Worked example 1
Identify the functional groups in CH₃CH₂COOH and in CH₃COOCH₃, and say which boils higher.
Try it first: Which of the two can donate a hydrogen bond?
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Functional GroupsCourse level: High School / HonorsCourse level: AP Chemistry
Functional Groups and Organic Families
What you'll be able to do: Identify the common functional groups from a structure and predict how each one affects polarity, hydrogen bonding and boiling point.
Introduction
A functional group is the reactive part of a molecule. The carbon skeleton is mostly inert, so knowing a dozen groups lets you predict properties and reactions for thousands of compounds.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Recognise alcohol, ether, aldehyde, ketone, carboxylic acid, ester, amine and amide groups
- Match each family to its IUPAC suffix or prefix
- Predict relative boiling point and water solubility from the group present
- Explain why ordinary carboxylic acids are much stronger acids than ordinary alcohols
What a functional group is
A functional group is a specific arrangement of atoms that gives a molecule characteristic chemistry. Ethanol and 1-butanol differ in chain length but behave alike, because both carry an –OH. Reactions happen at the group; the chain mostly comes along for the ride.
Identify a group by its connections, then consider how neighboring groups and the carbon skeleton affect its behavior.
Recognize groups by local connectivity
R and R′ denote carbon-containing groups; they may be different. Identify the atoms attached directly to a carbonyl carbon before naming the family.
| Family | Recognizable group | Introductory name pattern | Example |
|---|---|---|---|
| Alcohol | R–OH, not within COOH | -ol or hydroxy- | ethanol |
| Ether | R–O–R′ | alkoxy- parent | methoxyethane |
| Aldehyde | R–C(=O)H | -al | propanal |
| Ketone | R–C(=O)–R′ | -one | propanone |
| Carboxylic acid | R–C(=O)OH | -oic acid | ethanoic acid |
| Ester | R–C(=O)O–R′ | alkyl alkanoate | methyl ethanoate |
| Amine | N bonded to carbon, without adjacent carbonyl | -amine or amino- | ethanamine |
| Amide | R–C(=O)–N | -amide | ethanamide |
In methyl ethanoate, “methyl” names the group attached to the singly bonded oxygen; “ethanoate” names the acid-derived portion including the carbonyl carbon. An OH attached to a carbonyl is part of a carboxylic acid, not an independent ordinary alcohol group.
CH₃CH₂OH ethanol; CH₃CHO ethanal; CH₃COCH₃ propanone; CH₃COOH ethanoic acid.
The nitrogen families
| Family | Structure and behavior |
|---|---|
| Amine | R–NH₂ (or R₂NH, R₃N), basic because the nitrogen lone pair accepts a proton. |
| Amide | R–CO–NH₂, a carbonyl bonded straight to nitrogen, far less basic because the lone pair is delocalised into the C=O. |
Scroll horizontally if needed to read all columns.
An amine lone pair often accepts a proton; amide resonance greatly reduces nitrogen basicity. Strong-acid protonation of an amide can instead involve its oxygen.
Compare physical properties with controls
Alcohols and carboxylic acids can donate and accept hydrogen bonds. Primary and secondary amines have N–H donors, but tertiary amines have no N–H and only accept through their available lone pair. Primary and secondary amides can donate via N–H and accept at the carbonyl oxygen; a fully N-substituted amide has no N–H donor. The amide nitrogen is a poor acceptor because its lone pair is delocalized.
Ethers, aldehydes, ketones and esters are polar and can accept hydrogen bonds from water, even though they lack O–H/N–H donors for self-association. They are not equivalent to nonpolar hydrocarbons. For comparable size and shape, hydrogen-bond donation often raises boiling point, but dispersion, polarizability, branching and total molecular size can reverse a simplistic family ranking. Small members often mix well with water; a growing hydrocarbon portion commonly reduces solubility. Protonating an amine or deprotonating a carboxylic acid creates an ion and can substantially change water solubility.
Compare size, shape and polarizability before isolating functional-group effects. Ethers and carbonyl compounds can accept hydrogen bonds from water.
