IB Chemistry HLTopic 3 — Classification of MatterPaper 1 & 2Core idea~12 min read
Functional Groups
There are millions of organic compounds and only a handful of reactions you need to learn. That works because the long carbon chain is mostly dead weight — the chemistry happens at one small reactive spot. Find that spot and you already know how the molecule behaves.
📘 What you need to know
A functional group is an atom or group of atoms that gives a molecule its characteristic physical and chemical properties.
Compounds sharing the same functional group belong to the same class — ethanol and hexan-1-ol are both alcohols.
Each class has its own IUPAC prefix or suffix, which is how the group shows up in the name.
Saturated means only single bonds between carbons. Unsaturated means there is at least one C=C or C≡C.
Four of the classes are just a C=O with something different attached: aldehyde, ketone, carboxylic acid, ester. Amide makes a fifth.
A molecule can hold more than one functional group, and each one keeps its own chemistry.
“Same class” is not the same statement as “same homologous series” — that is the next page.
What a functional group actually does
Look at hexane and hexan-1-ol. They have almost identical carbon skeletons, but hexane is unreactive and barely dissolves in water, while hexan-1-ol can be oxidised, dehydrated, turned into an ester, and mixes with water far better. One OH group did all of that.
The reason is electron density. A plain C–C or C–H bond is close to non-polar, so nothing is attracted to it. Put an oxygen or a halogen in and you create a polar bond with a partial positive carbon — a target. Put in a double bond and you create a region of loose pi electrons — also a target. Reactivity follows the group, not the chain.
This is why the chain length barely changes the chemistry. Propan-1-ol and decan-1-ol undergo the same reactions, because the reacting part is the same. The chain length changes physical properties like boiling point, not chemical behaviour.
Position matters as much as the atoms
Here is the point that costs the most marks on this topic. Two molecules can be built from exactly the same atoms and still belong to different classes, purely because of where the group sits.
Ethanol and methoxymethane share a molecular formula, yet one is a liquid you can drink and the other is a gas. Ethanol’s OH can hydrogen bond to its neighbours; the ether’s buried oxygen cannot, because it has no hydrogen attached to it.
If you only remember one thing from this page, remember to check where the oxygen sits before you name the class. An oxygen with a hydrogen on it is an alcohol or an acid. An oxygen with carbons on both sides is an ether or an ester.
The carbonyl family: five classes, one group
The C=O group is called a carbonyl, and it turns up in five different classes. They all react in broadly similar ways, because they all have the same electron-poor carbon in the middle. What separates them is the single atom or group sitting on the right-hand side of that carbon.
The aldehyde is the odd one out: it is the only carbonyl with a hydrogen bonded straight to the carbonyl carbon. That single hydrogen is why aldehydes are oxidised easily and ketones are not.
Why the H is written first in RCHO. An aldehyde is often written as RCHO rather than RCOH, because RCOH looks like an alcohol with the O and H next to each other. Putting the H before the O makes it clear the hydrogen is on the carbon, not on the oxygen.
Saturated or unsaturated?
This pair of words is only ever about carbon–carbon bonds.
Saturated: only single C–C bonds. The molecule is holding as many hydrogens as it can, so it is “full up”.
Unsaturated: at least one C=C or C≡C. There is room to add more atoms across that multiple bond.
A C=O double bond does not make a molecule unsaturated in the way this syllabus uses the word. Propanone has a double bond, but both of its carbon–carbon bonds are single, so it counts as saturated. Only look at carbon-to-carbon.
The classes you need to know
Class
Group name
Group
Prefix or suffix
Simplest example
alkane
alkyl
C–C single bonds only
-ane
ethane, C2H6
alkene
alkenyl
C=C
-ene
ethene, C2H4
alkyne
alkynyl
C≡C
-yne
ethyne, C2H2
halogenoalkane
halogeno
–F, –Cl, –Br, –I
fluoro-, chloro-, bromo-, iodo-
chloroethane, C2H5Cl
alcohol
hydroxyl
–OH
-ol, or hydroxy-
ethanol, C2H5OH
ether
alkoxy
–O– between two carbons
alkoxy-
methoxymethane, CH3OCH3
aldehyde
carbonyl
–CHO at a chain end
-al
ethanal, CH3CHO
ketone
carbonyl
–CO– inside the chain
-one
propanone, CH3COCH3
carboxylic acid
carboxyl
–COOH
-oic acid
ethanoic acid, CH3COOH
ester
ester
–COO–
-yl …-oate
methyl methanoate, HCOOCH3
amine
amino
–NH2
-amine, or amino-
ethanamine, C2H5NH2
amide
amido
–CONH2
-amide
ethanamide, CH3CONH2
arene
phenyl
benzene ring, C6H5–
phenyl-, or -benzene
methylbenzene, C6H5CH3
🧩 Spotting the group in an unfamiliar molecule
Find every atom that is not carbon or hydrogen. Circle them. These are where all the interesting chemistry lives.
