Carboxylic acids, esters, acylation

What is the carboxylic acid functional group?

-COOH

What makes carboxylic acids weak acids?

Less than 100% of the H⁺ ions are ionised/dissociated in an aqueous solution.

Products of reaction between carboxylic acid and water

Carboxylate ion and hydroxonium (H₃O⁺) ion

How do you know there is a delocalised electron in the carboxylate ion?

The two C-O bonds are equal in length, therefore, there is not a double C=O bond.

Why are carboxylic acids more acidic than similar molecules?

The delocalisation of the electrons makes the carboxylate ion stable and more dissociative.

Effect of increasing chain length on strength of carboxylic acid

Increase in chain length leads to a weaker acid

Why does increase in chain length lead to a weaker carboxylic acid?

The alkyl groups push electrons towards the -COOH group, making it more stable and the O-H bond less likely to break

Effect of adding halogen groups to the strength of a carboxylic acid

Increase in halogen groups leads to a stronger acid

Why does an increase in number of halogen groups lead to a stronger carboxylic acid?

The halogen groups attract the electrons making the carboxylic acid group less stable.

General overall symbol equation for reaction between carboxylic acid and carbonate

2HA + CO₃^2- → H₂O + CO₂ + 2A^-

Step 1 general symbol equation for reaction between carboxylic acid and carbonate

HA + CO₃^2- → HCO₃^- + A^-

Step 2 general symbol equation for reaction between carboxylic acid and carbonate

HA + HCO₃^- → H₂O + CO₂ + A^-

Equation for esterification of carboxylic acids

Carboxylic acid + Alcohol → Ester + water

What part of an ester forms the ester linkage?

O-C=O

The esterification reaction is very slow, what is required to speed up the reaction?

A strong acid catalyst (e.g. sulphuric acid) and heating under reflux

Is the formation of esters a reversible reaction?

Yes

Conditions used to ensure a high yield of the ester product

• Using excess of acid/alcohol depending on cost

• Using more concentrated sulphuric acid than required for catalytic action as it reacts with water

• Distilling off the ester as it forms

What are esters commonly used as?

Solvents

Plasticisers

Perfumes

Food flavourings

How does solubility of ester molecules depend on their size?

Smaller ester molecules are soluble in water, but the larger molecules are insoluble in water

Why are larger ester molecules insoluble in water?

The Van der Waals forces are the predominant intermolecular forces (no hydrogen bonds can be formed)

What is the reverse reaction of esterification?

Hydrolysis

Hydrolysis reaction equation

Ester + water → Carboxylic acid + alcohol

How do you control the direction of the reversible hydrolysis/ esterification reaction?

By controlling the reagents and reaction conditions used

What is the catalyst required for hydrolysis of esters?

Refluxing with dilute aqueous acid or alkali.

Qualities of acid hydrolysis of esters

Reversible

Products of acid hydrolysis of esters

Carboxylic acid and alcohol

Qualities of alkali hydrolysis of esters

Irreversible

Products of alkali hydrolysis of esters

Carboxylate ion and alcohol

How do fatty acids become fats and oils?

They are long chain carboxylic acids which combine with glycerol

What makes some fatty acids saturated?

A lack of double bonds

Why do fats require higher temperatures to melt?

They have mainly saturated hydrocarbon chains and therefore, a neat and tightly packed structure, increasing the VdW forces between them.

What state are fats at room temperature?

Solid

Why do oils have a lower melting temperature than fats?

They have unsaturated hydrocarbon chains leading to bent chains that don’t pack together well. Therefore, decreasing the strength of the VdW forces.

What state are oils at room temperature?

Liquid

What kind of hydrolysis do fats and oils undergo?

Base hydrolysis with OH^- ions from sodium hydroxide to form a carboxylate ion and an alcohol.

What are the products formed from base hydrolysis of fats and oils?

Sodium salt (Na⁺ from sodium hydroxide)

Glycerol (propane-1-2-3-triol)

How are biodiesels formed?

Vegetable oils are reacted with methanol using a strong alkali as a catalyst.

What are the products of the reaction to form biodiesel?

A mixture of methyl esters of fatty acids

What is acylation?

The name given to a reaction resulting in the introduction of an acyl group (R-C=O) into a molecule

What substances are commonly used as an acylating agent?

Acyl chlorides and acid anhydrides

What is a carboxylic acid derivative?

Compounds in which the -OH group of the carboxylic acid has been replaced by another group of atoms

What are acylation reactions classified as?

Nucleophilic addition-elimination reactions.

What is the functional group for acyl chlorides?

-COCl

What is the suffix for acyl chlorides?

-oyl chloride

What are the intermolecular forces acting between molecules of acyl chlorides?

Permanent dipole-dipole forces

Van der Waals forces

What is the state of acyl chlorides at room temperature?

Liquid

Are acyl chlorides soluble in water?

No

Why are acyl chlorides insoluble in water?

There are no hydrogen bonds