Difficulty: Introductory to Intermediate | Prerequisites: Lewis structures, IUPAC nomenclature, molecular geometry.
This set of topics covers the classification skills that tie the rest of organic chemistry together. Knowing whether two structures are the same compound, constitutional isomers, stereoisomers, or unrelated is a skill you will use in every chapter. Redox classification in organic chemistry uses a different framework from general chemistry (counting bonds to electronegative atoms rather than assigning oxidation states). And understanding boiling points and bond lengths rounds out your ability to predict physical properties from structure. These are the "connecting tissue" topics that show up as quick multiple-choice questions on exams.
To classify two structures, check molecular formula first (different formula = not isomers), then connectivity (same connectivity = same compound or stereoisomers; different connectivity = constitutional isomers). Organic redox is tracked by counting C-H bonds gained (reduction) or C-O/C-X bonds gained (oxidation). Among alkanes, more carbons and less branching means a higher boiling point. Bond length decreases with smaller atoms and higher bond order.
Constitutional isomers (structural isomers)
Molecules with the same molecular formula but different connectivity (atoms bonded in a different order). Think of them as having the same Lego pieces assembled differently.
Stereoisomers
Molecules with the same molecular formula and the same connectivity, but different spatial arrangement of atoms. Cis/trans isomers are one type.
Same compound
Two drawings that look different but represent the identical molecule (same formula, same connectivity, same spatial arrangement). Often just drawn from a different angle or flipped.
Not isomers
Two molecules with different molecular formulas. They cannot be isomers of any kind.
Oxidation (organic)
A reaction that increases the number of bonds from carbon to electronegative atoms (O, N, halogen) or decreases the number of C-H bonds.
Reduction (organic)
A reaction that increases the number of C-H bonds or decreases the number of bonds from carbon to electronegative atoms.
Bond length
The distance between two bonded nuclei. Shorter bonds are stronger. Bond length decreases with higher bond order (triple < double < single) and with smaller atoms.
When two structures are placed side by side, work through this decision tree:
Same molecular formula? If no → not isomers. Full stop.
Same connectivity (same bonding order)? If no → constitutional isomers.
Same spatial arrangement? If no → stereoisomers. If yes → same compound.
Examples from the exam:
A cyclopentane (C₅, one ring) vs a pentene (C₅, no ring): different molecular formulas (the cyclopentane is C₅H₁₀, the open-chain pentane is C₅H₁₂). Not isomers.
A C₇ compound vs a C₆ compound: different molecular formulas entirely. Not isomers.
Two cyclohexanes with methyl groups drawn in different spatial orientations but identical connectivity: stereoisomers (they differ only in the 3D arrangement of the substituents).
Organic chemistry classifies redox differently from general chemistry. Rather than tracking formal oxidation states, you count the types of bonds gained or lost at the reacting carbon:
Oxidation: the carbon gains bonds to electronegative atoms (O, N, halogen) or loses C-H bonds.
Reduction: the carbon gains C-H bonds or loses bonds to electronegative atoms.
Not a redox reaction: when the changes balance out, or the number of C-H and C-O/C-X bonds stays the same overall.
Examples from the exam:
A reaction that adds both a C-H bond and a C-O bond simultaneously: not a redox reaction (the gain of a bond to H is offset by the gain of a bond to O).
A reaction where the product has gained C-H bonds but also gained two C-O bonds and a C-Cl bond: oxidation (net gain of bonds to electronegative atoms outweighs the C-H gain).
Two factors determine the boiling point of an alkane:
More carbon atoms = higher boiling point. Larger molecules have stronger London dispersion forces because they have more surface area for temporary dipole interactions.
Less branching = higher boiling point. A straight-chain alkane has more surface area contact between molecules than a highly branched isomer of the same formula. More contact means stronger London dispersion forces.
Example from the exam: among five alkane structures, the one with the most carbons and no branching has the highest boiling point.
Bond length depends on two factors:
Atom size: smaller atoms form shorter bonds. Hydrogen is the smallest atom, so H-H is the shortest bond among common single bonds.
Bond order: triple bonds are shorter than double bonds, which are shorter than single bonds (more electron density pulls the nuclei closer).
Example from the exam: among C-H, C-O, C-C, and H-H, the H-H bond is the shortest because hydrogen atoms are the smallest.
