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3.3.9 Carboxylic Acids and Derivatives (Carboxylic acids (structure, are…
3.3.9 Carboxylic Acids and Derivatives
Carboxylic acids
structure
are weak acids but will liberate CO₂ from carbonates:
carboxylic acid (aq) + NaHCO₃ (aq) --> sodium salt + water + carbon dioxide
react with alcohols in the presence of an acid catalyst to give esters; reversible reaction:
carboxylic acid + alcohol --> ester + water
functional group: -COOH
-OH groups in carboxylic acids are more acidic than -OH group in alcohols
suffix -oic acid
can form hydrogen bonds with water; completely soluble in water up to butanoic acid; higher melting point than alkanes of similar Mr
weak acid: Hδ+ may be lost as H⁺ so behaves as an acid; carboxylate ion is left and the negative charge is shared over the whole of the carboxylate group; delocalisation makes the resulting ion more stable; equilibrium is well on the left
Esters
structure
common uses:
solvents
plasticisers
perfumes
food flavourings
vegetable oils and animal fats are esters of propane-1,2,3-triol (glycerol); can be hydrolysed in alkaline conditions to give soap (salts of long-chain carboxylic acids) and glycerol
biodiesel: mixture of methyl esters of long-chain carboxylic acids; produced by reacting methanol in the presence of a catalyst
can be hydrolysed in acid or alkaline conditions to form alcohols and carboxylic acids or salts of carboxylic acids
functional group: RCOOR'
naming: alcohol-yl acid-oate
based on name of parent acid
hydrolysis with acid catalyst::
ester + water --> carboxylic acid + alcohol
forms an equilibrium mixture; does not go to completion; acid catalyst affects only rate at which the equilibrium is reached, not the position
room temperature with strong acid catalyst
carbonyl carbon atom of ester has Cδ+ atom so is attacked by water acting as a weak nucleophile
hydrolysis with base catalyst:
forms salt of carboxylic acid rather than acid itself
goes to completion; not an equilibrium
Acylation
Acid anhydrides:
structure
RCOOCOR'
Acyl chlorides:
structure
Amides:
structure
RCONH₂ or N-substituted: RCONHR'
manufacturing aspirin:
industrial advantages of ethanoic anhydride over ethanoyl chloride
ethanoic anhydride is a better acylating agent that ethanoyl chloride; cheaper, less corrosive, less reactive with water, safer
acyl group is introduced into another molecule
rate of reaction depends on:
magnitude of partial positive charge on carbonyl carbon
how easily Z is lost
how good the nucleophile is:
primary amine > ammonia > alcohol > water
Nucleophilic addition-elimination reactions
water
product is carboxylic acid
alcohols
product is ester
ammonia
product is amide
primary amines
product is N-substituted amide
with acyl chlorides (mechanism) and acid anhydrides
Sammer Sheikh