MCAT topics → Biologically Relevant Molecules and Organic Reactivity → Alkanes, Alkenes, Alkynes — Reactivity Overview

Alkanes, Alkenes, Alkynes — Reactivity Overview

MCAT trap: Applies Markovnikov's rule as a memorized pattern without understanding the carbocation stability basis. Hydrogen adds to the less-substituted carbon because this generates the more stable (more substituted) carbocation intermediate.

Alkanes, alkenes, and alkynes are the foundational hydrocarbon classes, and on the MCAT their reactivity differences come down to one thing: π bonds. The most common reasoning error: students memorize Markovnikov's rule as 'H goes to the carbon with more H's' without understanding the mechanism — that shortcut breaks down the moment a question asks you to justify the product or apply it to a novel alkene. The rule is a consequence of carbocation stability, not a pattern to match. Alkanes are fully saturated — no π bonds, low reactivity, essentially inert except under combustion or radical conditions. Alkenes and alkynes have π bonds that are electron-rich and weak relative to σ bonds, making them prime targets for electrophilic addition.

The exam tests this topic from several angles. Mechanism questions push you to explain WHY a product forms — specifically, why Markovnikov or anti-Markovnikov regiochemistry results, and whether addition is syn or anti. Passage-based questions may give you a novel reaction or biological context (e.g., fatty acid unsaturation, isoprenoid biosynthesis) and ask you to apply these principles without having seen that exact system before.

Students also confuse syn and anti addition because they assume both atoms in a diatomic reagent add simultaneously from the same face. The bromonium ion intermediate is the key concept that gets overlooked. Get the mechanisms solid and the rules follow automatically.

Common misconceptions

Common mistake
Wrong: Markovnikov's rule means the hydrogen adds to the carbon with more hydrogens as a memorized rule, not because of carbocation stability.
Right: Hydrogen adds to the less-substituted carbon because this generates the more stable (more substituted) carbocation intermediate.
Markovnikov's rule isn't just a memory trick — it's a consequence of carbocation stability. When HX adds to an asymmetric alkene, the proton adds first, generating a carbocation. The carbocation forms preferentially at the more substituted carbon because more alkyl groups stabilize positive charge through hyperconjugation and induction. So 'H goes to the less substituted carbon' is a description of the outcome, not the reason — and if a question asks you to justify the product or apply this to a novel alkene, you need the mechanistic logic, not the shortcut.
Common mistake
Wrong: Halogenation of alkenes (Br2) proceeds with syn addition because both bromines add simultaneously.
Right: Halogenation proceeds via a bromonium ion intermediate, giving anti addition (the nucleophile attacks from the opposite face).
Br2 halogenation does NOT proceed with both bromines attacking simultaneously from the same face. Instead, one bromine acts as an electrophile and forms a three-membered bromonium ion with the alkene π electrons — this intermediate locks the top face. The second bromide ion then attacks from the opposite (bottom) face in an SN2-like step, giving anti addition. This means that for a cis alkene, you'll get a different stereochemical outcome than for a trans alkene — something that only makes sense if you understand the bromonium ion mechanism.
Common mistake
Wrong: Radical halogenation preferentially occurs at primary carbons because they are more accessible.
Right: Radical halogenation preferentially occurs at tertiary carbons because the tertiary radical intermediate is most stable.
Steric accessibility is an ionic chemistry concept — it doesn't drive radical halogenation selectivity. Radical halogenation goes through a carbon radical intermediate, and radical stability follows the same trend as carbocation stability: tertiary > secondary > primary. The tertiary radical is more stable because the surrounding alkyl groups stabilize the unpaired electron through hyperconjugation. Primary carbons may be more exposed, but the radical formed there is less stable and therefore less likely to form. On the MCAT, always ask which intermediate is more stable — that drives selectivity.
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What the exam tests

  1. Predict and explain the difference in reactivity between alkanes (no π bonds, low reactivity) and alkenes/alkynes (π bonds, undergo addition reactions).
  2. Apply Markovnikov's rule by reasoning from carbocation stability — not just pattern-matching — and identify when anti-Markovnikov conditions (peroxides or BH3) reverse the regiochemistry.
  3. Interpret combustion energetics and predict selectivity in radical halogenation based on radical intermediate stability (tertiary > secondary > primary).
  4. Determine whether an addition reaction proceeds with syn or anti stereochemistry — for example, anti addition via a bromonium ion in Br2 halogenation versus syn addition in catalytic hydrogenation.

Can you avoid these mistakes?

HBr adds to 2-methylpropene (isobutylene). Which carbon does the Br end up on, and why? Now explain your answer using carbocation stability — not the memorized rule.
Br2 adds to cis-2-butene in CCl4. Draw the expected product(s) and specify their stereochemistry. Would the products differ if you started with trans-2-butene? Why or why not?
A radical halogenation reaction is run on 2-methylbutane with Cl2 and UV light. Rank the four types of hydrogens in the molecule by likelihood of substitution and explain the basis for that ranking.
BH3/THF followed by H2O2/NaOH is added to propene. Compare the regiochemistry and stereochemistry of this product to what you'd get from HBr with a peroxide initiator. What controls each outcome?

Related topics

Functional Groups and IUPAC Nomenclature Stereochemistry (Chirality, R/S, Enantiomers, Diastereomers) Alcohols — Oxidation, Substitution, Synthesis Aldehydes and Ketones (Nucleophilic Addition, Enolates, Aldol)

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