Difficulty: Intermediate | Prerequisites: Familiarity with Lewis structures, bond-line notation, carbocation stability, and elimination reactions (Ch. 5–7).
This chapter is where alkene chemistry shifts from "what are they" to "what do they do." Every reaction here adds something across the carbon–carbon double bond, and the differences come down to what gets added, where it lands (regiochemistry), and from which face (stereochemistry). If you are comfortable with carbocation stability, nucleophile/electrophile logic, and the basics of E1/E2 from earlier chapters, this material will click. If those are still shaky, shore them up first: nearly every mechanism here routes through a carbocation or relies on the same steric and electronic arguments.
Alkenes react by addition: something breaks the pi bond and two new sigma bonds form. The major reactions in this half of the chapter are catalytic hydrogenation (adds H₂), hydrogen halide addition (adds HX following Markovnikov's rule), and acid-catalysed hydration (adds water, also Markovnikov). Regiochemistry is governed by carbocation stability, stereochemistry by whether the addition is syn or anti, and equilibrium position by Le Chatelier's principle.
Hydrogenation
Addition of H₂ across a double bond in the presence of a metal catalyst (Pt, Pd, Ni, or Rh), converting an alkene to an alkane. Think of it as the reverse of an elimination reaction.
Catalytic hydrogenation
Hydrogenation carried out on a heterogeneous metal surface. The reaction is exothermic, and the energy released is called the heat of hydrogenation.
Heat of hydrogenation
The enthalpy change (ΔH) when one mole of an alkene is hydrogenated. A lower (less negative) value means the starting alkene is more stable.
Syn addition
Both new bonds form on the same face of the double bond. In simple terms, both hydrogens land on the same side of the molecule.
Anti addition
The two new bonds form on opposite faces of the double bond.
Stereospecific reaction
A reaction in which a particular stereoisomer of the starting material gives one specific stereoisomer (or set of stereoisomers) of the product. Catalytic hydrogenation is stereospecific for syn addition.
Stereoselective reaction
A reaction that produces two or more stereoisomeric products but favours one over the others. In simple terms, both products can form but one wins.
Electrophilic addition
An addition reaction in which the first step is attack of an electrophile on the pi electrons of the alkene. This is the main pattern for HX and H₃O⁺ additions.
Markovnikov's rule
When HX adds to an unsymmetrical alkene, the hydrogen goes to the carbon of the double bond that already has more hydrogens, and the halide goes to the more substituted carbon. Think of it as "the rich get richer" for hydrogen count.
Regioselective
A reaction that preferentially forms one constitutional isomer over another. When the preference is 100%, the reaction is called regiospecific.
Carbocation rearrangement
A 1,2-hydride or 1,2-methyl shift that converts a less stable carbocation into a more stable one. Always check for these whenever a mechanism passes through a carbocation intermediate.
Acid-catalysed hydration
Addition of water across an alkene double bond in the presence of an acid catalyst (typically H₂SO₄ or H₃PO₄), producing an alcohol. Follows Markovnikov regiochemistry.
Microscopic reversibility
The principle that the forward and reverse paths of an equilibrium reaction pass through the same intermediates and transition states, in opposite order. In simple terms, the reaction rewinds through exactly the same steps.
Le Chatelier's principle
If a system at equilibrium is disturbed, it shifts to counteract the disturbance. Adding more water pushes hydration forward; removing water favours the alkene.
Alkene + H₂, in the presence of a metal catalyst (Pt, Pd, Ni, or Rh), yields an alkane.
The reaction is the reverse of elimination and is exothermic.
Without a catalyst the reaction is extremely slow, because the H–H bond is strong and H₂ does not interact well with pi electrons on its own.
This is a heterogeneous reaction: the catalyst is a solid while the alkene and H₂ are in solution.
Common solvents: ethanol, hexane, or acetic acid (anything that dissolves the alkene).
Mechanism (four steps on the catalyst surface)
Step 1: H₂ adsorbs onto the metal surface. The strong H–H sigma bond breaks, forming two weaker metal–hydrogen bonds.
Step 2: The alkene adsorbs onto the surface. The pi bond breaks and is replaced by two carbon–metal sigma bonds.
Step 3: One hydrogen atom transfers from the metal to one of the double-bond carbons.
Step 4: The second hydrogen transfers, forming the alkane and freeing the catalyst surface for the next molecule.
Stereospecificity: syn addition
Catalytic hydrogenation is stereospecific for syn addition. Both hydrogen atoms are delivered from the catalyst surface to the same face of the alkene.
This is true for all catalytic hydrogenations, regardless of the catalyst metal.
Other alkene addition reactions can be syn, anti, or nonstereospecific, but hydrogenation is always syn.
Stereoselectivity and steric effects
When the two faces of a double bond are not equivalent (because of nearby substituents), the catalyst preferentially delivers H₂ to the less hindered face.
Example: hydrogenation of 2-methyl(methylene)cyclohexane over Pt gives 68% cis-1,2-dimethylcyclohexane and 32% trans-1,2-dimethylcyclohexane. The less crowded face of the double bond sits more easily on the catalyst surface.
