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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Organic ChemistryFunctional GroupsContent level: Core 18 min
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 carboxylic acids are acidic while alcohols are not
Lesson
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.
The oxygen families
Alcohol: R–OH, suffix -ol. Ether: R–O–R′, named as alkoxy. Aldehyde: R–CHO with the C=O at the chain end, suffix -al. Ketone: R–CO–R′ with the C=O inside the chain, suffix -one. Carboxylic acid: R–COOH, suffix -oic acid. Ester: R–COO–R′, suffix -oate.
CH₃CH₂OH ethanol; CH₃CHO ethanal; CH₃COCH₃ propanone; CH₃COOH ethanoic acid.
The nitrogen families
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.
Amines are bases; amides are not, even though both contain nitrogen.
Groups control physical properties
Alcohols, carboxylic acids, primary amines and amides have N–H or O–H bonds, so they hydrogen bond and have high boiling points. Aldehydes, ketones and esters are polar but cannot donate a hydrogen bond, so they boil lower. Ethers and hydrocarbons are close to nonpolar and boil lowest. Small polar molecules dissolve in water; solubility fades as the carbon chain grows.
For a similar molar mass: carboxylic acid > alcohol > ketone/aldehyde > ether > alkane in boiling point.
Why carboxylic acids are acidic
Removing H⁺ from R–COOH leaves a carboxylate whose negative charge is delocalised over two equivalent oxygens. That resonance stabilisation makes the anion much more stable than an alkoxide from an alcohol, so a carboxylic acid is roughly ten orders of magnitude more acidic.
R–COOH ⇌ R–COO⁻ + H⁺ (Ka ≈ 10⁻⁵)
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 is why –COOH is acidic and –OH is not.
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?
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Common mistakes
Calling any molecule with an oxygen an alcohol.
Why it's wrong: Ethers, aldehydes, ketones, acids and esters all contain oxygen without an –OH group.
Check instead: Look for a hydrogen bonded directly to oxygen before saying alcohol.
Ranking boiling points by molar mass alone.
Why it's wrong: Among isomers the molar mass is identical, so only the functional group and its intermolecular forces differ.
Check instead: Ask whether the molecule can donate a hydrogen bond, then whether it is polar.
Treating amides as basic because they contain nitrogen.
Why it's wrong: The nitrogen lone pair is delocalised into the adjacent C=O, so it is not available to accept a proton.
Check instead: Check whether the nitrogen sits next to a carbonyl before calling it a base.
Practice this skill
No practice questions are available for this topic yet. You can still practice the whole unit.
What you should now know
A functional group is the reactive site of an organic molecule and it, rather than the carbon skeleton, sets the chemistry. Alcohols (–OH), ethers (C–O–C), aldehydes (terminal C=O), ketones (internal C=O), carboxylic acids (–COOH), esters (–COO–), amines (–NH₂) and amides (–CONH₂) each have a signature suffix. Groups with O–H or N–H hydrogen bond and boil highest; polar groups without them come next; ethers and hydrocarbons boil lowest. Carboxylic acids are acidic because the carboxylate anion delocalises its charge over two oxygens.
- The functional group, not the chain, determines reactivity
- 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
Sources and further reading
This lesson is original Chem Help content. No external sources were adapted.