Carboxylic acids - Organic Chem II Flashcards
What is the functional group of carboxylic acids?
COOH
How can carboxylic acids be prepared?
- 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
What are the solubility properties of carboxylic acids?
- 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
What is the trend in boiling points for carboxylic acids?
- 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
What are the reactions of the carbonyl group?
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
What are the reactions of the OH group?
Very few reactions of the -OH group remain
- Reaction with PCl5
CH3CH2COOH + PCl5 -> POCl3 + HCl + CH3CH2CCl (propanoyl chloride)
Why is a carboxylic acid being a weak acid important behaviour? Why is there such a difference?
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.
What are the different carboxylic acid reactions?
- With indicators
- With alkalis to form salts: eg/ CH3CH2COOH + NaOH -> CH3CH2COONa + H2O
- With aqueous sodium carbonate: eg/ CH3COOH + Na2CO3 -> CH3COONa + CO2 + H2O (fizzing/effervescence)
What is the test for acidity?
Reagent: add to a solution of sodium carbonate (or sodium hydrogencarbonate)
Result: effervescence. The colourless gas evolved turns limewater cloudy
What is the test for -OH group?
- Add some PCl5
- Steamy fumes which turn damp blue litmus paper red
What type of bonding will exist between CH3COO and Na in sodium ethanoate, CH3COONa?
CH3COO-Na+ ionic
What is the appearance and main physical properties of these salts?
- Brittle
- Conducts electricity when molten/aqeuous solution
- High mpt/bpt
- Tend to be soluble in water
- White crystalline solid
What happens when a stronger acid (HCl) is added?
CH3COOH + H2O ⇌ H3O+ + CH3COO-
What happens when you increase [H3O+]?
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
How are esters formed?
Reagents: an alcohol and a carboxylic acid
Conditions: a few drops of acid catalyst such as concentrated H2SO4 heat under reflux
Esterfication: eg/ reaction of methanoic acid with propan-1-ol
HCOOH + HOCH2CH2CH3 –> HCOOCH2CH2CH3 + H2O
What does an ester smell like?
Fruity
What is the functional group in an ester?
R’-COO-R
How can esters be prepared?
- An alcohol and a carboxylic acid eg/ ethanol and butanoic acid
- An alcohol and an acyl chloride eg/ 3-methylbutan-1-ol and ethanoyl chloride
Which out of the 2 ester preparation methods give the highest yield and why?
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
What are the different uses of esters?
- Food flavourings
- Solvents (eg/ antibiotics, drugs, glues. printing inks)
- Plasticisers
What does acid ester hydrolysis look like?
Catalysed by a strong acid eg/ conc H2SO4 producing a carboxylic acid and an alcohol. Uses water
What does alkali ester hydrolysis look like (saponification)?
Ester + OH- –> (heat under reflux) Carboxylate ion + alcohol
How can the sodium salt of the carboxylic acid be converted into the carboxylic acid?
Add a strong acid eg/ HCl (aq)
Room temp.
What is a saturated fat?
The fatty acid part of the molecule that contains no C=C bonds
What is a monosaturated fat?
Each fatty acid part of the molecule contains one C=C bond
What is a polyunsaturated fat?
Each fatty acid part of the molecule contains several C=C bonds
What is the trend in melting points for fats?
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
What is margarine made up of?
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)
What are polyesters?
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
What is the most common way to form polyesters?
Dicarboxylic acids and diols
What’s another way of making a polyester?
React a diol with a diacyl chloride
What monomer could be used to make a polyester?
Bifunctional monomer
Eg/ lactic acid
Are polyesters biodegrable?
Yes - they can be decomposed by microorganisms
What are acyl chlorides derivatives of?
Carboxylic acids - bond angle = 120
How are acyl chlorides prepared?
Can be prepared relatively easily from the carboxylic acid and a chlorinating reagent eg/ PCl5
What are some of the characteristics regarding ethanoyl chloride?
- Corrosive
- Reacts violently with water, producing a corrosive gas - HCl
- Eye protection and gloves should be worn
- Fume cupboard
What reaction do acyl chlorides undergo?
Nucleophilic substitution
What are the different reactions of acyl chlorides?
- With water to form carboxylic acids + HCl
- With ammonia to form primary amides + HCl
- With primary amines to form secondary amides + HCl
- with alcohols to form esters + HCl
- Friedel-Crafts reactions to form ketones