This is a steric effect on reactivity: the bulky part of the molecule physically blocks one face of the alkene from reaching the catalyst.
The heat of hydrogenation tells you how stable the starting alkene is. A lower value (less exothermic) means a more stable double bond.
Stability trend by substitution (most stable last):
Ethylene (H₂C=CH₂): 136 kJ/mol
Monosubstituted (e.g. propene, 1-butene): ~125–126 kJ/mol
Cis-disubstituted (e.g. cis-2-butene): ~117–119 kJ/mol
Trans-disubstituted (e.g. trans-2-butene): ~114–115 kJ/mol
Trisubstituted (e.g. 2-methyl-2-pentene): ~112 kJ/mol
Tetrasubstituted (e.g. 2,3-dimethyl-2-butene): ~110 kJ/mol
Key pattern: more alkyl groups on the double bond = lower heat of hydrogenation = more stable alkene.
Trans isomers are more stable than their cis counterparts (less steric strain between substituents on the same side).
These data connect directly to what you already know about hyperconjugation and steric arguments from earlier chapters.
The alkene acts as a nucleophile; the hydrogen halide (H–X) is the electrophile.
Product: an alkyl halide.
Rate trends
More substituted alkenes react faster (more electron-rich pi bond).
Acid strength order for rate of addition: HI > HBr > HCl >> HF (strongest acid adds fastest).
These correlate with what you already know from SN1/SN2 and E1/E2.
Markovnikov's rule
Hydrogen adds to the carbon with more hydrogens; the halide ends up on the more substituted carbon.
This is because protonation forms the more stable (more substituted) carbocation.
When the selectivity is 100%, the reaction is termed regiospecific.
Mechanism (two steps)
Step 1 (slow, rate-determining): The pi electrons attack the proton of HX, forming a carbocation on the more substituted carbon and releasing X⁻.
Step 2 (fast): The halide ion (nucleophile, Lewis base) attacks the carbocation (electrophile, Lewis acid), forming the C–X bond.
Stereoselectivity
Addition of HBr to achiral alkenes (1-butene, cis-2-butene, trans-2-butene) produces a racemic mixture of 2-bromobutane.
Optically active products cannot form from optically inactive starting materials unless at least one chiral reagent, reactant, or catalyst is present.
Whenever a carbocation intermediate forms, there is a chance it will rearrange to a more stable carbocation before the nucleophile captures it.
Rearrangements happen by a 1,2-hydride shift or a 1,2-methyl shift (an H or CH₃ migrates with its bonding electrons to the adjacent positively charged carbon).
A secondary carbocation adjacent to a tertiary centre will rearrange to the tertiary carbocation.
Practical takeaway: if your predicted product from Markovnikov addition does not match the observed product, check whether a rearrangement of the initially formed carbocation could explain the difference.
Acid-catalysed addition of water converts an alkene into an alcohol. The acid (H₂SO₄ or H₃PO₄) is regenerated at the end, so it is a true catalyst.
Follows Markovnikov regiochemistry: the OH ends up on the more substituted carbon.
Mechanism (three steps)
Step 1 (slow, rate-determining): Protonation of the double bond forms the more stable carbocation.
Step 2 (fast): Water (nucleophile) attacks the carbocation, forming an oxonium ion (protonated alcohol).
Step 3 (fast): Deprotonation of the oxonium ion by another water molecule yields the alcohol and regenerates H₃O⁺.
Key points
More stable carbocations react faster, just as in HX addition.
Hydration and dehydration are the reverse of each other: the same mechanism runs in both directions (microscopic reversibility).
Pushing the equilibrium: excess water favours the alcohol (hydration); removing water or using concentrated acid favours the alkene (dehydration).
Le Chatelier's principle governs whether an addition or elimination product dominates at equilibrium.
For hydration/dehydration:
Adding water pushes the equilibrium towards the alcohol.
Keeping water concentration low favours the alkene.
Key equations
ΔG° = –RT ln K (relates Gibbs free energy to the equilibrium constant)
ΔG° = ΔH° – TΔS° (relates free energy to enthalpy and entropy)
Interpreting the signs
A negative ΔH means the products have stronger bonding than the reactants (exothermic, favoured enthalpically).
A negative ΔS means there are fewer molecules of product than reactant (the system becomes more ordered).
You can use these two factors together to work out whether a given addition or elimination is thermodynamically favoured under specific conditions.
General hydrogenation: C=C + H₂ → C–C (with Pt, Pd, Ni, or Rh catalyst)
General electrophilic addition: C=C + HX → alkyl halide (Markovnikov orientation)
General acid-catalysed hydration: C=C + H₂O (H₃O⁺ catalyst) ⇌ alcohol (Markovnikov orientation)
Gibbs free energy: ΔG° = –RT ln K
Gibbs–Helmholtz: ΔG° = ΔH° – TΔS°
Heats of hydrogenation decrease in the order: ethylene > monosubstituted > cis-disubstituted > trans-disubstituted > trisubstituted > tetrasubstituted
Catalytic hydrogenation is one of the most widely used reactions in industry. Margarine and shortening are made by partially hydrogenating vegetable oils (reducing some C=C double bonds in unsaturated fats to single bonds). Acid-catalysed hydration is the industrial method for producing simple alcohols such as ethanol and isopropanol from petrochemical alkenes.
