Carboxylic Acids: Structure, Properties and Preparation
A concise revision guide to carboxylic acids: the –COOH group, naming and drawing acids, hydrogen bonding and its effect on boiling point and solubility, and how acids are made by oxidation and by hydrolysis of nitriles.
- 4.1.1(a)
- 4.2.1(c)(i)-b
- 6.1.2(a)
- 6.1.3(a)
- 6.2.4(c)(ii)
What these spec points say
- 4.1.1(a) application of IUPAC rules of nomenclature for systematically naming organic compounds
- 4.2.1(c)(i)-b the oxidation of primary alcohols by an oxidising agent, e.g. Cr2O72–/H+ (i.e. K2Cr2O7/H2SO4), to form carboxylic acids
- 6.1.2(a) oxidation of aldehydes using Cr2O72–/H+ (i.e. K2Cr2O7/H2SO4) to form carboxylic acids
- 6.1.3(a) explanation of the water solubility of carboxylic acids in terms of hydrogen bonding
- 6.2.4(c)(ii) reaction of nitriles from (b) by acid hydrolysis to form carboxylic acids
Recap: Weak Acids and the Oxidation of Alcohols
Check what you remember about weak acids as partly dissociated proton donors, and about oxidising primary alcohols by distillation or under reflux, before you start carboxylic acids.
The Carboxyl Group
A carboxylic acid contains the carboxyl group, –COOH: a carbonyl group, C=O, and a hydroxyl group, O–H, joined to the same carbon atom.
- The general formula is RCOOH, or CₙH₂ₙ₊₁COOH for an acid with an alkyl group.
- The carboxyl carbon has three bonding regions, so the group is trigonal planar with bond angles of about 120°.
- Both the C=O and the O–H bonds are polar: oxygen is δ− in each, and the hydroxyl hydrogen is δ+.
- The two groups affect each other, so –COOH does not behave like a ketone or like an alcohol.
Why it is an acid
In water a small fraction of the molecules donate a proton. Carboxylic acids are weak acids: the equilibrium lies well to the left.
CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)
The anion formed is a carboxylate ion. Its negative charge is shared by both oxygen atoms, which makes it stable enough to form.
Key idea: The carboxyl group, –COOH, is a C=O and an O–H on the same carbon. It is polar, it can form hydrogen bonds, and it donates a proton to a small extent.
Naming and Drawing Carboxylic Acids
- Count the carbon atoms in the longest chain that includes the –COOH carbon.
- Replace the final -e of the alkane name with -oic acid.
- The carboxyl carbon is always carbon 1, so no number is needed for it.
- Number the chain from that carbon to locate any branches or other groups.
- Two carboxyl groups give a -dioic acid, and the final -e of the alkane name is kept.
| Name | Structural formula | Notes |
|---|---|---|
| methanoic acid | HCOOH | one carbon: H is joined to –COOH |
| ethanoic acid | CH₃COOH | the acid in vinegar |
| propanoic acid | CH₃CH₂COOH | three carbons including –COOH |
| butanoic acid | CH₃CH₂CH₂COOH | four carbons including –COOH |
| 2-methylpropanoic acid | (CH₃)₂CHCOOH | branch on carbon 2 |
| ethanedioic acid | HOOCCOOH | two –COOH groups |
| benzoic acid | C₆H₅COOH | –COOH joined to a benzene ring |
Displayed and skeletal formulae of six carboxylic acids. In a skeletal formula the –COOH carbon is the end of the zig-zag line, with =O and –OH drawn on it.
Drawing tips
- In a displayed formula draw the O–H bond of –COOH as well as the C=O bond.
- Write –COOH or –CO₂H in a structural formula, never –COHO or –OCOH.
- A carboxylic acid and an ester can share a molecular formula. C₂H₄O₂ is ethanoic acid, CH₃COOH, or methyl methanoate, HCOOCH₃.
Check: Naming and Drawing Carboxylic Acids
Name carboxylic acids from structural formulae and work out formulae from names, for acids not used on this page.
Hydrogen Bonding, Boiling point and Solubility
Carboxylic acids have higher boiling points than other organic compounds of similar relative molecular mass.
| Compound | Mr | Boiling point / °C | Strongest force between molecules |
|---|---|---|---|
| butane, C₄H₁₀ | 58 | −1 | weak forces between non-polar molecules |
| propanal, CH₃CH₂CHO | 58 | 49 | permanent dipole–dipole forces |
| propan-1-ol, CH₃CH₂CH₂OH | 60 | 97 | hydrogen bonds |
| ethanoic acid, CH₃COOH | 60 | 118 | hydrogen bonds, more of them |
Why the boiling point is so high
- The O–H hydrogen is strongly δ+ and both oxygen atoms have lone pairs, so the molecules form hydrogen bonds with each other.
- Each –COOH group can form two hydrogen bonds with one neighbouring molecule: C=O to H–O, twice.
- More energy is needed to overcome these forces than the single hydrogen bond between two alcohol molecules.
- The boiling point rises along the series as the chain grows: 101 °C, 118 °C, 141 °C and 164 °C from methanoic to butanoic acid.
Dimers
In the pure liquid, in the vapour just above the boiling point and in non-polar solvents, carboxylic acid molecules pair up as dimers held together by two hydrogen bonds.
Two ethanoic acid molecules joined as a dimer. Each hydrogen bond runs from a lone pair on a C=O oxygen to the δ+ hydrogen of the other molecule’s O–H group.
Solubility in water
- The –COOH group forms hydrogen bonds with water molecules, through both its C=O oxygen and its O–H group.
