Difficulty: Introductory to Intermediate | Prerequisites: General chemistry (atomic structure, electron configuration, periodic trends)
This material covers the two foundational skills of organic chemistry: naming molecules (IUPAC nomenclature) and drawing their electronic structures (Lewis structures). Nomenclature is the language you will use in every subsequent topic, from reactions to spectroscopy. Lewis structures underpin your understanding of reactivity, formal charge, and how molecules relate to one another. You should already be comfortable with electronegativity, valence electrons, and basic bonding from general chemistry before tackling this.
IUPAC nomenclature follows a strict set of rules: find the longest carbon chain, number it to give substituents the lowest locants, and add stereochemical prefixes (cis/trans) where geometry matters. Lewis structures require you to account for every valence electron, assign formal charges correctly, and show all lone pairs and bonds. Two molecules with the same molecular formula but different atom connectivity are constitutional isomers; same connectivity but different spatial arrangement makes them stereoisomers.
IUPAC nomenclature
The standardised system for naming organic compounds maintained by the International Union of Pure and Applied Chemistry. Names encode the parent chain length, substituent identity and position, and (where relevant) stereochemistry.
In simple terms, it is the set of rules that lets any chemist in the world draw the same molecule from the same name.
Parent chain
The longest continuous chain of carbon atoms in a molecule. The parent chain determines the base name (methane, ethane, propane, butane, pentane, hexane, heptane, octane, etc.).
Think of it as the backbone of the molecule; everything else hangs off it.
Substituent
An atom or group of atoms that replaces a hydrogen on the parent chain. Common substituents include methyl (-CH3), ethyl (-C2H5), isopropyl, and tert-butyl.
Think of substituents as the branches growing from the backbone.
Stereochemistry (cis/trans)
A description of the three-dimensional arrangement of atoms in a molecule. Cis means two groups are on the same side of a ring or double bond; trans means they are on opposite sides.
In simple terms, two molecules can have identical connectivity but look different in 3D space, and that difference matters.
Constitutional isomers (structural isomers)
Molecules that share the same molecular formula but differ in which atoms are bonded to which. They have different connectivity.
Think of it as two different buildings made from exactly the same set of bricks.
Stereoisomers
Molecules with the same molecular formula and the same connectivity, but a different arrangement of atoms in three-dimensional space.
In simple terms, same skeleton, different 3D shape.
Enantiomers
A specific type of stereoisomer: non-superimposable mirror images of one another, like left and right hands.
Lewis structure
A diagram showing all valence electrons in a molecule as bonds (shared pairs) and lone pairs (unshared pairs). A correct Lewis structure accounts for every valence electron and satisfies the octet rule where possible.
Formal charge
The charge assigned to an atom in a Lewis structure, calculated as: valence electrons minus lone-pair electrons minus half the bonding electrons. Formal charge is a bookkeeping tool, not a measure of actual electron density.
In simple terms, it tells you whether an atom "owns" more or fewer electrons than it brought to the molecule.
Lone pair
A pair of valence electrons on an atom that is not involved in bonding. Lone pairs are critical for determining molecular geometry and reactivity.
Identify the longest continuous chain of carbon atoms. This gives the root name (e.g., heptane = 7 carbons, octane = 8 carbons).
Number the chain so that the first point of difference gives the lowest locant to a substituent.
If there is a tie, use alphabetical order of substituents to break it.
Simple alkyl groups: methyl (1C), ethyl (2C), propyl (3C), butyl (4C).
Branched substituents: isopropyl (two-carbon branch with a methyl), tert-butyl (three methyls on one carbon).
Use multiplying prefixes (di-, tri-, tetra-) for identical substituents on the parent chain.
List substituents in alphabetical order in the final name. Multiplying prefixes (di-, tri-) do not count for alphabetical ordering, but iso-, tert-, sec- and cyclo- do.
For cycloalkanes and alkenes with two substituents, indicate cis (same side) or trans (opposite side).
Example from this exam: cis-4-methyl-1-isopropylcyclohexane describes a cyclohexane ring where the methyl and isopropyl groups sit on the same face.
Work backwards: draw the parent chain first, number it, then attach substituents at the specified positions.
Example: 4-(tert-butyl)-3-methylheptane means a 7-carbon chain with a tert-butyl group at C4 and a methyl group at C3.
Common errors: choosing the wrong parent chain (not the longest one), misnumbering carbons, and drawing a branch group in the wrong position.
Parent chain: pentane (5 carbons).
Substituent: one methyl group on carbon 3.
Name: 3-methylpentane.
Note: numbering from the other end would give "3-methylpentane" as well (symmetric), but in asymmetric cases always choose the direction giving the lowest locant.
Count total valence electrons. For anions, add one electron per negative charge; for cations, subtract one per positive charge.
Draw the skeletal structure (which atoms connect to which).
Place bonding pairs between connected atoms, then distribute remaining electrons as lone pairs to satisfy octets.
Hydrogen always gets 2 electrons (one bond). Carbon gets 8 (four bonds). Nitrogen gets 8 (typically three bonds + one lone pair). Oxygen gets 8 (typically two bonds + two lone pairs).
