26.4 Reactions of carboxylic acids and esters

Question 1

Why are carboxylic acids reactive

Answer 1

The C=O bond is very polar so C is f+ and O is f-

So the C f+ is open to attack from nucleophiles

also there is an OH bond where the H can be lost as a H+ ion in which case it is behaving as an acid

Question 2

What is formed is a hydrogen from OH group on carboxylic acid is lost

Is it stable?

Answer 2

A carboxylate ion just consists of
R- C =O and another O- attached to the C
So the whole carboxylate is negatively charged due to this loss of a proton

Yes because delocalisation makes the resulting ion more stable, so this stability is what allows the H+ ion to be released

Question 3

Where is the position of equilibrium in carboxylic acid dissociating to release H+ ions

Answer 3

It is over to the left because carboxylic acids are weak acids

Question 4

How can you test for carboxylic acids

Answer 4

Sodium hydrogencarbonate will release CO2 gas and water when reacted with a carboxylic acid

Question 5

How do carboxylic acids react

Answer 5

They are proton donors

So form ionic salts with the more reactive metals, alkalis, metal oxides or metal carbonates

So methanoic acid gives methanoates and ethanoic acid gives ethanoates

Question 6

How does ethanoic acid react with sodium hydroxide

Answer 6

It makes sodium ethanoate CH3COONa because the H+ ion is released, making water

Question 7

How does ethanoic acid react with aqueous sodium carbonate

Answer 7

Sodium ethanoate is also formed, but CO2 and H2O are formed as products also
Two ethanoic acids react with one sodium carbonate

Question 8

Reaction of ethanoic acid and ethanol

What is used to speed up reaction

Answer 8

Ethyl ethanoate and water are formed

This reaction is speeded up by a strong acid catalyst

Equilibrium reaction

Question 9

How are esters hydrolysed

What are they hydrolysed into

Answer 9

The carbonyl of an ester has f+ charge so is attacked by water acting as a weak nucleophile

This hydrolysis forms carboxylic acid and alcohol

A strong acid catalyst needs to be used

Question 10

Does the hydrolysis reaction go to completion?

Answer 10

No so an equilibrium mixture is made containing alcohol, carboxylic acid, water and ester

Question 11

How do bases catalyse hydrolysis of esters

Answer 11

The salt of the acid is produced rather than the acid itself

This removes acid from reaction mixture so an equilibrium is NOT established and reaction goes to completion
So more product is made in mixture

Question 12

What is another way esters can be hydrolysed

Answer 12

They can be boiled with sodium hydroxide

Products are glycerol and a combination of sodium salts of the three acids that form the ester called a soap

Question 13

What are esters used for

Difference between fats and oils

Answer 13

Animal and vegetable oils are the esters made from alcohol propane-1,2,3-triol (aka glycerol)

Difference between fat and oil is that fat is solid at room temp and oil is liquid

Question 14

How are fats and oils hydrolysed

Answer 14

. Hydrolysed in acidic conditions to give a mixture of glycerol and and the fatty acids
Or can be hydrolysed by boiling with NaOH

Question 15

What is made when the triglyceride esters are hydrolysed by NaOH

Answer 15

. Sodium salts of the three acids, and glycerol are formed

These salts are called soaps
The type of salt depends on fatty acids initially present in ester

Question 16

What type of bonds are present in triglycerides

Why does this make them good at cleaning

Answer 16

Ionic bonds: The sodium salts have two distinct ends : A non polar hydrocarbon chain, and a COO- group which is polar

So hydrocarbon chain mixes with grease, and the COO- mixes with water

So they are good cleaning agents because they allow water and oil to mix

Question 17

Is glycerol soluble in water

Answer 17

It contains three OH bonds so they can hydrogen bond with water, making it soluble

Question 18

What is glycerol used for in industry

Answer 18

. Ointments and creams to prevent them drying out
. Solvent in medicines and is present in toothpaste
. Solvent in food industry eg food colouring

. It can plasticise various materials, plasticisers go between the molecules and allow them to slip over each other

Question 19

What is biodiesel

Answer 19

Renewable fuel as it is made from oils derived from crops such as rape seed

Biodiesel is usually made from rapeseed oil which is a triglyceride ester

Question 20

How is biodiesel made

Answer 20

• The lipid is broken down into three fatty acids and glycerol by reaction with KOH and then these fatty acids are converted into three new esters using methanol

So potassium hydroxide is not used up so is a catalyst