Source: Exam 4 Key, University of Minnesota Twin Cities
Difficulty: Intermediate | Prerequisites: Functional group basics, Lewis structures, bond energies Tags: IUPAC naming, nomenclature, alkene naming, alkyne naming, carbocations, radicals, reaction rates, intermediates, organic chemistry I
IUPAC nomenclature asks you to identify the longest carbon chain, number it to give the lowest set of locants to substituents and functional groups, and assemble the name with correct prefixes, suffixes, and stereochemical labels (E/Z). Reactive intermediates, particularly carbocations and radicals, determine which reaction pathway is faster or more favourable. The stability of the intermediate controls the rate: a more stable carbocation forms faster, and the difference in starting-material stability controls how exothermic the overall reaction is.
IUPAC nomenclature
The systematic method for naming organic compounds established by the International Union of Pure and Applied Chemistry. It uses a parent chain, locants, prefixes, and suffixes so every structure maps to exactly one unambiguous name. In simple terms, it is the "official" naming system that tells you the structure from the name alone.
Locant
A number assigned to each carbon in the parent chain to specify where substituents and functional groups are attached. Think of it as the address of each carbon on the chain.
E/Z stereodescriptor
A label that specifies the geometry of a double bond. Z (zusammen) means the higher-priority groups are on the same side; E (entgegen) means they are on opposite sides. Assigned using Cahn-Ingold-Prelog priority rules. Think of it as "Z = same side, E = opposite side" when you look at the two highest-priority substituents across the double bond.
Carbocation
A reactive intermediate bearing a formal positive charge on a trivalent carbon. Stability order: 3° > 2° > 1° > methyl, because alkyl groups donate electron density through hyperconjugation and induction. In simple terms, a carbon that has lost a bond and carries a positive charge. The more alkyl groups around it, the happier (more stable) it is.
Radical (free radical)
A reactive intermediate with an unpaired electron on carbon. Stability follows the same order as carbocations (3° > 2° > 1° > methyl). Think of it as a carbon with a lonely, unpaired electron instead of a charge.
Exothermic reaction
A reaction that releases energy, meaning the products are lower in energy than the reactants. Delta H is negative. In simple terms, the reaction gives off heat because the products sit in a deeper energy well than the starting materials.
Hammond's postulate
States that the transition state of a reaction step resembles the species (reactant or product) it is closest to in energy. For an endothermic step, the transition state resembles the product; for an exothermic step, it resembles the reactant. This is why a more stable carbocation intermediate forms faster: its transition state is also lower in energy.
When a molecule contains more than one functional group (e.g. an alkene, an alkyne, and an alcohol), the suffix priority determines the parent chain's ending:
Alcohols (-ol) outrank alkenes (-en) and alkynes (-yn) as the principal characteristic group.
Number the chain to give the lowest locant to the highest-priority suffix group first.
Multiple unsaturations are listed together in the name: e.g. "pent-4-en-2-yn-1-ol" names a five-carbon chain with a double bond at C4, a triple bond at C2, and a hydroxyl at C1.
Parent chain: pentane (5 carbons).
Functional groups: -OH at C1 (suffix -ol), triple bond at C2 (-yn), double bond at C4 (-en).
The alcohol gets C1 to keep the lowest locant on the highest-priority group.
Full name: pent-4-en-2-yn-1-ol.
Parent chain: pentane (5 carbons).
Substituent: chloro at C3.
Unsaturations: double bond at C2 (-en), triple bond at C4 (-yne).
The Z descriptor indicates the two higher-priority groups on C2 and C3 are on the same side of the double bond.
Identify the longest chain containing the principal characteristic group.
Number from the end that gives the lowest locant to that group.
List substituents alphabetically as prefixes.
Assign E/Z for each double bond with different groups on each carbon.
Use commas between numbers, hyphens between numbers and letters.
When a halogen radical abstracts a hydrogen from an alkane, the identity of the halogen controls the rate.
Cl· + CH₃CH₃ is faster than Br· + CH₃CH₃.
Reason: the Cl-H bond that forms (431 kJ/mol) is stronger than the Br-H bond (366 kJ/mol), so the chlorine abstraction step is more exothermic and, by Hammond's postulate, has a lower activation energy.
Both reactions produce an ethyl radical (·CH₂CH₃), but the transition state leading there is lower in energy for the chlorine pathway.
When an alkene reacts with HCl, a carbocation intermediate forms. The more substituted (more stable) the carbocation, the faster it forms.
A more substituted alkene that generates a 2° or 3° carbocation reacts faster with HCl than a less substituted alkene that generates a 1° carbocation.
This follows directly from Hammond's postulate: a more stable intermediate means a lower-energy transition state leading to it.
