Difficulty: Intermediate | Prerequisites: Lewis structures, functional group identification, basic hybridisation (sp, sp², sp³).
IUPAC nomenclature is the universal language for naming organic molecules, and this section of the course tests whether you can both read and write it fluently for compounds with multiple functional groups (alcohols, alkenes, alkynes). Alongside naming, you need to rank the stability of reactive intermediates, specifically carbocations and radicals, because that ranking determines which products actually form in nearly every reaction from here on. If you missed the earlier material on hybridisation and functional groups, go back to that first: everything here builds on knowing what sp, sp², and sp³ carbons look like and how they behave.
Name organic compounds by finding the longest chain that includes the highest-priority functional group, number from the end that gives the principal group the lowest locant, and list substituents alphabetically. Carbocation stability goes: vinylic (least) < ordinary secondary < allylic secondary (most), because delocalisation through an adjacent pi system stabilises the positive charge. C-H bond strength is the inverse of radical stability: a bond that breaks to give a more stable radical (allylic, more substituted) is weaker.
IUPAC nomenclature
The systematic naming convention maintained by the International Union of Pure and Applied Chemistry. It assigns every organic compound an unambiguous name based on its longest carbon chain, functional groups, and substituent positions. In simple terms, it is the official set of rules so that "2-methylpent-3-yn-2-ol" means exactly one structure to every chemist on the planet.
Carbocation
A reactive intermediate bearing a formal positive charge on carbon, typically sp² hybridised with an empty p orbital. Think of it as a carbon that desperately wants electrons, and everything about its stability (substitution, resonance, hybridisation) determines how long it lasts and which products form.
Vinylic carbocation
A carbocation where the positive charge sits on a carbon that is part of a C=C double bond (sp hybridised in context of the cation). These are notably unstable. The sp-hybridised carbon holds its electrons more tightly, making it a poor host for a positive charge.
Allylic carbocation
A carbocation located on a carbon adjacent to a C=C double bond, allowing the positive charge to be delocalised across the pi system via resonance. In simple terms, the empty p orbital overlaps with the neighbouring pi bond, spreading the charge over two carbons instead of one, which is a significant stabilisation.
Bond dissociation energy (BDE)
The enthalpy required to homolytically break a specific bond in the gas phase, producing two radicals. A weaker bond (lower BDE) breaks more easily and produces a more stable radical. This is the direct link between radical stability and bond strength.
Radical
A species with an unpaired electron on carbon. Radical stability follows the same general trend as carbocation stability: more substituted and allylic radicals are more stable. Think of it as the mirror image of carbocation reasoning, but with an unpaired electron instead of an empty orbital.
Homolytic cleavage
Bond breaking where each atom takes one electron from the shared pair, producing two radicals (shown with fishhook arrows). This is the opposite of heterolytic cleavage, where one atom takes both electrons. Homolysis is the process behind radical reactions and is what BDE measures.
Finding the parent chain
Identify the longest continuous carbon chain that includes the principal characteristic group (the highest-priority functional group). Priority order for groups you will see on this exam: carboxylic acid > aldehyde > ketone > alcohol > amine.
If the molecule has both a double bond and a triple bond, include both in the parent chain even if a longer chain exists that does not.
Numbering the chain
Number from the end that gives the principal characteristic group the lowest locant.
If the principal group is tied, give the lowest locant to the first point of difference among double/triple bonds, then substituents.
Naming unsaturation
Double bonds are indicated by "-en-" and triple bonds by "-yn-" in the name. When both are present, "-en-" comes before "-yn-" alphabetically, and the final "e" of the parent alkane name is dropped before a vowel.
E/Z or cis/trans stereodescriptors go in parentheses at the front of the name: (E)-3-methyl-pent-2-en-4-yn-1-ol.
Naming the alcohol suffix
The "-ol" suffix replaces the terminal "-e" of the parent name. The locant for -ol goes directly before the suffix (pent-3-yn-2-ol) in current IUPAC 2013 recommendations.
Worked example from the exam
The compound drawn as a five-carbon chain with a triple bond at C3 and an -OH at C2, plus a methyl branch at C2, is named 2-methylpent-3-yn-2-ol.
Reading the name (E)-3-methyl-pent-2-en-4-yn-1-ol: parent chain = pentane (5 carbons), double bond at C2 (E configuration), triple bond at C4, hydroxyl at C1, methyl substituent at C3.
The stability of a carbocation depends on how well the positive charge (the empty p orbital) is stabilised. Three main factors apply in this exam's scope:
Degree of substitution
Tertiary > secondary > primary > methyl. More alkyl groups donate electron density through hyperconjugation and induction.
Resonance (allylic stabilisation)
An allylic carbocation sits next to a C=C double bond. The empty p orbital overlaps with the pi system, delocalising the charge over two carbons.
This resonance stabilisation is substantial. A secondary allylic carbocation is more stable than a plain secondary carbocation.
Hybridisation (vinylic destabilisation)
A vinylic carbocation has the positive charge on a carbon that is part of a double bond. The higher s-character of sp² (or effectively sp) carbon makes it hold electrons more tightly and tolerate a positive charge poorly.
A secondary vinylic carbocation is less stable than a plain secondary carbocation.
Exam ranking (from the key)
Given three secondary carbocations: A (vinylic), B (allylic), C (plain secondary).
Order from least to most stable: A < C < B.
All three share the same degree of substitution, so the deciding factor is whether the cation gets resonance stabilisation (B does) or suffers from vinylic destabilisation (A does).
Bond dissociation energy (BDE) and radical stability are inversely related. A C-H bond that breaks to give a more stable radical is a weaker bond (lower BDE), because the products are lower in energy.
