Reaction	Typical Reagents/Conditions	Mechanism	Product / Regiochemistry	Common Pitfall
Hydrogenation	H₂, Pd or Pt catalyst	Syn addition across a metal surface	Alkane; both H atoms add to the same face	Forgetting that a catalyst is required — H₂ does not add to alkenes uncatalyzed
Hydrohalogenation	HX (HCl, HBr, HI)	Carbocation intermediate	Markovnikov addition — H adds to the less substituted carbon, X to the more substituted (more stable carbocation) carbon	Placing the halogen on the wrong carbon by not identifying the more stable carbocation first
Acid-catalyzed hydration	H₃O⁺ / H₂O	Carbocation intermediate	Markovnikov addition — OH ends up on the more substituted carbon; rearrangements possible	Forgetting that this pathway can rearrange, unlike the anti-Markovnikov alternative below
Hydroboration–oxidation	BH₃ (or BH₃·THF), then H₂O₂/NaOH	Concerted, single-step addition (no carbocation)	Anti-Markovnikov addition — OH ends up on the less substituted carbon; syn addition; no rearrangement	Confusing this with acid-catalyzed hydration — same overall transformation (alkene → alcohol), opposite regiochemistry
Halogenation	X₂ (Cl₂, Br₂)	Bridged halonium ion intermediate	Vicinal dihalide; anti addition	Predicting syn addition — the bridged intermediate forces the second halogen to attack from the opposite face

Regiochemistry note: the Markovnikov vs. anti-Markovnikov contrast above is the classic example of the regiochemistry reasoning introduced in Chapter 9 — identifying which pathway proceeds through the more stable intermediate (a carbocation, for Markovnikov addition) versus which pathway avoids a carbocation altogether (hydroboration).

Aromatic rings do not undergo these reactions — see Electrophilic Aromatic Substitution, below, and the Chapter 9 Common Mistake on this point.

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