Difficulty: Introductory to Intermediate | Prerequisites: Lewis structures, basic bonding, familiarity with carbon-chain drawing conventions.
Functional groups are the reactive centres of organic molecules, and identifying them is the first step toward predicting how a compound will behave in any reaction. IUPAC nomenclature is the universal naming system that encodes a molecule's backbone length, substituents, stereochemistry and functional groups into a single unambiguous name. If you can name a structure and draw a structure from a name, you can communicate with any chemist on the planet. This material underpins everything that follows in the course, from substitution and elimination to synthesis planning.
Organic chemistry sorts molecules by functional groups (alcohol, amine, ester, aldehyde, ketone, etc.), each of which has a characteristic reactivity pattern. IUPAC naming follows a strict recipe: find the longest carbon chain, number it to give the highest-priority group the lowest locant, then list substituents alphabetically with their positions. Master the naming rules and the common functional-group structures, and the rest of the course clicks into place.
Functional group
A specific arrangement of atoms within a molecule that determines the compound's chemical reactivity and properties. Think of it as the "business end" of the molecule, the part that actually does something in a reaction.
Alcohol (-OH)
A compound containing a hydroxyl group bonded to a saturated carbon. In simple terms, this is the -OH group attached to a carbon that is not part of a carbonyl.
Aldehyde (-CHO)
A carbonyl group (C=O) bonded to at least one hydrogen atom, always found at the end of a carbon chain. Think of it as a terminal C=O with an H on one side.
Ketone (C=O, internal)
A carbonyl group flanked by two carbon atoms (not at the chain terminus). In simple terms, a C=O buried inside the chain rather than at the end.
Ester (-COOR)
A carbonyl group bonded to an oxygen that is itself bonded to another carbon. Think of it as an acid whose acidic H has been swapped for a carbon group.
Amine (-NH2, -NHR, -NR2)
A nitrogen atom bonded to one, two, or three carbon/hydrogen atoms (derived from ammonia by replacing H with carbon groups). In simple terms, an organic version of ammonia.
Ether (R-O-R)
An oxygen atom bonded to two carbon groups. Think of it as water where both hydrogens have been replaced by carbons.
Amide (-CONHR or -CONR2)
A carbonyl group bonded directly to a nitrogen. In simple terms, a hybrid of an acid and an amine, where N is bonded to C=O.
Haloalkane (alkyl halide, R-X)
A carbon bonded to a halogen (F, Cl, Br, or I). The halogen is the reactive site in substitution and elimination reactions.
Carboxylic acid (-COOH)
A carbonyl group bonded to a hydroxyl group, giving the -COOH unit. This is the group that makes vinegar sour and fatty acids fatty.
Alkene (C=C)
A carbon-carbon double bond. The pi bond is the reactive part, acting as a nucleophile-rich region.
IUPAC nomenclature
The International Union of Pure and Applied Chemistry's systematic naming convention. It gives every organic compound a unique, unambiguous name based on its structure.
When a problem shows you a structure and asks which functional groups are present, work through it bond by bond:
Look for C=O first. If the carbon of C=O is bonded to an -OH, it is a carboxylic acid. If bonded to an -OR, it is an ester. If bonded to -NR2 or -NHR or -NH2, it is an amide. If bonded to an H, it is an aldehyde. If bonded to two carbons, it is a ketone.
Look for -OH bonded to a saturated carbon (no adjacent C=O). That is an alcohol.
Look for nitrogen: -NH2, -NHR, or -NR2 without an adjacent C=O is an amine. With an adjacent C=O it is an amide.
Look for C-O-C linkages where neither oxygen neighbour is a carbonyl. That is an ether.
Look for C=C (a double bond between two carbons). That is an alkene.
Look for C-X where X is F, Cl, Br, or I. That is a haloalkane.
From the exam key: a compound containing both an ester linkage, an aldehyde, and an amine was tested. Students needed to tick all three (plus recognise the absence of alcohol, ketone, ether, amide, alkene, haloalkane and carboxylic acid).
Find the parent chain. Identify the longest continuous carbon chain that includes the highest-priority functional group.
Number the chain. Assign locants so that the principal functional group gets the lowest possible number. If there is a tie, use the first point of difference rule.
Name the parent. Use the root name for the chain length (meth-, eth-, prop-, but-, pent-, hex-, etc.) and the suffix for the principal group (-ol for alcohol, -al for aldehyde, -one for ketone, -oic acid for carboxylic acid, -ene for alkene, etc.).
List substituents. Each substituent gets a locant and a prefix (methyl-, ethyl-, chloro-, bromo-, etc.), listed alphabetically. Multiplying prefixes (di-, tri-, tetra-) do not affect alphabetical order.
