There are millions of organic compounds, which sounds impossible to learn until you notice that they nearly all behave like one of about a dozen families. The carbon chain is mostly scaffolding. The functional group is where the chemistry happens.
📚 What you need to know
A functional group is an atom or group of atoms that gives a molecule its characteristic properties.
Compounds sharing a functional group belong to the same class, and behave in similar ways.
Each class has its own IUPAC prefix or suffix — –ol for alcohols, –al for aldehydes, and so on.
Molecules with only single C–C bonds are saturated; those with a C=C or C≡C are unsaturated.
A molecule can contain more than one functional group.
Carbon always forms four bonds — use that to check any structure you draw.
Why groups matter more than chains
Take ethanol and hexan-1-ol. One is a small molecule, the other is three times the size — but both have an –OH group, so both react in the same ways. They differ in boiling point and solubility, but chemically they are the same family.
Now compare ethanol (CH3CH2OH) with ethanoic acid (CH3COOH). Almost the same size, almost the same formula — but one is a neutral alcohol and the other is an acid that reacts with carbonates and neutralises bases. The group is doing all the work.
This is the single most useful idea in organic chemistry. When you meet an unfamiliar molecule in an exam, don’t panic at the size of it — find the functional group, and you already know how it behaves.
The groups you need
Learn these as families rather than as individual molecules. Note how many of them are built around a C=O.
Class
Functional group
Formula
Ending
Example
alkane
alkyl
—
–ane
ethane, C2H6
alkene
alkenyl
C=C
–ene
ethene, C2H4
alkyne
alkynyl
C≡C
–yne
ethyne, C2H2
halogenoalkane
halogeno
–F, –Cl, –Br, –I
chloro– etc.
chloroethane
alcohol
hydroxyl
–OH
–ol
ethanol
ether
alkoxy
–O–
no suffix
ethoxyethane
aldehyde
carbonyl
–CHO
–al
ethanal
ketone
carbonyl
–CO–
–one
propanone
carboxylic acid
carboxyl
–COOH
–oic acid
ethanoic acid
ester
ester
–COO–
–oate
methyl ethanoate
amine
amino
–NH2
–amine
ethanamine
amide
amido
–CONH2
–amide
ethanamide
Careful with the carbonyls. Aldehydes, ketones, carboxylic acids, esters and amides all contain a C=O. What tells them apart is what else is attached to that carbon — an H, another carbon, an –OH, an –O– or an –NH2.
Telling the carbonyl family apart
This is where most marks are lost, so it is worth being precise. Look at the carbon carrying the double-bonded oxygen and ask what else is bonded to it:
Aldehyde — at least one hydrogen. Always at the end of a chain. Written RCHO.
Ketone — two carbons. Always in the middle, so it needs at least three carbons. Written RCOR.
Carboxylic acid — an –OH. Written RCOOH.
Ester — an –O– leading to another carbon chain. Written RCOOR.
Amide — a nitrogen. Written RCONH2.
Note the writing convention: an aldehyde is written –CHO, with the H before the O, so it is not mistaken for an alcohol. Writing “COH” is a genuine ambiguity, not just untidiness.
Saturated and unsaturated
These two words describe the carbon–carbon bonds, not the functional group.
Saturated means the molecule is holding all the hydrogen it can. A double bond costs it two.
Saturated — only single C–C bonds. The molecule holds the maximum possible number of hydrogens.
Unsaturated — contains at least one C=C or C≡C. Each double bond means two fewer hydrogens.
WORKED EXAMPLE
Identify the functional group in each and name the class: (a) CH3CH2CHO, (b) CH3COCH3, (c) CH3CH2COOH, (d) CH3CH2NH2.
(a) C=O carbon carries an H, and sits at the chain endcarbonyl → aldehyde(b) C=O carbon sits between two carbonscarbonyl → ketone(c) C=O carbon also carries an –OHcarboxyl → carboxylic acid(d) nitrogen with two hydrogens on the endamino → amine
WORKED EXAMPLE
A molecule has the formula C3H6O and reacts as a ketone. Draw its condensed structure and explain why C2H4O could not be a ketone.
A ketone needs C=O with a carbon on each sideCH₃COCH₃ — propanoneWhy 2 carbons cannot workWith only 2 carbons, the C=O must sit at the end, so there is a hydrogen on it — that makes it an aldehyde (ethanal), not a ketone.ketones need at least 3 carbons
💡 Exam tip
Learn the name of the group as well as the class. The class is “alcohol”; the group is “hydroxyl”. Questions ask for both.
When identifying a carbonyl compound, always state what else is attached to the C=O carbon. That is the reasoning mark.
A molecule can have several groups — check the whole structure before answering.
Check every carbon has four bonds in anything you draw.
⚠️ Common mix-up
Aldehyde vs ketone. Aldehydes are at the end of the chain and carry an H; ketones are in the middle.
Write –CHO, not –COH. The second looks like an alcohol.
Ethers have no suffix — they are named with an alkoxy prefix, like ethoxyethane.
Saturated and unsaturated describe C–C bonds, not whether a functional group is present.
Class is not the same as homologous series — a class shares a group; a series also shares a general formula.
Up next: Homologous Series — what happens when you keep the group the same and just make the chain longer, one CH2 at a time.
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