
What is hydrolysis?
• A reaction with water (or in the presence of an aqueous solution) that breaks a chemical compound into 2 compounds
What can esters be hydrolysed by?
• Aqueous acid • Aqueous alkali
Give the general equation and conditions for the hydrolysis of esters by aqueous acid.
• Ester + water -> carboxylic acid + alcohol • Conditions: reflux, aqueous H2SO4
Give the general equation and conditions for the hydrolysis of esters by aqueous alkali.
• Ester + hydroxide ion -> carboxylate ion + alcohol • Conditions: reflux, aqueous NaOH
What are 3 differences between the hydrolysis of esters by aqueous acid and aqueous alkali?
• The hydrolysis of esters by aqueous acid is reversible, but with aqueous alkali, it’s irreversible • The acid is a catalyst, but the alkali is used up • In the reversible reaction, some ester will remain in the mixture, but in the irreversible reaction, provided that there is enough NaOH, the ester will be completely converted
Other than carboxylic acids, what else can produce esters?
• Acid anhydrides • Acyl chlorides
What can phenols be esterified by, and why?
• Only acid anhydrides and acyl chlorides • Carboxylic acids are not reactive enough to esterify them
How are acid anhydrides named?
• If it is formed from 2 of the same carboxylic acid, the word acid is replaced with anhydride • For example, 2 ethanoic acid molecules would form ethanoic anhydride • If 2 different carboxylic acids are used, put their names in alphabetical order (e.g. butanoic propanoic anhydride)
How do acid anhydrides react to form esters? Give a general equation.
• Acid anhydride + alcohol -> ester + carboxylic acid