Carboxylic Acids

This is part of the HSC Chemistry course under the topic Reactions of Organic Acids and Bases.

HSC Chemistry Syllabus

• investigate the structural formulae, properties and functional group including:

• explain the properties within and between the homologous series of carboxylic acids, amines and amides with reference to the intermolecular and intramolecular bonding present

• investigate the differences between an organic acid and organic base

Carbonyl Compounds: Structure, Properties & Reactions

This video introduces a new group of organic compounds – carbonyl compounds, including the structure, properties and reactions of

• aldehydes and ketones
• carboxylic acids (organic acid).

 

Structure and Nomenclature

Aldehyde, ketone and carboxylic acids all contain a carbonyl carbon that is sp² This means both functional groups contain a C=O bond, of which one is a reactive π-bond, the other is an unreactive s-bond. 

The carbonyl carbon in a carboxylic acid is also bonded to a –OH group. Note that this should not be referred to as an alcohol group. The –OH in a carboxylic acid functional group should not be considered separately, but as part of the entire group of atoms (–COOH). 

 

Functional group	Suffix	Prefix	Generic structure	Example
Aldehyde	-al	Formyl-
Ketone	-one	Oxo-
Carboxylic acid	-oic acid	Carboxyl-

 

Nomenclature priority - In order of decreasing priority: carboxylic acid, aldehyde, ketone, alcohol, alkene, alkyne and alkanes. - In the presence of carboxylic acid, aldehyde or ketone functional group, an alcohol will be referred to by its prefix ‘hydroxyl’

  

Properties of Carboxylic Acid

Acidity of Carboxylic Acids

• Carboxylic acids are organic weak acids.

• The deprotonation of hydrogen from a carboxylic acid forms a carboxylate ion (conjugate base).

 

Why is carboxylic acid acidic?

The proton or hydrogen atom attached to oxygen is acidic because:

• O–H bond is polarised and weak due to oxygen’s high electronegativity

• Resonance stabilisation of the conjugate base (carboxylate ion)

 

What is resonance stabilisation? 

 

The negative charge of the carboxylate ion is delocalised. This means the extra electron lone pair of the negatively charged oxygen atom can move to the adjacent bond position to form a C=O bond with the adjacent carbon. When this occurs, the electrons in the old C=O moves to the other oxygen atom to become an electron lone pair. 

The delocalisation of this electron lone pair is referred to as resonance stabilisation. The new chemical structure formed from resonance stabilisation is called the resonance structure of carboxylate ion.

Resonance stabilisation of carboxylate ions explains why carboxylic acids are willing to give away their protons (and hence are acidic). It also explains why alcohols, despite having –OH group, do not behave as proton donors in water. The conjugate base of alcohols (alkoxide) does not have an additional oxygen atom to allow for resonance stabilisation. Therefore, alkoxides are more unstable than carboxylate ions.

Strength of Carboxylic Acids

When carboxylic acids are halogenated, the O–H bond becomes more polarised. This means pK_a decreases and their acid strength increases.

 

As the carbon chain of carboxylic acids increases in length, acidity decreases and pK_a This is because alkyl groups have the opposite effect to that of halogens.

 

 

 

Boiling and Melting Points of Carboxylic Acids

Carboxylic acids are polar compounds. They are generally considered more polar than aldehydes and ketones due to the presence of an additional electronegative oxygen atom.   Carboxylic acids can form hydrogen bonds – hydrogen atom attached to oxygen acts as a bond donor while an electron lone pair acts as a bond acceptor. The electron lone pair can either be from –OH or C=O.   Carboxylic acids have much higher boiling and melting points than hydrocarbons, alcohols, aldehydes and ketones of similar molecular weight.   The formation of two hydrogen bonds between two molecules of carboxylic acid forms a dimer configuration which further increases the strength of dispersion forces between the two molecules.   As a result of this dimer configuration, carboxylic acids have higher boiling points than alcohols of similar molar mass, despite both functional groups being able to form hydrogen bonds.

Hydrogen boding between molecules of carboxylic acids creates a dimer.   Hydrogen bonding between molecules of ethanol does not produce dimers.

  

Table: compounds that can form hydrogen bonds have, in general, stronger intermolecular force and higher boiling and melting points than those that do not.

 

Compound	Functional group	Molar mass (g mol–1)	Type of intermolecular force	Boiling point (ºC)
Butane	Alkane	58	Dispersion	–1
Butanal	Aldehyde	72	Strong dipole	49
Butanone	Ketone	72	Stronger dipole	56
Butanol	Alcohol	74	Hydrogen bonding	97
Butanoic acid	Carboxylic acid	88	Hydrogen bonding	118

  

Solubility in Water

Similar to alcohols, aldehydes and ketones, small carboxylic acids are soluble in water. - Carboxylic acids are more soluble in water than alcohol, aldehydes and ketones of similar molecular weight because they can form more hydrogen bonds.   Carboxylic acids’ solubilities in water decrease with molecular weight (number of carbons in its chain). The extension of the carbon chain decreases the overall polarity of the molecule.

Oxidation

Oxidation of alcohols produces aldehyde, ketone and carboxylic according to the following table

 

Reactant	Reagent/catalyst/condition	Product
Primary alcohol	Mild oxidising agent pyridinium chlorochromate (PCC)	Aldehyde
Primary alcohol	Strong oxidising agent Acidified potassium permanganate (H+/KMnO4) Acidified sodium dichromate (H+/NaCr2O7) Jones Reagent (CrO3/H+) Tollens’ Reagent (Ag(NH3)2+) (silver mirror test)	Carboxylic acid
Secondary alcohol	Any oxidising agent Acidified potassium permanganate (H+/KMnO4) Acidified sodium dichromate (H+/NaCr2O7) Jones Reagent (CrO3/H+) Tollens’ Reagent (Ag(NH3)2+) (silver mirror test)	Ketone

 

Oxidation of an aldehyde produces a carboxylic acid: 

Carboxylic Acid and Base Reactions

Carboxylic acids are weak acids that react with Arrhenius and Brønsted-Lowry bases. Each carboxylic acid functional group is monoprotic i.e. donates one proton. Some organic molecules may contain more than one carboxylic acid functional group in which case the substance may be polyprotic.

Carboxylic acid + metal hydroxide →  salt + water

Example: reaction between acetic acid (C₂H₄O₂) and sodium hydroxide to produce sodium acetate and water

 

 

Carboxylic acid + metal carbonate → salt + carbon dioxide + water

Example: reaction between acetic acid (C₂H₄O₂) and sodium carbonate to produce sodium acetate, carbon dioxide and water

 

 

Carboxylic acid + metal hydrogen carbonate →  salt + carbon dioxide + water

Example: reaction between acetic acid (C₂H₄O₂) and sodium hydrogen carbonate to produce sodium acetate, carbon dioxide and water

 

 

  

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