- Methanoic, ethanoic, propanoic and butanoic acid mix with water in all proportions.
- A longer hydrocarbon chain is non-polar and cannot hydrogen bond. It disrupts the hydrogen bonding between water molecules.
- So solubility falls as the chain grows: hexanoic acid is only slightly soluble and octadecanoic acid is insoluble.
Boiling points of four compounds of similar Mr, showing the effect of hydrogen bonding in ethanoic acid, and the solubility of carboxylic acids in water falling as the hydrocarbon chain grows.
Exam wording: “Carboxylic acids form hydrogen bonds between molecules. These are stronger than the forces between aldehyde or alkane molecules of similar Mr, so more energy is needed to separate the molecules.”
Quick Check: Rank the Four Compounds
Four new compounds have almost the same Mr, so the forces between the molecules decide the order.
Making Carboxylic Acids
1. Oxidation of a primary alcohol or an aldehyde
Heat the alcohol or aldehyde under reflux with an excess of acidified potassium dichromate(VI), K₂Cr₂O₇ in dilute H₂SO₄. The orange solution turns green as Cr₂O₇²⁻ is reduced to Cr³⁺.
CH₃CH₂OH + 2[O] → CH₃COOH + H₂O
CH₃CHO + [O] → CH₃COOH
- Reflux keeps the aldehyde, formed first, in the flask until it is oxidised further.
- Distilling instead would remove the aldehyde as it forms.
- The alcohol chemistry is in 4.2.1 Alcohols; the oxidation of aldehydes is on Oxidation and Tests for Carbonyl Compounds.
2. Hydrolysis of a nitrile
A nitrile, RCN, is hydrolysed when it is heated under reflux with dilute acid or with aqueous alkali. The C≡N group becomes –COOH.
| Reagent | Equation | Then |
|---|---|---|
| dilute HCl, reflux | CH₃CH₂CN + 2H₂O + HCl → CH₃CH₂COOH + NH₄Cl | the acid is formed directly |
| NaOH(aq), reflux | CH₃CH₂CN + H₂O + NaOH → CH₃CH₂COONa + NH₃ | add dilute HCl: CH₃CH₂COONa + HCl → CH₃CH₂COOH + NaCl |
- Nitriles are made from a haloalkane and KCN, which adds one carbon atom to the chain.
- The hydroxynitriles on Reduction and Nucleophilic Addition hydrolyse in the same way: CH₃CH(OH)CN gives 2-hydroxypropanoic acid, CH₃CH(OH)COOH.
Routes to a carboxylic acid: oxidation of a primary alcohol or an aldehyde under reflux with acidified dichromate(VI), and hydrolysis of a nitrile with dilute acid or with alkali followed by acid.
Worked example
Outline how butanoic acid can be made from 1-bromopropane in two steps.
Answer. Heat butanenitrile under reflux with dilute HCl: CH₃CH₂CH₂CN + 2H₂O + HCl → CH₃CH₂CH₂COOH + NH₄Cl
Exam focus: For a full oxidation mark give the reagent (acidified K₂Cr₂O₇), the condition (heat under reflux) and the colour change (orange to green). Use [O] in the equation.
Check: Properties and Preparation of Carboxylic Acids
Explain boiling points and solubility using hydrogen bonding, and choose reagents, conditions and equations for making carboxylic acids, using compounds not used on this page.
Common Exam Points
Say
- “Hydrogen bonds form between carboxylic acid molecules” and state which atoms are involved.
- “Heat under reflux with acidified potassium dichromate(VI); orange to green.”
- “Solubility falls as the non-polar hydrocarbon chain becomes longer.”
Do not say
- “The O–H and C=O bonds break when the acid boils” (only intermolecular forces are overcome).
- “Carboxylic acids are strong acids because they contain two oxygen atoms” (they are weak acids).
- “The nitrile is oxidised to the acid” (it is hydrolysed).
Watch for
- Carbon counting: the –COOH carbon is part of the chain, so CH₃CH₂COOH is propanoic acid.
- Hydrogen-bond diagrams: show lone pairs, δ+ and δ− charges and a straight O–H···O.
- Nitrile hydrolysis: acid gives an ammonium salt; alkali gives the carboxylate salt and ammonia.
FAQs
Use these quick answers to check the structure, properties and preparation of carboxylic acids.
Why do carboxylic acids have high boiling points?
The COOH group has both a polar C=O and an OH group, so each molecule can form two hydrogen bonds with a neighbour, giving hydrogen-bonded pairs called dimers. More energy is needed to separate the molecules than for an alcohol of similar size.
How are carboxylic acids named?
Count the longest chain including the COOH carbon, which is always carbon 1, and use the ending -oic acid: CH₃CH₂COOH is propanoic acid. Two COOH groups give -dioic acid, as in ethanedioic acid, HOOCCOOH.
Why are small carboxylic acids soluble in water?
The COOH group forms hydrogen bonds with water molecules through its C=O oxygen and its OH group. Acids with up to four carbon atoms mix with water; solubility then falls as the non-polar hydrocarbon chain grows.
How do I make a carboxylic acid from an alcohol?
Heat a primary alcohol under reflux with excess acidified potassium dichromate(VI). Reflux keeps the aldehyde, formed first, in the flask until it is oxidised further. The solution turns from orange to green: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O.
How is a nitrile turned into a carboxylic acid?
Heat it under reflux with dilute hydrochloric acid: CH₃CH₂CN + 2H₂O + HCl → CH₃CH₂COOH + NH₄Cl. Refluxing with NaOH(aq) gives the carboxylate salt and ammonia instead, so dilute acid is then added to release the carboxylic acid.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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