For each oxygen, ask what is on either side. H on one side means alcohol or acid. Carbons on both sides means ether or ester.
Look for any C=O. If you find one, check the fourth thing on that carbon: H gives aldehyde, carbon gives ketone, OH gives acid, O–C gives ester, N gives amide.
Scan the carbon backbone for multiple bonds. A C=C makes it an alkene, a C≡C makes it an alkyne, a ring with delocalised electrons makes it an arene.
Do not stop at the first group you find. If more than one is present, name them all — the question is usually testing exactly that.
Worked examples
WORKED EXAMPLE
Identify all the functional groups in HOCH2CH2COOH and name the classes they belong to.
Step 1: mark every non-carbon, non-hydrogen atomThere are three oxygens: one in the HO at the left, and two in the COOH at the right.Step 2: work out the left-hand oxygenHO–CH2– : an oxygen with an H on one side and a carbon on the otherThat is a hydroxyl group, so this end is an alcohol.Step 3: work out the right-hand pair of oxygens–COOH : a C=O with an OH bonded to the same carbonThat is a carboxyl group, so this end is a carboxylic acid.hydroxyl (alcohol) and carboxyl (carboxylic acid)Two groups, so this molecule shows both sets of reactions. It is 3-hydroxypropanoic acid — the acid wins the suffix and the alcohol becomes the hydroxy- prefix.
WORKED EXAMPLE
A compound is CH3CH=CHCH2Cl. State its functional groups, and say whether it is saturated or unsaturated.
Step 1: look for atoms that are not C or Hone Cl, bonded to a carbonA halogen on a carbon chain makes this a halogenoalkane. The group is halogeno, and it would appear in the name as chloro-.Step 2: scan the backbone for multiple bondsCH=CH between carbons 2 and 3A C=C is an alkenyl group, so it is also an alkene.Step 3: decide saturated or unsaturatedRule: unsaturated means at least one carbon-to-carbon multiple bond. There is one here.alkenyl and halogeno; the compound is unsaturatedBoth groups react. The C=C will do addition, the C–Cl will do substitution.
WORKED EXAMPLE
A student says butanal and butanone must be in the same class because both contain a carbonyl group. Explain why they are wrong.
Step 1: write out both structuresbutanal: CH3CH2CH2CHObutanone: CH3COCH2CH3Step 2: check what the carbonyl carbon is bonded toIn butanal the carbonyl carbon carries a hydrogen and one carbon chain. In butanone it carries two carbon chains and no hydrogen.Step 3: name the two classesCarbonyl at a chain end with an H gives an aldehyde. Carbonyl inside the chain gives a ketone. These are two different classes.aldehyde and ketone — different classes, so different chemistryThey do share the molecular formula C4H8O, which makes them functional group isomers. Coming up in a couple of pages.
💡 Exam tip
Name the group, not just the class, when asked for a functional group. “Hydroxyl” is the group; “alcohol” is the class. Some mark schemes want the group name specifically.
Circle the non-carbon atoms first. It takes three seconds and it means you never miss a second functional group hiding at the other end.
Check what is bonded to a carbonyl carbon before you commit. Aldehyde, ketone, acid, ester and amide all look similar at a glance.
“Saturated” only refers to carbon-to-carbon bonds. A C=O or a C–Cl does not affect it.
Learn the prefixes and suffixes together with the groups. Half the marks on this topic come through the naming rather than the structure.
The IB data booklet does not list functional groups for you. This table has to be memorised.
⚠ Common mix-up
Confusing an alcohol with an ether. An alcohol’s oxygen has a hydrogen on it; an ether’s oxygen sits between two carbons and has none.
Confusing a carboxylic acid with an ester. The acid ends in OH; the ester has that H replaced by a carbon chain.
Calling an aldehyde a ketone. Check for the hydrogen on the carbonyl carbon — the aldehyde has one, the ketone does not.
Writing an aldehyde as RCOH. That reads as an alcohol. Always write RCHO.
Calling propanone unsaturated because it has a double bond. The C=O does not count; only C=C and C≡C do.
Assuming same class means same homologous series. Aldehydes and ketones are different classes but share a general formula, and cyclic compounds share formulas with alkenes.
Stopping after finding one functional group. If a question awards two marks for identifying groups, there are almost certainly two groups.
Up next: Homologous Series — you have the groups, so now we line up all the compounds that share one and see what changes as the chain grows.
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