Formula: Formal charge = (valence electrons of the free atom) minus (lone pair electrons) minus (half of bonding electrons).
A neutral nitrogen with three bonds and one lone pair has formal charge 0. A nitrogen with four bonds and no lone pair has formal charge +1.
An oxygen with one bond and three lone pairs has formal charge -1.
The sum of all formal charges in a molecule must equal the overall charge of the species.
Constitutional isomers: same molecular formula, different connectivity. Example from this exam: two C4H8NO2- anions where the atoms are bonded in different arrangements (one has an amide linkage, the other an amino acid-like structure).
Stereoisomers: same formula, same connectivity, different 3D arrangement (cis/trans, or enantiomers).
Enantiomers: mirror-image stereoisomers that are non-superimposable.
Bronsted acid and conjugate base: related by loss of one proton (H+). If two structures differ by more than a proton, they are not an acid-base pair.
When an exam gives you a molecular formula and structural constraints, work systematically:
Use the degree of unsaturation (DoU) formula: DoU = (2C + 2 + N - H - X) / 2, where X = halogens.
A ring or a double bond each count as one degree of unsaturation.
Example: C5H10 has DoU = 1, so the molecule has either one ring or one double bond.
If the question specifies "1 ring and two alkyl groups in a trans relationship," draw a cyclopropane (or cyclobutane, etc.) ring with substituents on opposite sides. For C5H10 with one ring, a common answer is trans-1,2-dimethylcyclopropane.
Students often pick a shorter chain with more substituents instead of the longest continuous chain. The parent chain must always be the longest, even if it is not drawn in a straight line.
Forgetting to include stereochemical prefixes (cis/trans) when the molecule has a ring or double bond with two distinct groups is a frequent point-loser.
Many students forget lone pairs or leave out formal charges on Lewis structures. Every valence electron must be accounted for. If the sum of your formal charges does not match the overall molecular charge, something is wrong.
Confusing constitutional isomers with stereoisomers: if the connectivity (which atoms are bonded to which) is different, they are constitutional isomers, full stop. Stereoisomers have identical connectivity.
⚠️ IUPAC naming questions appear on virtually every Org Chem I exam. Know the rules cold, including stereochemical prefixes.
⚠️ Lewis structure problems test formal charge, lone pairs, and hydrogen placement. Missing any one of these costs points.
⚠️ You will be asked to classify relationships between molecules (constitutional isomers vs stereoisomers vs enantiomers). Read the molecular formula and connectivity carefully before answering.
True or False: The parent chain is always the chain drawn horizontally in a structural formula.
False. The parent chain is the longest continuous chain, regardless of how it is drawn.
Fill in the blank: Two molecules with the same molecular formula but different atom connectivity are called ______.
Constitutional isomers.
True or False: A nitrogen atom with four bonds and no lone pair has a formal charge of +1.
True.
Fill in the blank: Formal charge = valence electrons minus ______ minus half of ______.
Lone pair electrons; bonding electrons.
True or False: Cis and trans isomers are constitutional isomers.
False. They are stereoisomers (same connectivity, different 3D arrangement).
Q: A molecule has the molecular formula C5H10 and contains one ring. It has two alkyl groups in a trans relationship. Draw a possible structure.
A: trans-1,2-Dimethylcyclopropane. Draw a three-membered ring with one methyl on C1 pointing up and one methyl on C2 pointing down (or vice versa). The remaining carbon on the ring accounts for the fifth carbon.
Q: Two molecules both have the formula C4H8NO2-. Molecule A has the nitrogen bonded to a carbonyl carbon. Molecule B has the nitrogen bonded to a CH2 group next to a carboxylate. What is their relationship?
A: They are constitutional isomers. The molecular formula is identical but the connectivity differs.
Q: What is the formal charge on an oxygen atom that has one bond and three lone pairs?
A: Formal charge = 6 (valence) minus 6 (lone pair e-) minus 1 (half of 2 bonding e-) = -1.
Q: Provide the IUPAC name for an eight-carbon alkane with methyl groups at positions 2, 4, and 7.
A: 2,4,7-Trimethyloctane.
Q: What is the degree of unsaturation for C5H10?
A: DoU = (2(5) + 2 - 10) / 2 = 1. The molecule has one ring or one double bond.
Nomenclature is the language of every reaction mechanism, synthesis problem, and spectroscopy question you will encounter later in the course. If you cannot name or draw a molecule correctly, you will struggle to communicate your answers.
Lewis structures feed directly into resonance, molecular geometry (VSEPR), and acid-base chemistry. Formal charge, in particular, comes up again when you evaluate resonance contributors and predict reactivity.
IUPAC naming, organic nomenclature, parent chain, substituent, alkyl group, methyl, ethyl, isopropyl, tert-butyl, cis, trans, stereochemistry prefix, Lewis dot structure, formal charge calculation, lone pair, bonding pair, constitutional isomers, structural isomers, stereoisomers, enantiomers, mirror image, degree of unsaturation, index of hydrogen deficiency, molecular formula, valence electrons, octet rule