Difficulty: Intermediate | Prerequisites: Functional groups, bond-line structures, stereochemistry basics (R/S, cis/trans)
Alkenes and alkynes undergo addition reactions where new atoms or groups attach across the pi bond. The product you get depends on the reagent (which controls what adds), the regiochemistry (which carbon gets what), and the stereochemistry (whether addition is syn or anti). Mastering these three variables lets you predict the product of nearly every reaction in this unit.
Stereospecific
A reaction whose mechanism forces a single stereochemical outcome. The geometry of the starting material dictates the geometry of the product, with no room for variation.
In simple terms, this means the 3D arrangement of the product is locked in by how the reaction works, not by chance.
Regiospecific
A reaction that produces only one constitutional isomer from among the possible options. One carbon of the double bond receives one group, the other carbon receives the other, with complete selectivity.
Think of it as: the reagent "knows" exactly which carbon to attach to, every time.
Markovnikov addition
In the addition of HX to an alkene, the hydrogen adds to the less substituted carbon and the X (halide or other group) adds to the more substituted carbon. This follows from formation of the more stable carbocation intermediate.
In simple terms: "the rich get richer" – the carbon with more hydrogens gets the new hydrogen, and the carbon with more R groups gets the X.
Anti-Markovnikov addition
The opposite regiochemistry to Markovnikov: the hydrogen adds to the more substituted carbon and the OH (or other group) adds to the less substituted carbon. This occurs in hydroboration-oxidation.
Think of it as the exception to the Markovnikov rule, driven by a completely different mechanism (no carbocation).
Syn addition
Both new groups add to the same face of the double bond. Hydrogenation (H2/Pd) and hydroboration are classic examples.
In simple terms, both groups come in from the same side of the flat alkene.
Anti addition
The two new groups add to opposite faces of the double bond. Halogenation (X2) and halohydrin formation work this way, because a cyclic intermediate (bromonium or chloronium ion) blocks one face.
Think of it as one group landing on top and the other on the bottom.
Lindlar catalyst
A "poisoned" palladium catalyst (Pd on CaCO3, treated with lead acetate and quinoline) that reduces alkynes to cis-alkenes and stops there, rather than going all the way to an alkane.
In simple terms, it is a deliberately weakened catalyst that does half the job: triple bond to double bond, cis geometry.
Dissolving metal reduction
Reduction of an alkyne to a trans-alkene using an alkali metal (Li or Na) in liquid ammonia at -33 °C. The mechanism involves radical anion intermediates rather than surface catalysis.
Think of it as the complementary method to Lindlar: where Lindlar gives you cis, dissolving metal gives you trans.
Alkene to alkane: H2, Pd/C (or PtO2, THF, or alcohol as solvent)
Stereospecific: both hydrogens add syn (same face)
Product is an alkane with cis addition of the two H atoms
Alkyne to alkane: H2, Pd/C (excess H2)
Same syn addition, both H atoms cis to each other
Requires 2 equivalents of H2 to go all the way through
Alkyne to cis-alkene: H2, Lindlar catalyst
Stereoselectivity: H atoms add cis to each other
The poisoned catalyst stops reduction at the alkene stage
Alkyne to trans-alkene: Li or Na, liquid NH3, -33 °C
Stereoselectivity: H atoms add trans to each other
Dissolving metal reduction, radical anion mechanism
Alkene to vicinal dihalide: X2 (Cl2 or Br2) in CH2Cl2 or CCl4
Regio- and stereospecific: the two X atoms add anti (trans to each other)
Proceeds through a cyclic halonium ion intermediate
With cis alkenes, products may be racemic
With trans alkenes where both carbons bear the same substituents, the product may be meso
Alkene to haloalkane: HX (HBr, HCl, HI), no solvent or ether
Regiospecific: X adds to the more substituted carbon (Markovnikov)
Product may be racemic if a new chiral centre is generated
Alkyne to vinylic halide (1 eq HX):
Regiospecific: X adds to the more substituted carbon
Stereospecific: H and X add anti to each other
Alkyne to geminal dihalide (excess HX):
Regiospecific: both X atoms end up on the more substituted carbon
Each addition is anti, following Markovnikov each time
Alkene to halohydrin: X2 in H2O, or NBS in aqueous DMSO (for Br)
Stereospecific: X and OH add anti (trans to each other)
Regiospecific: OH goes on the more substituted carbon, X on the less substituted carbon
Products may be racemic
Acid-catalysed hydration: H3O+ (dilute acid)
Markovnikov: OH adds to the more substituted carbon
Proceeds through a carbocation, so rearrangements are possible
Oxymercuration-demercuration: 1. Hg(OAc)2, H2O, THF; 2. NaBH4
Regiospecific: OH on the more substituted carbon (Markovnikov)
No carbocation rearrangement
Hydroboration-oxidation: 1. BH3, THF; 2. H2O2, NaOH, H2O
Regiospecific: OH on the less substituted carbon (anti-Markovnikov)
Stereospecific: syn addition of OH and H
This is the key way to get the anti-Markovnikov alcohol
Terminal alkyne to methyl ketone: H2O, H2SO4, HgSO4
Markovnikov: OH adds to the more substituted carbon
The enol intermediate tautomerises to the keto form
Terminal alkyne to aldehyde: 1. BH3, THF; 2. H2O2, H2O, pH 8
Anti-Markovnikov: OH adds to the less substituted carbon
Enol tautomerises to an aldehyde
Symmetric internal alkyne to ketone: either set of reagents works, because both carbons are equivalent
NBS, CCl4, hv
Regioselective: Br adds to the allylic position (the carbon next to the double bond)
Radical mechanism (note the light/heat initiation)
Hydrogenation is used industrially to convert unsaturated vegetable oils into saturated fats (margarine production). Hydroboration-oxidation is a workhorse in pharmaceutical synthesis for installing OH groups with precise regiochemistry and stereochemistry.
