Source: CHEM 237 Practice Exam 1, University of Washington | Chapters 1, 2
Tags: IUPAC nomenclature, structural formulas, Lewis structures, formal charge, resonance, line-angle structures, cycloalkanes, organic chemistry, CHEM 237
Difficulty: Introductory to Intermediate
Prerequisites: General chemistry (atomic structure, electron configuration, bonding basics). Familiarity with the periodic table and electronegativity trends.
Organic chemistry begins with learning to speak its language: how molecules are named, drawn, and interpreted at the electron level. This material underpins every reaction mechanism you will encounter later in CHEM 237 and beyond. If you cannot draw a correct structure from a name (or name one from a structure), you will struggle to follow reaction problems. Similarly, Lewis structures and resonance are the foundation for understanding reactivity, acidity, and stability, topics that dominate the rest of the course.
If you are coming in cold, make sure you are comfortable with covalent bonding, electronegativity, and the octet rule from general chemistry before diving in.
IUPAC nomenclature gives every organic molecule a unique, systematic name built from its longest carbon chain, substituent names, and locator numbers. Drawing structures means translating those names into line-angle (skeletal) formulas, and vice versa. Lewis structures map out every bonding and lone pair of electrons, and when more than one valid Lewis structure exists for a molecule, those are resonance forms, with the most stable form carrying the fewest formal charges and placing negative charge on the most electronegative atoms.
IUPAC nomenclature
The standardised system for naming organic compounds, maintained by the International Union of Pure and Applied Chemistry. Names are built from a parent chain, substituent prefixes, and numerical locators.
In simple terms, this is the "official" naming system that tells you exactly what a molecule looks like from its name alone.
Parent chain
The longest continuous chain of carbon atoms in a molecule, which determines the root name (methane, ethane, propane, butane, pentane, hexane, and so on).
Think of it as the backbone of the molecule. Everything else hangs off it.
Substituent (alkyl group)
A branch or side group attached to the parent chain, named by replacing the "-ane" ending with "-yl" (e.g. methyl, ethyl, propyl, isopropyl).
In simple terms, these are the bits that stick out from the main chain.
Line-angle (skeletal) structure
A shorthand drawing convention where carbon-carbon bonds are drawn as a zigzag line. Carbon atoms sit at each vertex and line end, and hydrogens on carbon are omitted (assumed to fill each carbon to four bonds). Heteroatoms (O, N, etc.) and their hydrogens are always shown.
Think of it as the standard "stick figure" for organic molecules.
Lewis structure
A diagram showing all valence electrons in a molecule as dots or lines (bonding pairs) and dots (lone pairs). Every atom should satisfy the octet rule where possible (duet for hydrogen).
In simple terms, it is the full electron map of a molecule.
Formal charge
The charge assigned to an atom in a Lewis structure, calculated as: valence electrons minus lone-pair electrons minus half of bonding electrons.
Think of it as a bookkeeping tool. It tells you whether an atom "owns" more or fewer electrons than it brought to the molecule.
Resonance structures
Two or more valid Lewis structures for the same molecule that differ only in the placement of electrons (not atoms). The real molecule is a weighted average (hybrid) of all resonance forms.
In simple terms, when you can push electrons around to draw the molecule a different way without moving any atoms, those are resonance structures.
Constitutional (structural) isomers
Molecules with the same molecular formula but different connectivity of atoms.
Think of it as same ingredients, different recipe. The atoms are the same, but they are bonded together in a different order.
The process is mechanical once you know the steps:
Identify the parent chain from the root name. "Propane" = 3 carbons, "pentane" = 5 carbons, "hexane" = 6 carbons. If the name includes "cyclo", the parent chain forms a ring.
Number the parent chain so that substituents get the lowest possible locator numbers.
Attach each substituent at its numbered position. Common substituents: methyl (1C), ethyl (2C), propyl (3C), isopropyl (3C, branched).
Add hydrogens to fill each carbon to four bonds (or omit them in a line-angle drawing).
Example: isopropylcyclopropane
A three-membered carbon ring (cyclopropane) with an isopropyl group attached. The isopropyl group is a three-carbon branch where the attachment point is the middle carbon, giving it a "Y" shape.
Example: 3-ethylpentane
A five-carbon straight chain (pentane) with a two-carbon branch (ethyl) at carbon 3.
Example: 1-methyl-4-propylcyclohexane
A six-membered ring (cyclohexane) with a methyl group at position 1 and a three-carbon straight chain (propyl) at position 4.
Working backwards from a drawn structure:
Find the longest continuous carbon chain in the molecule. This is the parent chain.
Number from the end that gives substituents the lowest set of locator numbers.
Name each substituent and list them alphabetically in the final name, each with its locator number.
If the same substituent appears more than once, use di-, tri-, tetra- prefixes (these prefixes do not affect alphabetical ordering).
Drawing Lewis structures step by step
Count total valence electrons. For ions, add one electron per negative charge or subtract one per positive charge.
Draw the skeletal structure (the arrangement of atoms). Hydrogen is always terminal.
Place a bonding pair (one line) between each pair of bonded atoms.
Distribute remaining electrons as lone pairs, starting with outer atoms, to satisfy octets.
If an atom lacks an octet, convert a lone pair on a neighbouring atom into a bonding pair (forming a double or triple bond).
