6.1.3 Carboxylic acids, esters and acyl chlorides

Question 1

Structure of carboxylic acid

Answer 1

The carboxyl functional group is polar because oxygen is more electronegative than C and H

Question 2

Solubility of carboxylic acids

Answer 2

• Shorter chained carboxylic acids are soluble in water because the O-H bond is able to make Hydrogen bonds with water
• Carboxylic acids with longer carbon chains become less soluble in water beaus they have bigger induced dipoles which break up any H bonds that form with water

Question 3

Acidity of carboxylic acids

Answer 3

• Carboxylic acids are weak acids rest partially dissociate in water and have a ka value
• ka proportional to [p]/[r]

Question 4

Test for carboxylic acids

Answer 4

Reagent: any carbonate solution or solid e.g. NaCO3

• fizzing due to CO2 gas made
• makes salt, water and carbon dioxide

Question 5

Why is the ester functional group polar?

Answer 5

COOC

- oxygen is more electronegative than carbon

Question 6

How are esters named?

Answer 6

e. g. methyl ethan oate
- methyl - comes from alcohol methanol
- ethan comes from carboxylic acid ethnic acid
- oate - family name

Question 7

2 methods of esterification

Answer 7

1. joining alcohol and carboxylic acid

2. joining alcohol and acid anhydride

Question 8

Esterification - alcohol + carboxylic acid

Answer 8

• makes ester and water
• requires conc H2SO4 catalyst (dehydrating agent), reflux at 100 c
• remove H from alcohol, remove OH from carboxyl group, join by ester bond

Disadvantages:

• equilibrium reaction so 100% yield never obtained
• conc H2SO4 is corrosive and dangerous to handle, must be separated from the reaction method at the end

Question 9

Esterification - alcohol + acid anhydride

Answer 9

• makes ester and carboxylic acid (no water)
• requires room temperature, no catalyst, unheated reflux
• split acid anhydride into 2 pieces, central O aside
• one piece into ester, other into carboxylic acid

Advantages

• not an equilibrium reaction so can obtain 100% yield
• no dangerous chemical to separate out at the end

Question 10

What is an acid anhydride?

Answer 10

• think of as 2 carboxylic acid joined by a singe O atom

- oic anhydride

Question 11

Definition of hydrolysis

Answer 11

uses water to break a bond in a molecule which splits the molecule into 2 pieces

Question 12

Acid hydrolysis of esters

Answer 12

• requires water, dilute HCl catalyst, reflux 100 c
• products are the original alcohol and carboxylic acid
• C-O ester bond breaks, H-OH bond in water breaks
• H to O (alcohol part)
• OH to C in C=O (carboxylic acid part)

Question 13

Base hydrolysis of esters

Answer 13

• requires water, dilute NaOH catalyst, reflux 100 c
• products overall are alcohol and sodium salt of carboxylic acid (carboxylate) and water
• but initially alcohol and carboxylic acid is made
• carboxylic acid reacts with NaOH - neutralisation which produces water and carboxylate

Question 14

Acyl chlorides naming and functional group

Answer 14

• COCl (C=O)
• named after parent carboxylic acid
• oic acid removed and replaced with -oyl chlorides

Question 15

How are acyl chlorides made?

Answer 15

• reacting the parent carboxylic acid with thionyl chloride SOCl2
• product are acyl chloride, HCl and SO2 gases which escape leaving only the pure acyl chloride