Students often think Markovnikov addition means "the halide goes to the carbon with more hydrogens." It does not. Markovnikov says the hydrogen adds to the carbon with more H atoms, and the halide (or other electrophilic group) goes to the more substituted carbon. The logic follows the stability of the carbocation intermediate.
Students confuse "regiospecific" and "stereospecific." Regio tells you which carbon gets which group. Stereo tells you which face of the molecule the groups add to (syn vs anti). They are independent questions.
Hydroboration-oxidation is often remembered as "anti-Markovnikov and anti addition." The regiochemistry is anti-Markovnikov, yes, but the stereochemistry is syn, not anti. Both the H and the OH end up on the same face.
Students sometimes forget that acid-catalysed hydration can give rearranged products (because it goes through a carbocation), while oxymercuration-demercuration gives Markovnikov product without rearrangement. If the question says "no rearrangement," oxymercuration is the answer.
True or False: Hydrogenation of an alkene with H2/Pd adds the two hydrogens anti to each other.
Answer: False. Hydrogenation is a syn addition.
Fill in the blank: Hydroboration-oxidation places the OH group on the ______ substituted carbon.
Answer: less
True or False: Treating an alkyne with excess HBr gives a geminal dibromide with both Br atoms on the more substituted carbon.
Answer: True.
Fill in the blank: To convert an internal alkyne to a trans-alkene, use ______, liquid NH3, at -33 °C.
Answer: Li or Na (dissolving metal reduction)
True or False: Oxymercuration-demercuration can cause carbocation rearrangements.
Answer: False. That is acid-catalysed hydration. Oxymercuration avoids rearrangement.
⚠️ You will almost certainly be asked to predict the product of an addition reaction, including both regiochemistry and stereochemistry. Know which reagent set gives Markovnikov vs anti-Markovnikov, and syn vs anti.
⚠️ Expect a question comparing acid-catalysed hydration with oxymercuration-demercuration. The key differentiator is rearrangement.
⚠️ Lindlar vs dissolving metal reduction is a favourite: both start from the same alkyne, but one gives cis and the other gives trans. Be ready to pick the correct reagent for a target alkene geometry.
⚠️ Halohydrin questions will test whether you remember that OH goes on the more substituted carbon and X goes on the less substituted carbon, with anti stereochemistry.
Q: What reagents convert 1-butyne into butanal (an aldehyde)?
A: 1. BH3, THF; 2. H2O2, H2O, pH 8. Hydroboration-oxidation of a terminal alkyne places OH on the less substituted carbon, and the resulting enol tautomerises to the aldehyde.
Q: You treat 2-butyne with H2 and Lindlar catalyst. What is the product, and what is its geometry?
A: cis-2-Butene. Lindlar catalyst gives syn addition of H2, producing the cis alkene.
Q: An alkene is treated with Br2 in CH2Cl2. Is the addition syn or anti? Could the product be meso?
A: Anti addition (trans). The product can be meso if the starting alkene is trans and both carbons of the double bond bear the same functional groups.
Q: You want to convert 1-methylcyclohexene into 1-methylcyclohexanol with OH on the more substituted carbon, without any risk of carbocation rearrangement. Which reagents do you choose?
A: 1. Hg(OAc)2, H2O, THF; 2. NaBH4 (oxymercuration-demercuration). This gives Markovnikov addition without rearrangement.
Q: What is the product when cyclohexene reacts with NBS, CCl4, and light (hv)?
A: 3-Bromocyclohexene. NBS brominates at the allylic position (the carbon adjacent to the double bond), not at the double bond itself.
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These addition reactions connect directly to the elimination reactions (E1, E2) covered in the substitution/elimination unit, because addition and elimination are reverse processes. Understanding carbocation stability here also prepares you for SN1 and E1 mechanisms. The stereochemistry concepts (syn vs anti addition) carry over to epoxide formation and ring-opening reactions covered in the next set of notes.