Formal charge calculation
Formal charge = (valence electrons of the neutral atom) - (lone pair electrons) - (half of bonding electrons).
A correct Lewis structure has formal charges that sum to the overall charge of the molecule or ion.
Acetic acid (CH3COOH)
The central carbon is double-bonded to one oxygen (which carries two lone pairs) and single-bonded to the other oxygen (which carries two lone pairs and is also bonded to H). All formal charges are zero in the most stable form.
Methylammonium ion (CH3NH3+)
Nitrogen has four bonds (three to H, one to C) and no lone pairs. Formal charge on N = 5 - 0 - 4 = +1. This single positive formal charge accounts for the ion's overall +1 charge.
Ozone (O3)
Two resonance structures, each with the central oxygen double-bonded to one terminal oxygen and single-bonded to the other. In each form, the central oxygen has a formal charge of +1, and the single-bonded terminal oxygen has a formal charge of -1. Both resonance forms are equally stable (they are equivalent by symmetry).
Identifying the most stable resonance form
Minimise the number of formal charges.
Place negative formal charges on more electronegative atoms.
Maximise the number of atoms with full octets.
Equivalent resonance structures contribute equally to the hybrid.
Constitutional isomers share a molecular formula but differ in which atoms are bonded to which. When asked to draw constitutional isomers:
An isomer with the same functional groups rearranges the carbon skeleton or moves the functional group's position without changing what the functional group is (e.g. moving a C=O along a chain while keeping it an aldehyde).
An isomer with different functional groups changes the identity of the functional group itself (e.g. converting an aldehyde into a ketone, or an ether into an alcohol), while keeping the same molecular formula.
Students often number the parent chain from whichever end is closest to the top of the page. The rule is to number from the end that gives the lowest set of locator numbers to the substituents, regardless of how the structure is drawn.
A common mistake with cycloalkane nomenclature is forgetting that the ring itself is the parent chain when it contains more substituent carbons than any single chain attached to it.
Students frequently confuse resonance structures with constitutional isomers. Resonance forms differ only in electron placement; the atoms stay in the same positions. If you have moved an atom, you have drawn an isomer, not a resonance structure.
When calculating formal charge, students sometimes use total electrons instead of valence electrons. Always use valence electrons only (the group number for main-group elements).
⚠️ Drawing structures from names and naming structures from drawings are near-guaranteed exam questions. These are free points if you drill the process.
⚠️ Lewis structures with correct formal charges appear on virtually every CHEM 237 exam. Expect at least one molecule with resonance.
⚠️ You will be asked to identify the most stable resonance form. Know the three rules: fewest formal charges, negative charge on the most electronegative atom, full octets on all atoms.
⚠️ Constitutional isomer questions often ask for two isomers: one with the same functional groups and one with different functional groups. Read the prompt carefully.
True or false: in IUPAC nomenclature, substituent prefixes like "di-" and "tri-" affect alphabetical ordering. (False. They are ignored when alphabetising.)
Fill in the blank: the formal charge on an atom equals its number of ______ electrons minus its lone-pair electrons minus half its bonding electrons. (Valence.)
True or false: resonance structures differ in the arrangement of atoms. (False. Only electron placement differs; atoms stay put.)
Fill in the blank: in a line-angle structure, each vertex and each line end represents a ______ atom. (Carbon.)
True or false: two molecules with the same molecular formula but different atom connectivity are called resonance structures. (False. They are constitutional isomers.)
Q: Draw the structure of 3-ethylpentane. How many carbon atoms are in the longest chain?
A: Five. The parent chain is pentane (5 carbons), with an ethyl group (2 carbons) branching off carbon 3.
Q: A molecule has the formula C4H10. Draw two constitutional isomers.
A: Butane (a straight four-carbon chain) and 2-methylpropane (isobutane, a three-carbon chain with a methyl branch at carbon 2).
Q: Draw the Lewis structure of ozone (O3) with all lone pairs and formal charges. How many resonance structures does it have?
A: Two equivalent resonance structures. In each, the central oxygen is double-bonded to one terminal oxygen and single-bonded to the other. The central O carries a +1 formal charge, and the singly bonded terminal O carries a -1 formal charge.
Q: What is the formal charge on nitrogen in the methylammonium ion (CH3NH3+)?
A: +1. Nitrogen has 5 valence electrons, 0 lone pairs, and 4 bonds (8 bonding electrons). Formal charge = 5 - 0 - 4 = +1.
Q: You are given a molecule with the nitrile functional group (C≡N). Draw a constitutional isomer with the same functional group and one with a different functional group.
A: Same functional group: move the nitrile to a different position on the carbon chain (e.g. change a straight-chain nitrile to a branched-chain nitrile). Different functional group: an isocyanide (C-N≡C connectivity) or, if the formula allows, an oxime or other nitrogen-containing functional group with the same molecular formula.
This material connects directly to functional group identification (covered in the next set of notes), because you need to recognise structural features before you can name or classify them. Lewis structures and resonance feed into acid-base chemistry: the stability of a conjugate base is often judged by how well it delocalises charge through resonance. Later in CHEM 237, reaction mechanisms are drawn as electron-pushing (curved arrow) steps, which only make sense if you are fluent with Lewis structures and lone pairs.
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