Difficulty: Foundational | Prerequisites: General chemistry (Lewis structures, bonding, electronegativity)
Organic chemistry naming follows a set of strict IUPAC rules that let you go from a molecular structure to an unambiguous name and back again. The system works by identifying the longest carbon chain (parent chain), numbering it to give substituents the lowest possible locants, and listing substituents alphabetically. Once you can name a molecule, you can read any reaction or mechanism in the course.
IUPAC nomenclature
The standardised system for naming organic compounds, maintained by the International Union of Pure and Applied Chemistry. In simple terms, it is the universal language chemists use so that a name points to exactly one structure.
Parent chain
The longest continuous chain of carbon atoms in an organic molecule. This chain determines the base name (methane, ethane, propane, butane, pentane, hexane, and so on). Think of it as the backbone the rest of the molecule hangs off.
Substituent
An atom or group of atoms that replaces a hydrogen on the parent chain. Common examples: methyl (CH3), ethyl (C2H5), propyl (C3H7), isopropyl, sec-butyl, tert-butyl. In simple terms, these are the branches.
Locant
The number assigned to a carbon on the parent chain to indicate where a substituent is attached. You number the chain so that substituents get the lowest possible set of locants.
Constitutional isomers (structural isomers)
Molecules with the same molecular formula but different connectivity of atoms. For example, butane and 2-methylpropane are both C4H10 but have different structures. Think of it as same ingredients, different wiring.
Cycloalkane
An alkane whose carbon chain forms a ring. Named by adding the prefix "cyclo-" to the parent name. Cyclohexane (C6H12) is the most commonly examined example.
Degree of unsaturation (index of hydrogen deficiency, IHD)
A calculation that tells you how many rings or pi bonds a molecule contains. Formula: IHD = (2C + 2 + N - H - X) / 2, where C = carbons, N = nitrogens, H = hydrogens, X = halogens. Each ring or double bond costs one degree; a triple bond costs two.
Alkyl group
A substituent derived from an alkane by removing one hydrogen. Methyl, ethyl, propyl, isopropyl, butyl, sec-butyl, isobutyl, tert-butyl are the ones you need cold for exams.
Common vs. IUPAC names
Some older names persist in everyday use (isopropyl alcohol, tert-butyl). IUPAC names are systematic and unambiguous. Exams may ask you to convert between the two.
Find the longest continuous carbon chain that includes the principal characteristic group (if any). This is your parent chain.
Number the chain from the end that gives the lowest set of locants to substituents. If there is a tie, alphabetical order of substituents breaks it.
Identify and name each substituent. Use multiplying prefixes (di-, tri-, tetra-) for identical substituents. These prefixes do not affect alphabetical order.
List substituents alphabetically before the parent name. Each substituent gets its locant.
Add the appropriate suffix for the functional group (-ane for alkanes, -ene for alkenes, -ol for alcohols, and so on).
If the ring has more substituents or more carbons than any open chain attached to it, the ring is the parent.
Number the ring to give the lowest set of locants to substituents.
Example from the review: 1-sec-butyl-4-isopropyl-2-methylcyclohexane. Here the cyclohexane ring is the parent, with three substituents listed alphabetically (isopropyl before methyl before sec-butyl, since "sec-" is not considered in alphabetical ordering of the base name "butyl").
sec-Butyl: the point of attachment is on the second carbon of a four-carbon chain. Written as "sec-" followed by "butyl."
tert-Butyl (t-butyl): a central carbon bonded to three methyl groups. Tertiary carbon is the attachment point.
Isopropyl: a three-carbon group with the attachment at the middle carbon. Equivalent to the IUPAC name 1-methylethyl.
Neopentyl: a 2,2-dimethylpropyl group.
When these appear as substituents, their common names are acceptable in IUPAC nomenclature.
Same molecular formula, different connectivity.
The number of constitutional isomers increases rapidly with carbon count. Butane (C4H10) has 2; pentane (C5H12) has 3; hexane (C6H14) has 5; heptane (C7H16) has 9.
From the review: 2,2-dimethylpentane is one isomer of C7H16. Being able to draw all isomers for a given formula is a standard exam task.
IHD = (2C + 2 + N - H - X) / 2
Each double bond = 1 degree. Each triple bond = 2 degrees. Each ring = 1 degree.
Oxygen does not affect the calculation. Nitrogen adds one to the numerator. Each halogen subtracts one.
From the review, answer 21 indicates 5 degrees of unsaturation, consistent with a benzene ring (4 degrees from 3 double bonds + 1 ring) plus one additional degree.
Alkanes are saturated hydrocarbons: all C-C single bonds, no functional groups.
General formula: CnH(2n+2) for acyclic; CnH(2n) for monocyclic.
From the review (Q30): alkanes are the least reactive hydrocarbon class because they lack pi bonds and functional groups. Their chemistry is limited to radical reactions (combustion, halogenation).
