Addition reactions (alkenes & alkynes)
The idea
The π bond of an alkene or alkyne is electron-rich and exposed, so it behaves as a nucleophile toward electrophiles — the opposite stance from the saturated halides of substitution chemistry. Addition reactions break that π bond and attach two new groups across the former double or triple bond: hydrogen halides, water (acid-catalyzed hydration), halogens, and hydrogen (catalytic hydrogenation) all add this way. You already know functional groups and reaction types; here the focus is regiochemistry — which carbon gets which group — and the carbocation intermediates that govern it.
For additions that begin by protonating the alkene, Markovnikov's rule is the organizing principle, and its real basis is carbocation stability. The proton adds so as to generate the more stable (more substituted) carbocation — tertiary beats secondary beats primary — because alkyl groups stabilize positive charge through hyperconjugation and induction. The nucleophile then attaches to that carbon. Reverse conditions exist: radical addition of HBr with peroxides gives the anti-Markovnikov product, a reminder that the rule describes the ionic mechanism, not a law of nature.
The misconception to discard is that 'Markovnikov' is a memorized hydrogen-counting slogan ('the rich get richer'). Treat it instead as a consequence: identify the two possible carbocations, pick the more stable one, and the regiochemistry follows automatically — which also tells you immediately how rearrangements or radical conditions will change the answer.
Worked example
2-methylpropene (CH₂=C(CH₃)₂) reacts with HBr under ordinary ionic conditions (no peroxides). Predict the major product and justify the regiochemistry through the carbocation intermediate.
- Identify the two carbons of the double bond: the CH₂ end (carbon bearing two H) and the C(CH₃)₂ end (carbon bearing two methyl groups). The proton from HBr can add to either.
- Compare the two possible carbocations: adding H⁺ to the CH₂ carbon places the positive charge on the central carbon, which is bonded to three other carbons — a tertiary cation. Adding H⁺ to the central carbon instead would leave a primary cation on the CH₂.
- Apply the stability ordering: the tertiary carbocation is far more stable than the primary one, so the proton adds to the terminal CH₂ carbon, in line with Markovnikov's rule.
- Attach the nucleophile: bromide then bonds to the tertiary cationic carbon, giving (CH₃)₃CBr, that is 2-bromo-2-methylpropane (tert-butyl bromide).
- Sanity-check the logic: the bromine ends up on the more substituted carbon precisely because that is where the stable intermediate carried the charge — confirming this is the Markovnikov product, and predicting that peroxide radical conditions would instead place Br on the CH₂ end.
Answer. The major product is 2-bromo-2-methylpropane, formed via the more stable tertiary carbocation (Markovnikov addition).
Check your understanding
- Why does carbocation stability, rather than a hydrogen-counting rule, ultimately decide the regiochemistry of HBr addition?
- How would adding peroxides change the product, and what kind of intermediate replaces the carbocation in that case?
- What would you expect if the carbocation could rearrange to an even more stable one, and how would that show up in the product mixture?
- How does acid-catalyzed hydration of the same alkene parallel this mechanism, and what product would it give?
Build the foundations first
Addition reactions (alkenes & alkynes) builds on these concepts. If any feel shaky, start there.