Include stereochemistry. Use (E)/(Z) for alkene geometry, (R)/(S) for chiral centres, placed in parentheses before the locant or name segment they describe.
Work backwards through the name:
Decode the parent chain length and principal group from the suffix.
Place the principal group at the locant given.
Attach substituents at their locants.
Add stereochemistry where indicated.
Exam key examples:
(E)-2-chloropenta-2,4-dien-1-ol: five-carbon chain (penta), -OH at C1 (-1-ol), double bonds at C2 and C4 (dien), chloro at C2, E geometry at the C2 double bond.
4-bromo-2-methyl-1-butene: four-carbon chain (but), double bond at C1 (-1-butene), methyl at C2, bromo at C4.
Exam key example: a structure with -OH, a double bond, a methyl branch, and a bromine was named 4-bromo-2-methyl-4-penten-2-ol. The parent chain is five carbons (pent), the double bond is at C4 (-4-penten), the -OH is at C2 (-2-ol), and substituents are 4-bromo and 2-methyl.
Students often confuse esters and ethers. An ester has a C=O adjacent to the C-O-C linkage; an ether does not. If you see C(=O)-O-C, that is an ester.
Students sometimes call an amide an amine (or vice versa). The distinguishing feature is the carbonyl: amide = nitrogen bonded to a C=O. Amine = nitrogen with no adjacent C=O.
When numbering the parent chain, students sometimes number from the wrong end and give substituents unnecessarily high locants. Always number so the principal functional group gets the lowest locant.
Forgetting to include (E)/(Z) designations on alkenes that require them. If a double bond has two different groups on each carbon, it needs an (E) or (Z) label.
⚠️ Functional group identification is typically tested as a "tick all that apply" question. Missing one group or ticking an extra one loses marks. Scan every bond in the molecule systematically.
⚠️ IUPAC naming questions appear as both "name this structure" and "draw this name." Both directions are fair game, and stereochemistry labels are expected where relevant.
⚠️ If a molecule contains multiple functional groups, the highest-priority group determines the suffix. All others become prefixes (e.g. hydroxy- instead of -ol when an aldehyde or acid is the principal group).
⚠️ Substituent alphabetical ordering ignores multiplying prefixes: "dimethyl" is alphabetised under M, not D. "tert-butyl" is alphabetised under B, not T.
True or false: an amide contains a nitrogen atom bonded directly to a carbonyl carbon. True.
Fill in the blank: the suffix for an alcohol in IUPAC nomenclature is ____. -ol
True or false: in the name "3-bromo-2-methylpentane," the substituents are listed alphabetically. True (bromo before methyl).
Fill in the blank: a carbonyl group at the end of a chain with a hydrogen on the carbonyl carbon is called an ____. aldehyde
True or false: (E)-2-chloropenta-2,4-dien-1-ol has two double bonds. True ("dien" indicates two double bonds).
Q: A compound has the structure: O=C(OC)c1ccccc1N. Which functional groups are present?
A: Ester (the C(=O)-O-C linkage), amine (the -NH2 on the ring). There is no free alcohol, no aldehyde, no ketone, and no carboxylic acid.
Q: Draw the structure of (E)-2-chloropenta-2,4-dien-1-ol.
A: Five-carbon chain with -OH at C1, double bonds at C2-C3 and C4-C5, Cl at C2, and (E) geometry across the C2=C3 double bond (the larger groups on opposite sides).
Q: Give the IUPAC name for a five-carbon chain with -OH at C2, a double bond at C4, a methyl branch at C2, and Br at C4.
A: 4-bromo-2-methyl-4-penten-2-ol.
Q: What is the difference between an ester and a carboxylic acid?
A: Both have a carbonyl bonded to an oxygen, but in a carboxylic acid the oxygen bears a hydrogen (-COOH), while in an ester the oxygen bears a carbon group (-COOR).
Q: Draw 4-bromo-2-methyl-1-butene.
A: Four-carbon parent chain, C=C double bond between C1 and C2, a methyl group at C2, and Br at C4.
Functional group identification feeds directly into predicting reaction mechanisms. The presence of an alcohol (-OH) determines whether a molecule is a candidate for E1 elimination under acidic conditions, or for conversion to a haloalkane with HBr or HCl. Likewise, recognising a haloalkane is the first step in deciding whether a substitution (SN1 or SN2) or elimination (E1 or E2) pathway will dominate, topics covered in the next set of study notes.
IUPAC naming conventions also extend to more complex functional groups you will meet later in the course, including epoxides, thiols, and polyfunctional compounds with competing naming priorities.
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