Explain acidity by comparing conjugate bases
Carboxylic acids are weak acids in water, often with pKa near 4–5. Ordinary alcohols are much weaker acids, often near pKa 16, rather than literally incapable of losing a proton. Their difference follows the conjugate bases: carboxylate delocalizes negative charge over two oxygens through resonance, while an ordinary alkoxide lacks that equivalent two-oxygen delocalization. A lower-energy conjugate base makes proton loss more favorable.
Compare ethanoic acid with ethanol under the same solvent convention: their pKa separation is about eleven units, corresponding to roughly 10¹¹ in acid-equilibrium constants. Actual values depend on structure and solvent. A functional-group label predicts a pattern; neighboring electron-withdrawing groups or other structural changes can alter the strength.
R–COOH ⇌ R–COO⁻ + H⁺ (Ka ≈ 10⁻⁵)
One molecule can contain several functional groups
Classify each recognizable part of a molecule rather than giving the whole molecule one family label. A molecule bearing both an alcohol and an amine can interact through hydrogen bonding and can act as a base at the amine nitrogen.
The functional groups can influence one another, but their presence is identified from local connectivity.
Circle local groups first, then predict behavior.
Key ideas
Definition
Functional group
The atoms in a molecule responsible for its characteristic reactions.
Definition
Carbonyl
A C=O group; the shared feature of aldehydes, ketones, acids, esters and amides.
Rule
Aldehyde vs ketone
A terminal carbonyl is an aldehyde; a carbonyl between two carbons is a ketone.
Rule
Hydrogen bonding
Only O–H and N–H groups let a molecule donate a hydrogen bond, which raises boiling point sharply.
Key concept
Resonance and acidity
The carboxylate ion spreads its charge over two oxygens, which makes carboxylic acids much stronger acids than ordinary alcohols.
Equation
Acid dissociation
R–COOH ⇌ R–COO⁻ + H⁺
- R = any carbon group
Equation
Esterification
R–COOH + R′–OH ⇌ R–COO–R′ + H₂O
- R′–OH = the alcohol supplying the ester's second half
Equation
Amine as a base
R–NH₂ + H₂O ⇌ R–NH₃⁺ + OH⁻
- R–NH₂ = primary amine with a nitrogen lone pair
Worked examples
Worked example 2
Name and classify CH₃CH₂CHO, and explain why it is not a ketone.
Try it first: Where in the chain does the C=O sit?
0 of 3 steps revealed.
Worked example 3
Identify the functional groups and likely water interactions of HO-CH₂-CH₂-NH₂.
Try it first: Inspect the O-H and N-containing ends separately.
0 of 3 steps revealed.
Common mistakes
| Mistake | Why it's wrong | Check instead |
|---|---|---|
| Calling any molecule with an oxygen an alcohol. | Ethers, aldehydes, ketones and esters can contain oxygen without an alcohol OH; the OH within COOH belongs to a carboxylic acid group. | Look for a hydrogen bonded directly to oxygen before saying alcohol. |
| Ranking boiling points by molar mass alone. | Isomers share molar mass, but shape, branching and functional-group interactions can all differ. | Ask whether the molecule can donate a hydrogen bond, then whether it is polar. |
| Treating amides as basic because they contain nitrogen. | The nitrogen lone pair is delocalised into the adjacent C=O, so it is not available to accept a proton. | Check whether the nitrogen sits next to a carbonyl before calling it a base. |
Scroll horizontally if needed to read all columns.
After the lesson · Question bank
Independent practice
Work through new problems on your own. The lesson's quick checks and worked examples stay above if you need to review a step.
17 topic questions across 2 difficulty levels.
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What you should now know
Identify functional groups from local connectivity, especially the atoms attached to a carbonyl. Use each family’s suffix or prefix and retain multiple groups when present. Hydrogen-bond donation, acceptance, polarity, size and shape jointly influence physical properties. Carboxylate resonance makes carboxylic acids much stronger acids than ordinary alcohols; amide delocalization makes its nitrogen much less basic than an ordinary amine.
- Functional groups guide reactivity, while the carbon skeleton and neighboring groups can modify it
- Terminal C=O is an aldehyde; internal C=O is a ketone
- O–H and N–H groups hydrogen bond, raising boiling point and water solubility
- Acid > alcohol > ketone/aldehyde > ether > alkane for boiling point at similar mass
- –COOH is acidic because of resonance in the carboxylate ion
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