Carboxylic acids - Organic Chem II

Question 1

What is the functional group of carboxylic acids?

Answer 1

COOH

Question 2

How can carboxylic acids be prepared?

Answer 2

• Oxidation of primary alcohols and aldehydes
• Hydrolysing nitriles (-CN). Heating a nitrile under reflux with dilute hydrochloric acid. Heating with dilute sodium hydroxide also results in hydrolysis but the sodium salt of the carboxylic acid is formed instead

Question 3

What are the solubility properties of carboxylic acids?

Answer 3

• Acids are very soluble in organic solvents
• Solubility in water is due to hydrogen bonding
• Small carboxylic acids dissolve readily in water
• As the Mr of the acid increases, the solubility decreases as the proportion of the molecule that can form hydrogen bonds decreases
• Benzoic acid is pretty insoluble in cold water but soluble in hot water

Question 4

What is the trend in boiling points for carboxylic acids?

Answer 4

• Increase as the Mr of the acid increases
• Carboxylic acids have high boiling points for their Mr values arising from intermolecular hydrogen bonding due to polar O-H bonds leading to the formation of dimers

Question 5

What are the reactions of the carbonyl group?

Answer 5

Most reactions of the carbonyl group are absent
- There is no reaction with 2,4-DNPH
- There are no addition reactions with HCN
- Reduction to a primary alcohol is, however, possible using LiAlH4 in dry ether at room temperature, although the yield can be poor. The reaction proceeds by nueclophilic addition in the same way as with carbonyl compounds. Aldehyde cannot be isolated

Question 6

What are the reactions of the OH group?

Answer 6

Very few reactions of the -OH group remain
- Reaction with PCl5
CH3CH2COOH + PCl5 -> POCl3 + HCl + CH3CH2CCl (propanoyl chloride)

Question 7

Why is a carboxylic acid being a weak acid important behaviour? Why is there such a difference?

Answer 7

The ionisation of ethanoic acid in aqueous solution is
CH3COOH ⇌ H+ + CH3COO- (⇌ means that only a small proportion of the molecules dissociate)
Whereas a similar reaction for ethanol does not happen

Difference = stability of the anion. The more stable something is, the more likely it is to form.

Question 8

What are the different carboxylic acid reactions?

Answer 8

1. With indicators
2. With alkalis to form salts: eg/ CH3CH2COOH + NaOH -> CH3CH2COONa + H2O
3. With aqueous sodium carbonate: eg/ CH3COOH + Na2CO3 -> CH3COONa + CO2 + H2O (fizzing/effervescence)

Question 9

What is the test for acidity?

Answer 9

Reagent: add to a solution of sodium carbonate (or sodium hydrogencarbonate)
Result: effervescence. The colourless gas evolved turns limewater cloudy

Question 10

What is the test for -OH group?

Answer 10

• Add some PCl5
• Steamy fumes which turn damp blue litmus paper red

Question 11

What type of bonding will exist between CH3COO and Na in sodium ethanoate, CH3COONa?

Answer 11

CH3COO-Na+ ionic

Question 12

What is the appearance and main physical properties of these salts?

Answer 12

• Brittle
• Conducts electricity when molten/aqeuous solution
• High mpt/bpt
• Tend to be soluble in water
• White crystalline solid

Question 13

What happens when a stronger acid (HCl) is added?

Answer 13

CH3COOH + H2O ⇌ H3O+ + CH3COO-

Question 14

What happens when you increase [H3O+]?

Answer 14

Shift equilibrium position to the left. Therefore the carboxylic acid will be ‘displaced’ from its salt. So adding a strong acid to the salt of a carboxylic acid will liberate the weaker acid from its salt

Question 15

How are esters formed?

Answer 15

Reagents: an alcohol and a carboxylic acid
Conditions: a few drops of acid catalyst such as concentrated H2SO4 heat under reflux

Question 16

Esterfication: eg/ reaction of methanoic acid with propan-1-ol

Answer 16

HCOOH + HOCH2CH2CH3 –> HCOOCH2CH2CH3 + H2O

Question 17

What does an ester smell like?

Answer 17

Fruity

Question 18

What is the functional group in an ester?

Answer 18

R’-COO-R

Question 19

How can esters be prepared?

Answer 19

1. An alcohol and a carboxylic acid eg/ ethanol and butanoic acid
2. An alcohol and an acyl chloride eg/ 3-methylbutan-1-ol and ethanoyl chloride

Question 20

Which out of the 2 ester preparation methods give the highest yield and why?

Answer 20

Method 2 because it’s not reversible
Other advantages:
- No catalyst is needed and the reaction happens at room temperature
Therefore acyl chlorides more reactive than carboxylic acids

Question 21

What are the different uses of esters?

Answer 21

• Food flavourings
• Solvents (eg/ antibiotics, drugs, glues. printing inks)
• Plasticisers

Question 22

What does acid ester hydrolysis look like?

Answer 22

Catalysed by a strong acid eg/ conc H2SO4 producing a carboxylic acid and an alcohol. Uses water

Question 23

What does alkali ester hydrolysis look like (saponification)?

Answer 23

Ester + OH- –> (heat under reflux) Carboxylate ion + alcohol

Question 24

How can the sodium salt of the carboxylic acid be converted into the carboxylic acid?

Answer 24

Add a strong acid eg/ HCl (aq)
Room temp.

Question 25

What is a saturated fat?

Answer 25

The fatty acid part of the molecule that contains no C=C bonds

Question 26

What is a monosaturated fat?

Answer 26

Each fatty acid part of the molecule contains one C=C bond

Question 27

What is a polyunsaturated fat?

Answer 27

Each fatty acid part of the molecule contains several C=C bonds

Question 28

What is the trend in melting points for fats?

Answer 28

The more unsaturated the fat, the lower its melting point.
Saturated fats are solids at room temperature, while polyunsaturated fats are liquids at room temperature

Question 29

What is margarine made up of?

Answer 29

Reacting polyunsaturated fats with hydrogen over a nickel catalyst to form solid saturated fats. The finely divided nickel catalyst is suspended throughout the oil and removed by filtration (hydrogenation)

Question 30

What are polyesters?

Answer 30

Polymers in which the monomers are linked together by ester groups. The formation of polyesters is an example of condensation polymerisation, where a small molecule is eliminated each time two monomer molecules join together

Question 31

What is the most common way to form polyesters?

Answer 31

Dicarboxylic acids and diols

Question 32

What’s another way of making a polyester?

Answer 32

React a diol with a diacyl chloride

Question 33

What monomer could be used to make a polyester?

Answer 33

Bifunctional monomer
Eg/ lactic acid

Question 34

Are polyesters biodegrable?

Answer 34

Yes - they can be decomposed by microorganisms

Question 35

What are acyl chlorides derivatives of?

Answer 35

Carboxylic acids - bond angle = 120

Question 36

How are acyl chlorides prepared?

Answer 36

Can be prepared relatively easily from the carboxylic acid and a chlorinating reagent eg/ PCl5

Question 37

What are some of the characteristics regarding ethanoyl chloride?

Answer 37

• Corrosive
• Reacts violently with water, producing a corrosive gas - HCl
• Eye protection and gloves should be worn
• Fume cupboard

Question 38

What reaction do acyl chlorides undergo?

Answer 38

Nucleophilic substitution

Question 39

What are the different reactions of acyl chlorides?

Answer 39

1. With water to form carboxylic acids + HCl
2. With ammonia to form primary amides + HCl
3. With primary amines to form secondary amides + HCl
4. with alcohols to form esters + HCl
5. Friedel-Crafts reactions to form ketones