\text{IHD} = \frac{2C + 2 + N - H - X}{2}Where C = number of carbons, H = number of hydrogens, N = number of nitrogens, X = number of halogens. Oxygen and sulfur are ignored in the formula.
\text{Acyclic alkane: } C_nH_{2n+2}\text{Monocyclic alkane: } C_nH_{2n}IUPAC nomenclature is how pharmaceutical companies name drug molecules, how chemical suppliers label reagents, and how patent filings identify novel compounds. If two labs on different continents need to work with the same molecule, the IUPAC name is the one thing that removes ambiguity.
Degree of unsaturation is a practical first step when you receive a molecular formula from mass spectrometry data. Before you even look at an NMR spectrum, the IHD tells you whether to expect rings, double bonds, or aromatic systems.
Students often think "longest chain" means the chain drawn horizontally across the page. It does not. The longest chain can zig-zag through bends in the drawing. Always trace every possible path.
Multiplying prefixes (di-, tri-, tetra-) are ignored for alphabetical ordering, but "iso-", "neo-", and "cyclo-" are not. "sec-" and "tert-" are also ignored when alphabetising. This catches people on exams regularly.
Students frequently forget that a cycloalkane ring with more carbons than any attached chain becomes the parent, not the substituent. A cyclohexane with a methyl group is methylcyclohexane, not cyclohexylmethane.
When computing IHD, students sometimes include oxygen in the formula. Oxygen has no effect on the hydrogen deficiency index.
⚠️ Naming cycloalkanes with multiple complex substituents (sec-butyl, isopropyl, tert-butyl) is a high-frequency exam question. The final review answer key includes this exact pattern.
⚠️ Alphabetical ordering of substituents, especially those with prefixes (iso-, sec-, tert-, di-, tri-), is tested almost every term.
⚠️ Degree of unsaturation calculations appear on nearly every Organic Chemistry I final. Practise with molecules containing nitrogen and halogens, not just C and H.
⚠️ Being asked to draw all constitutional isomers for a given molecular formula is a staple. Work systematically: start with the longest chain, then shorten the chain by one carbon and place that carbon as a branch, and so on.
True or False: The prefix "di-" is considered when alphabetising substituents in an IUPAC name.
False. Multiplying prefixes are ignored for alphabetical ordering.
Fill in the blank: The formula for degree of unsaturation is IHD = (2C + 2 + N - ___ - X) / 2.
H (number of hydrogens)
True or False: A cyclohexane ring with a propyl substituent is named "propylcyclohexane" because cyclohexane has more carbons.
True. The ring (6 carbons) has more carbons than the chain (3 carbons), so the ring is the parent.
Fill in the blank: Alkanes have the general formula CnH___ for acyclic (non-ring) structures.
2n+2
True or False: Oxygen atoms affect the degree of unsaturation calculation.
False. Oxygen is ignored in the IHD formula.
Q: Give the IUPAC name for a cyclohexane ring bearing a sec-butyl group at C1, an isopropyl group at C4, and a methyl group at C2.
A: 1-sec-Butyl-4-isopropyl-2-methylcyclohexane. Substituents are listed alphabetically by their base name: butyl, then propyl (as isopropyl), then methyl.
Q: What is the IUPAC name for the simplest alkane isomer with the molecular formula C7H16 that has a quaternary carbon?
A: 2,2-Dimethylpentane. The quaternary carbon is C2, which bears two methyl groups, one hydrogen-less bond to C1, and one to C3.
Q: Calculate the degree of unsaturation for C6H5Cl.
A: IHD = (2(6) + 2 - 5 - 1) / 2 = (12 + 2 - 5 - 1) / 2 = 8/2 = 4. This is consistent with a benzene ring (three double bonds + one ring = four degrees).
Q: A molecule has the formula C4H8. How many degrees of unsaturation does it have, and what structural features could account for this?
A: IHD = (2(4) + 2 - 8) / 2 = 1. This molecule contains either one double bond (e.g., butene) or one ring (e.g., cyclobutane).
Q: When naming a molecule, how do you determine whether a ring or a chain is the parent structure?
A: The parent is whichever has more carbon atoms. If they have the same number of carbons, the ring is the parent. Complex substituent rules may apply, but the carbon-count rule handles most exam questions.
Nomenclature is the foundation for everything else in the course. Every reaction chapter assumes you can name reactants and products. Stereochemistry (R/S, E/Z) builds directly on the IUPAC numbering system and Cahn-Ingold-Prelog priority rules.
Degree of unsaturation connects forward to spectroscopy, where it is the first calculation you perform when given a molecular formula from mass spectrometry data.
IUPAC naming, organic nomenclature, naming alkanes, naming cycloalkanes, constitutional isomers, structural isomers, degree of unsaturation, index of hydrogen deficiency, IHD, parent chain, longest chain rule, substituent alphabetical order, methyl ethyl propyl butyl, sec-butyl, tert-butyl, isopropyl, neopentyl, complex substituents, naming organic compounds, alkane general formula, saturated hydrocarbons, Organic Chemistry I final review