3.3.9 Carboxyllic acids and Derivatives
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What two functional groups do carboxyllic acids have?
• the carbonyl group (-C=O) • the hydroxyl group (-OH)
How does having two groups influence the properties of the two groups? (In terms of the OH group and acidicty)
-Oh group in carboxylic acids is much more acidic than the -OH group in alchols.
Why is the -OH in carboxyllic acids much more acidic than in alcohols?
• X-ray diffraction studies of the carboxylate ion show that the two carbon-oxygen bonds are of equal length. • This suggests the negative charge is distributed evenly over the carboxylate group and is delocalised. • This delocalisation means the carboxylate is less likely to join up with the H+ ion again (the carboxylate ion is made more stable). • When alcohols OH bond breaks to release H atom, there is no way of delocalising the -ve charge, which reains firmly on the oxygen atom, that intense negative charge will be highly attractive towards H+ ions so the alchol will instantle reform. This is why carboxylic acids are more acidic than alcohols.
What is an ester? [general formula]
a derivative of carboyllic acids RCOOR’
How do you name an ester? + what is the structure
alkyl group + -oate eg: methanol + ethanoic acid = methyl ethanoate CH3OH+CH3COOH = CH3COOCH3 + H2O
Uses of short chain esters
• flavourings and perfumes (have pleasant fruity smell) • solvents • plastercisers
What are esters with longer carbon chains?
fats and oils
Why are carboxyllic acids weak acids?
because they only partially dissociate in solution RCOOH ⇌ RCOO- + H+ equilibirum lies well over to the left
How do you test for carboxyllic acids?
add sodium hydrogencarbonate to release CO2, causes effervesence CH3COOH (aq) +NaHCO3(aq) –> CH3COONa (aq) + H2O (l) + CO2 (g)
What is a salt produced by a carboxyllic acid called?
carboxylate
General acid base reactions with carboxylic acids theres 4
• metal + carboxylic acid –> salt + hydrogen • metal oxide/hyrdoxide + carboxylic acid —> salt + water • metal carbonate + carboxylic acid –> salt + water + carbon dioxide • ammonia + carboxylic acid –> ammonium salt
How to name carboxylates (salts) vs how to write them
name : metal + carboxylic acid stem +oate (magnesium ethanoate) write : the convention is to put the metal at the end of the salt’s formula (CH3COO-)2 Mg2+
How do you make an ester? Do you need a catalyst? what is it? What kind of reaction is it? also what’s the byproduct?
• carboxylic acid + alcohol • strong acid catalyst • reversible reaction (forms an equilibrium mixture of reactants and products) • byproduct is water
Explain the hydrolysis of esters
The carbonyl carbon atom (C=O) of an ester has a δ+ charge and is therefore attachked by water acting as a (weak) nucleophile
What makes a nucleophile weak?
A highly electronegative atom is a poor nucleophile because it is unwilling to share its electrons. As electronegativity increases, nucleophilicity decreases. oxygen is very electronegative
Give a general equation for the hydrolysis of esters
(H+ catalyst) RCOOR’ + H2O ⇌ RCOOH + R’OH
What’s the difference between fats and oils? why?
Oils are liquid at RT Oils (liquids at room temperature) contain more carbon to carbon double bonds than fats (solid at room temperature).
Desribe the structure of oils and fats?
Contain three molecules of long chain carboxylic acids (fatty acids) attatched to a glycerol molecule. They are esters of propane-1,2,3-triol (glycerol)
What happens when you hydrolyse fats and oils in alkaline conditions
produces glycerol and soap
Uses of glycerol
• used in pharmatceuticals and cosmetics, because it attracts water it can be used to prevent ointments and creams from drying out. • solvent in many medicines, present in toothpaste • solvent in food industry, ie solvent in food colourings • used to plasticise various materials like sheets and gaskets, cellophane.
How do plasticisers work?
Plastisicers are introduced between molecules of a poloymer which makes up the material and by allowing them the molecules to slip over each other the material becomes flexible and smooth . PVS may contain up to 50% plasticiser such as hexandioic acid. Over time the plasticiser leaks away leaving the plastic brittle and inflexible.
What is biodiesel
renewable fuel as it is made from oils derived from crops such as rape seed. (rape seed oil is a triglyceride ester)
How is biodiesel made?
oil reacted with methanol (with a strong alkali as a catalyst) to form a mixture of methyl ester which can be used as fuel in diesel vehicles
Can oils and fats be hydrolysed?
Yes (oil/fat + 3H2o–> glycerol + 3 fatty acid chains 3R-COOH)