The principle
Weaker C-H bond = more stable radical formed upon homolysis.
Stronger C-H bond = less stable radical formed upon homolysis.
Radical stability order
Tertiary > secondary > primary > methyl (same trend as carbocations, driven by hyperconjugation).
Allylic radicals are additionally stabilised by resonance with the adjacent pi bond, just like allylic carbocations.
Combining substitution and allylic effects
A 2° allylic radical is more stable than a 1° allylic radical, which is more stable than a plain 1° radical.
This means: the C-H bond that gives a 2° allylic radical is the weakest, and the C-H bond that gives a plain 1° radical is the strongest.
Exam ranking (from the key)
Bond B: breaks to give a 2° allylic radical (most stable radical, weakest bond).
Bond A: breaks to give a 1° allylic radical (middle).
Bond C: breaks to give a 1° radical, no allylic stabilisation (least stable radical, strongest bond).
Weakest to strongest: B < A < C.
Students often assume all secondary carbocations are equally stable. They are not. A secondary vinylic carbocation is significantly less stable than a plain secondary, and a secondary allylic carbocation is significantly more stable. The label "secondary" tells you substitution only; you still need to check for resonance or hybridisation effects.
Students frequently confuse bond strength with bond order. A C-H bond can be on a carbon next to a double bond without itself being a double bond. The question is what radical forms when that specific C-H bond breaks, not how many bonds the adjacent carbon has.
When naming compounds with multiple unsaturations, students sometimes number from the wrong end. The principal functional group (e.g. -OH) takes priority for the lowest locant, not the first double or triple bond encountered.
Students mix up E/Z assignment with cis/trans. E/Z uses Cahn-Ingold-Prelog priority rules (higher atomic number wins), while cis/trans is based on whether similar groups are on the same or opposite sides. For IUPAC names of alkenes with more than two different substituents on the double bond, E/Z is required.
⚠️ Naming with multiple functional groups (alcohol + alkene + alkyne) is a high-frequency exam question. Expect to both write a name from a structure and draw a structure from a name.
⚠️ Carbocation stability ranking with a twist (vinylic vs allylic vs plain) is a classic exam differentiator. The exam key explicitly tests whether you know that "secondary" alone is insufficient.
⚠️ Bond strength questions require you to link radical stability back to BDE. The exam does not ask you to memorise specific BDE values; it asks you to rank bonds and explain your reasoning using radical stability.
⚠️ Explanation questions ("briefly explain your choice") are worth marks. A correct ranking with no explanation, or a vague explanation, loses points. State: (1) the shared feature (all secondary), (2) the differentiating feature (allylic resonance, vinylic destabilisation), and (3) why that feature matters.
True or false: a secondary allylic carbocation is less stable than a tertiary carbocation with no resonance. ___
Fill in the blank: the C-H bond that produces the most stable radical upon homolysis is the ___ (weakest / strongest) bond.
True or false: when naming a compound with both -OH and a triple bond, the triple bond gets the lowest locant. ___
Fill in the blank: a vinylic carbocation is less stable than a plain secondary carbocation because of the higher ___ character of the carbon bearing the charge.
True or false: E/Z stereodescriptors use the same rules as cis/trans. ___
Answers: 1. Could be either, but generally true (substitution often outweighs resonance). 2. Weakest. 3. False, -OH takes priority. 4. s-character. 5. False, E/Z uses CIP priority rules.
Q: Write the IUPAC name for a five-carbon compound with a hydroxyl group at C2, a methyl branch at C2, and a triple bond between C3 and C4.
A: 2-methylpent-3-yn-2-ol.
Q: Three secondary carbocations are presented: one is vinylic, one is allylic, and one is a plain secondary cation. Rank them from least to most stable and explain.
A: Vinylic < plain secondary < allylic. All share the same degree of substitution. The vinylic cation is destabilised by higher s-character, while the allylic cation is stabilised by resonance delocalisation of the positive charge into the adjacent pi bond.
Q: Given three labelled C-H bonds on a molecule containing a double bond, rank them from weakest to strongest. Bond B gives a 2° allylic radical, Bond A gives a 1° allylic radical, Bond C gives a 1° radical.
A: B (weakest) < A < C (strongest). The more stable the radical produced, the lower the BDE and the weaker the bond.
Q: Draw the structure of (E)-3-methyl-pent-2-en-4-yn-1-ol.
A: A five-carbon chain with -OH at C1, a double bond at C2 in E configuration, a methyl group at C3, and a triple bond at C4. The E descriptor means the two higher-priority groups on each carbon of the double bond are on opposite sides.
Carbocation stability feeds directly into predicting products of electrophilic addition reactions (HBr, HCl, H₂O/H₂SO₄ additions to alkenes), which form the bulk of this exam's remaining problems. Markovnikov's rule is just a restatement of "the more stable carbocation forms preferentially."
Radical stability connects to the radical reactions section: radical halogenation (Br₂/light, NBS/peroxides) and anti-Markovnikov HBr addition (HBr/peroxides) both depend on knowing which radical intermediate is most stable.
IUPAC naming is a skill that carries through every organic chemistry course. Exam 4 adds alkynes and stereodescriptors to the naming toolkit you built in earlier exams with simple alkanes, alkenes, and alcohols.
IUPAC naming, organic nomenclature, alkyne naming, alcohol naming, enyne nomenclature, E/Z configuration, Cahn-Ingold-Prelog, carbocation stability, vinylic cation, allylic cation, secondary carbocation, resonance stabilisation, hyperconjugation, bond dissociation energy, BDE, radical stability, homolytic cleavage, homolysis, allylic radical, C-H bond strength, reactive intermediates, organic chemistry I, CHEM 202, exam 4 review