Students often think Markovnikov's rule is about "the halide goes to the carbon with fewer hydrogens." That is the result, but the reason is carbocation stability: the proton adds to give the more stable carbocation, and the halide simply follows.
Students sometimes confuse stereospecific with stereoselective. Stereospecific means one stereochemical outcome (syn or anti) is required by the mechanism. Stereoselective means one outcome is favoured, but the other can still form.
A common mistake is forgetting to check for carbocation rearrangements. Any time a secondary carbocation forms next to a tertiary centre, a 1,2 shift is likely.
Students occasionally assume that hydration and HX addition use different logic. They do not: both go through a carbocation intermediate, both follow Markovnikov's rule, and both can rearrange.
⚠️ Markovnikov's rule and the carbocation stability argument behind it are tested constantly. Be ready to predict the major product of HX or H₂O addition to any unsymmetrical alkene and to justify your answer with the mechanism.
⚠️ Expect a question where the "obvious" Markovnikov product is wrong because a carbocation rearrangement intervenes. Draw the initial carbocation, check for possible 1,2 shifts, then complete the mechanism.
⚠️ Syn vs. anti addition: know that catalytic hydrogenation is always syn, and be able to predict the stereochemical outcome of hydrogenating a cyclic or E/Z alkene.
⚠️ The relationship between heats of hydrogenation and alkene stability appears on exams as a ranking or comparison question. Remember: lower heat of hydrogenation = more stable alkene.
True or False: Catalytic hydrogenation can proceed with anti addition under certain conditions. Answer: False. It is always syn.
Fill in the blank: According to Markovnikov's rule, H adds to the carbon with ______ hydrogens. Answer: more
True or False: trans-2-Butene has a higher heat of hydrogenation than cis-2-butene. Answer: False. Trans is more stable, so it has a lower heat of hydrogenation.
Fill in the blank: The rate-determining step in electrophilic addition of HBr to an alkene is formation of the ______. Answer: carbocation
True or False: Acid-catalysed hydration and acid-catalysed dehydration share the same intermediates and transition states. Answer: True (microscopic reversibility).
Q: Predict the major product when 2-methylpropene reacts with HBr. Explain your reasoning.
A: tert-Butyl bromide (2-bromo-2-methylpropane). Protonation of the double bond occurs to form the more stable tertiary carbocation rather than the less stable primary carbocation. Bromide then attacks the tertiary carbon.
Q: When 1-butene is hydrogenated over platinum, what is the product and what is the stereochemistry of the addition?
A: Butane. The addition is syn (both hydrogens delivered from the same face), but because butane has no stereocentres in this case, the syn selectivity does not produce distinguishable stereoisomers.
Q: Rank the following alkenes in order of increasing stability: ethylene, trans-2-butene, 2-methyl-2-butene, propene.
A: Ethylene (least stable) < propene < trans-2-butene < 2-methyl-2-butene (most stable). Stability increases with increasing substitution of the double bond.
Q: 3,3-Dimethyl-1-butene reacts with HCl. The expected Markovnikov product would be 2-chloro-3,3-dimethylbutane, but the actual major product is 2-chloro-2,3-dimethylbutane. Explain.
A: Initial protonation gives a secondary carbocation at C-2. This rearranges via a 1,2-methyl shift to give a more stable tertiary carbocation at C-2 (now bearing three alkyl groups). Chloride captures the tertiary carbocation, giving the rearranged product.
Q: In acid-catalysed hydration, what experimental change would you make to push the equilibrium towards the alcohol product?
A: Use a large excess of water. By Le Chatelier's principle, increasing the concentration of a reactant shifts the equilibrium towards the products.
The carbocation intermediates here are the same species you encountered in SN1 and E1 reactions (Ch. 5–7). If you can rank carbocation stability for substitution and elimination, you can predict regiochemistry for electrophilic addition, and vice versa. The thermodynamic arguments (ΔG, ΔH, ΔS, Le Chatelier) will return in alcohol chemistry (Ch. 9–10), where dehydration of alcohols is the exact reverse of the hydration covered here. Stereochemistry of addition sets up the more complex stereochemical arguments in halogenation and epoxidation (covered in Part 2 of these notes, sections 8.8–8.14).
Alkene addition reactions, hydrogenation, catalytic hydrogenation, Pt Pd Ni Rh catalyst, syn addition, anti addition, stereospecific, stereoselective, heat of hydrogenation, alkene stability, electrophilic addition, hydrogen halide addition, HBr HCl HI addition to alkenes, Markovnikov's rule, Markovnikov addition, regioselective, regiospecific, carbocation, carbocation rearrangement, 1 2 hydride shift, 1 2 methyl shift, acid-catalysed hydration, hydration of alkenes, Le Chatelier's principle, microscopic reversibility, thermodynamics of addition, ΔG ΔH ΔS, Organic Chemistry I, OChem Chapter 8