If two reactions start from the same alkene + HCl, the pathway that forms the more stable carbocation is faster.
Example from the exam: both pathways start with the same alkene + HCl, but pathway B forms a 2° carbocation while pathway A forms a 1° carbocation. B is faster.
If two reactions form the same product but start from different alkenes, the one starting from the less stable (less substituted) alkene releases more energy.
A monosubstituted alkene is higher in energy than a disubstituted alkene. Converting the higher-energy starting material to the same product means a larger energy drop, so the reaction is more exothermic.
Example from the exam: both reactions form the same chloroalkane product. Reaction A starts from a monosubstituted alkene (less stable), so A is more exothermic than B, which starts from a disubstituted alkene.
Students often number the parent chain from the wrong end. Always start numbering from the end that gives the lowest locant to the highest-priority suffix group (e.g. -ol beats -en and -yn).
Students confuse E/Z with cis/trans. E/Z uses Cahn-Ingold-Prelog priority rules and works for all substitution patterns, while cis/trans only works cleanly when identical groups sit on each carbon of the double bond.
Students assume that a more substituted alkene always reacts faster with HCl. The rate depends on the stability of the carbocation intermediate that forms, not the stability of the starting alkene directly. They usually correlate, but the reasoning must go through the intermediate.
Students mix up "faster" and "more exothermic." A reaction can be fast but not very exothermic, or slow but highly exothermic. Rate depends on the activation energy (transition state); exothermicity depends on the energy difference between reactants and products.
⚠️ Naming compounds with multiple functional groups (alkene + alkyne + alcohol) is a classic exam question. Know the suffix priority order.
⚠️ You will be asked to compare reaction rates by evaluating intermediate stability. Always state which intermediate forms and why it is more or less stable.
⚠️ "Faster" vs "more exothermic" questions test whether you understand the difference between kinetics (activation energy, transition state) and thermodynamics (energy of products vs reactants).
⚠️ When comparing exothermicity for reactions that give the same product, focus on which starting material is higher in energy. The higher-energy starting material leads to the more exothermic reaction.
True or False: In IUPAC naming, an -ol suffix takes priority over -en and -yn suffixes when assigning the lowest locant. True.
Fill in the blank: A 3° carbocation is ______ stable than a 1° carbocation and therefore forms ______. More; faster.
True or False: If two reactions produce the same product, the one starting from the more stable reactant is more exothermic. False. The less stable reactant releases more energy reaching the same product.
Fill in the blank: Cl· abstracts hydrogen from ethane faster than Br· because the ______ bond is stronger, making the step more exothermic. H-Cl.
True or False: E/Z descriptors are assigned using the same priority rules as R/S (Cahn-Ingold-Prelog). True.
Q: Draw the structure of pent-4-en-2-yn-1-ol and explain how you determined the numbering.
A: Five-carbon chain. C1 bears the -OH (alcohol takes priority for lowest locant). C2-C3 is a triple bond. C4-C5 is a double bond. The structure has a terminal alkene and an internal alkyne with a primary alcohol.
Q: An alkene reacts with HCl by two different pathways. Pathway A produces a 1° carbocation; pathway B produces a 2° carbocation. Which pathway is faster, and why?
A: Pathway B is faster. The 2° carbocation is more stable than the 1° carbocation, so by Hammond's postulate the transition state leading to it is lower in energy, giving a smaller activation barrier.
Q: Two different alkenes both react with HCl to give the same chloroalkane product. Alkene X is monosubstituted; alkene Y is disubstituted. Which reaction is more exothermic?
A: The reaction starting from alkene X (monosubstituted) is more exothermic. A monosubstituted alkene is less stable (higher energy) than a disubstituted alkene. Since both reactions end at the same product energy, the larger energy drop from the less stable starting material means greater exothermicity.
Q: Why does Cl· abstract hydrogen from ethane faster than Br· does?
A: The H-Cl bond formed in the products is stronger than the H-Br bond. This makes the chlorine abstraction step more exothermic, which by Hammond's postulate lowers the activation energy, speeding up the reaction.
Nomenclature underpins every reaction problem: if you cannot name the starting material or product, you cannot communicate the chemistry. E/Z descriptors reappear when you study elimination reactions (E2 gives specific alkene geometry) and addition stereochemistry.
Carbocation stability connects directly to Markovnikov's rule in electrophilic addition, to rearrangements (1,2-hydride and methyl shifts), and to SN1 reaction rates in the substitution chapter.
The "faster vs more exothermic" distinction recurs in every kinetics-vs-thermodynamics comparison, including regioselectivity of radical halogenation and product distributions under kinetic vs thermodynamic control.
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