Carboxylic Acids and Derivatives

Question 1

-> How do you recognise a carboxylic acid?
-> Draw an example.

Answer 1

-> The end of the chain would have a -COOH group, one C = O bond and one C-O-H bond coming from the same carbons.

Question 2

-> How do you name a carboxylic acid?

• What if there are carboxylic acids on both ends of the chain?

Answer 2

-> They end in -oic acid and numbering of carbons always starts from the carboxylic acid end.
-> both ends have carboxylic acid would be ending in -dioic acid and if the chain if for example ethane -> then ethanedioic acid , so the “e” remains.

Question 3

-> Describe the acidity of carboxylic acids:

• give an equation as well for the displacement.

Answer 3

-> They are weak acids in water and slightly dissociate
-> But they are strong enough to displace CO2 from carbonates.

• CH3CO2H (aq) -> CH3CO2- (aq) + H+ (aq)

Question 4

-> Describe the solubility of carboxylic acids in water:
- Limit of chain and why?

Answer 4

-> Smaller carboxylic acids dissolve in water - up to 4 Carbons
-> After 4 Carbons the solubility reduces rapidly
-> They dissolve because they can form hydrogen bonds with water molecules.

Question 5

-> How are carboxylic acid salts stabilised to make dissociation (splitting) more likely?

-Talk about bond lengths
- The bonds present in the structure

Answer 5

-> They are stabilised by delocalisation.
-> The delocalised ion has equal C-O bond lengths.
-> No delocalisation -> C=O bond would be shorter than C-O bond.
-> Pi charge cloud has delocalised and spread out electrons and this makes the ion more stable -> more likely to form.

Question 6

-> Describe how the strength of a carboxylic acid varies with different atoms:

• Mention electron density
• stability
• acidity

Answer 6

1. Weak acid
   - Increasing chain length -> increases alkyl groups
   - increase in pushing of electron density towards the COO- ion
   - COO- becomes more negative and less stable -> acid is less strong.
2. Strong acid
   - More electronegative atom is on the carbon chain e.g. Chlorine
   - Chlorine atoms withdraw the electron density from the COO- ion
   - This makes it less negative and more stable -> acid is more strong.

Question 7

-> Describe the reactions of carboxylic acids with different compounds to form salts and other byproducts:

• mention the reaction
• give an equation

Answer 7

> acid + metal => salt + H2

2CH3CO2H + 2Na => 2CH3CO2- Na+ + H2

> acid + alkali => salt + water

CH3CO2H + NaOH => CH3CO2- Na+ + H2O

> acid + carbonate => salt + water + CO2

2CH3CO2H + Na2CO3 => 2CH3CO2- Na+ + H2O + CO2

Question 8

-> What would you observe if the carboxylic acid reacts with a carbonate?

• Mention the compound produced
• reaction

Answer 8

-> When the carboxylic acid reacts with solid Na2CO3 or aqueous NaHCO3 which are both carbonates -> a salt, water and CO2 is produced.

-> However, to recognise that the mixture is a carboxylic acid reacting with a carbonate -> you would see effervescence caused by the production of CO2

Question 9

-> Give an exception to the oxidation of carboxylic acids:

• Mention the products

Answer 9

-> Usually, carboxylic acids can’t be oxidised.
-> Except: methanoic acid => has an aldehyde group in the structure.

-> Methanoic acid -> HCOOH + [O] => HOCOOH
-> The product H2CO3 is called a carbonic acid -> decomposes to give CO2

Question 10

-> How are esters formed?

• Reaction of Esterification
• condition
• equation

Answer 10

-> Carboxylic acid + alcohol -> esters + water

-> Condition : strong acid catalyst (e.g. H2SO4)
- heating under reflux : as reaction is slow

-> reversible reaction

-> There is a H+ on the reversible reaction arrow which comes from the sulfuric acid catalyst.

Question 10

-> How are esters formed?

• Reaction of Esterification
• condition
• equation

Answer 10

-> Carboxylic acid + alcohol -> esters + water

-> Condition : strong acid catalyst (e.g. H2SO4)
- heating under reflux : as reaction is slow

-> reversible reaction

-> There is a H+ on the reversible reaction arrow which comes from the sulfuric acid catalyst.

Question 11

-> Give 3 uses of esters:

• examples
• reasons

Answer 11

-> used in perfumes and flavourings
- because they are sweet smelling compounds
- for perfumes they need to be non-toxic, soluble in solvents, volatile (turns into gas), and not react with water.

-> used as solvents for polar organic substances - e.g. ethyl ethanoate -> solvent in glues and printing inks.
- esters don’t form hydrogen bonds although they are polar as they have no H atom attached to a very electronegative atom.
- lower boiling point than hydrogen bonded carboxylic acids they are derived from
- almost insoluble in water

-> used as plasticiers for polymers
- pure polymers have limited flexibility as the polymer chains can’t move each other.
- inserting some plasticiers into the polymer allows the chains to move more easily and polymer is more flexible.

Question 12

-> How do you name esters?

Answer 12

-> name the bit that is attached to the single bonded oxygen by counting the carbons in the chain.
-> the name should end in -yl as it is like an alkyl group.

-> the second part of the name comes from the chain attached to the carbon of the carboxylic acid.
-> this ends in -anoate

e.g methyl propanoate

Question 13

-> Describe the hydrolysis of esters:

• Base Hydrolysis
• Acid Hydrolysis
• Reagents
• Conditions
• Equation

Answer 13

-> esters can be split up in two ways:

1. using a base (Sodium Hydroxide)
   - Reagents: Dilute NaOH
   - Heated under reflux

• ester + NaOH => carboxylic acid salt + alcohol
  > reaction not reversible ~> because the carboxylic acid salt product is the anion of the carboxylic acid.
  > The anion (a negative ion) is resistant to attack by weak nucleophiles such as alcohols
• the salt can be converted to the original carboxylic acid form when a strong acid is added.
  > Carboxylic Acid salt (CH3CH2CO2- Na+) + HCl => carboxylic acid + NaCl
• reaction is complete when excess NaOH is used to completely hydrolyse the ester.

2. Using an acid
   - Reagents: dilute acid (HCl)
   - Heated under reflux

• ester + H2O <-> H + -> carboxylic acid + alcohol
  > reaction is reversible
  > not a good yield of products gained

Question 14

Practical of forming esters and hydrolysing

Answer 14

later

Question 15

-> What are fats and oils?

• Draw the structure and how it’s formed
• uses
• products of hydrolysis

Answer 15

-> fats and oils are esters of glycerol + long chain carboxylic acids.

      fats+ oils                                            glycerol + ....

CH2OCO(CH2)16CH3                          H2COH
|                                                                    | H-COCO(CH2)16CH3          + 3KOH =>      HCOH 
 |                                                                    |
 CH2OCO(CH2)16CH3                           H2COH

Products :
- glycerol + 3 CH3(CH2)16CO2- K+

-> Glycerol - forms hydrogen bonds very easily + soluble in water => used in cosmetics, food and glues

-> vegetable oils and animal fats can be hydrolysed to give -> soap, glycerol, long chain carboxylic (fatty) acids.