When drawing curved arrows for resonance interconversion:
Arrows always show electron movement, not atom movement.
A π bond breaking moves electrons to an adjacent atom as a lone pair.
A lone pair forming a new π bond moves electrons from one atom into the space between two atoms.
The arrow must start from a bond or lone pair (a source of electrons) and point toward where those electrons are going.
Example from the exam: for a protonated enol system, the correct electron flow shows the π bond electrons moving to form a lone pair on the adjacent atom, while a lone pair on the other side forms a new π bond. The arrow from the π bond points toward the atom that will gain the lone pair.
Students often confuse constitutional isomers with stereoisomers. The test is connectivity: if the atoms are bonded in the same sequence, the difference must be spatial (stereoisomers). If the bonding order itself differs, they are constitutional isomers.
A common error is calling two compounds with different molecular formulas "constitutional isomers." They are not isomers at all. Always check the formula first.
In organic redox, students sometimes try to assign formal oxidation numbers as they did in general chemistry. The bond-counting method (C-H gained vs C-O/C-X gained) is simpler and is what this course expects.
Students assume that all reactions that involve oxygen are oxidations. A reaction can add an oxygen without being an oxidation if a C-H bond is also added in the same step.
⚠️ The isomer classification question (same, constitutional, stereo, or not isomers) appears on every exam. Practise the decision tree until it is automatic.
⚠️ Redox classification is a quick-points question. Count bonds to electronegative atoms before and after, and the answer follows directly.
⚠️ Boiling point ranking of alkanes tests your understanding of London dispersion forces. More carbons and less branching = higher boiling point.
⚠️ Bond length questions test atom size and bond order. H-H is the shortest common bond (smallest atoms); triple bonds are shorter than double bonds.
⚠️ Electron flow for resonance is tested as a multiple-choice question. Be sure the arrow starts from electrons (a bond or lone pair), not from an atom.
True or false: two molecules with different molecular formulas can be constitutional isomers. (False. Different formulas = not isomers.)
Fill in the blank: a reaction that increases the number of C-H bonds on a carbon is a ____. (reduction)
True or false: a more branched alkane has a higher boiling point than its straight-chain isomer. (False. Less branching = higher boiling point.)
Fill in the blank: the H-H bond is shorter than the C-H bond because hydrogen atoms are ____. (smaller)
True or false: resonance arrows can show atoms moving to new positions. (False. Only electrons move in resonance.)
Q: Compound A is a cyclopentane (C₅H₁₀). Compound B is an open-chain pentane (C₅H₁₂). How do they relate?
A: Not isomers. They have different molecular formulas (different numbers of hydrogen atoms).
Q: Two cyclohexane structures have the same connectivity and the same substituents, but the methyl groups point in different spatial directions. What is their relationship?
A: Stereoisomers. Same formula, same connectivity, different spatial arrangement.
Q: A reaction converts an aldehyde (R-CHO) to an alcohol (R-CH₂OH). Is this an oxidation, reduction, or neither?
A: Reduction. The carbon has gained a C-H bond (going from CHO to CH₂OH) while the number of C-O bonds stayed the same.
Q: Rank the following bonds from shortest to longest: C-C, C-H, H-H, C-O.
A: H-H < C-H < C-O < C-C (approximately). H-H is shortest because both atoms are the smallest. C-C is longest because carbon atoms are larger.
Q: Among five alkane isomers, which has the highest boiling point?
A: The one with the most carbon atoms and the least branching. Larger surface area increases London dispersion forces.
Isomer classification becomes more detailed when you study stereochemistry (enantiomers, diastereomers, meso compounds). The redox framework introduced here is used throughout organic reactions: every time you learn a new reaction, you should be able to classify it as oxidation, reduction, or neither. Boiling point trends connect to intermolecular forces from the electronic structure notes and come back when discussing solvent choice in reactions.
Isomers, constitutional isomers, structural isomers, stereoisomers, cis-trans isomers, same compound, not isomers, molecular formula, connectivity, oxidation, reduction, redox, organic redox, C-H bonds, C-O bonds, bond counting, boiling point, London dispersion forces, surface area, branching, alkane boiling point, bond length, bond order, H-H bond, atom size, electron flow, resonance arrows, curved arrows